Crop Profile for Corn (Field, Grain) in Wyoming

Revised: October, 2004

Wyoming Facts

Data from 1999-2003 Wyoming Agricultural Statistics Service

Northwest District Counties:
Big Horn, Park, Washakie

Southeast District Counties:
Goshen, Laramie, Platte

Description of crop
An annual, monoecious crop. Production focused on grain for sale or storing the plant - referred to as silage. Used for corn oil, livestock feed, and ethanol production.

Cropping system
Corn can be rotated with the following crops: small grains, alfalfa, sugarbeets, potatoes, and beans. A common rotation for dryland corn is wheat-corn-fallow.

Most corn acreage is irrigated, either by furrow or sprinkler systems. Dryland corn is profitable in areas where conservation and/or reduced tillage planting techniques are used, and in areas where the average precipitation is 9" or more during the months of May, June, July and August. With irrigated corn, pest problems may arise when corn is rotated with wheat or other small grains. Conservation and/or reduced tillage strategies that reduce runoff and erosion increase the amount of off-season moisture stored in the soil profile. The reproductive stage is the most important time to avoid water stress (two weeks before silking to two weeks after silking). Tillage is common in furrow-irrigated production.

Planting dates range from April 15 to May 15. Soil temperatures should reach 55° F before planting corn seed and germination of corn seed requires a soil temperature of 65° F. The maximum temperature for optimal growth is 90° F. In early spring it is important to plant at least 2" deep into the soil. Thirty inch row widths are common. Planting densities range from 26,000 - 36,000 plants per irrigated acre of corn, whereas dryland corn planting densities range from 12,000 - 18,000 plants per acre. Often, 5 -10% more seed than the target population are planted to compensate for germination or seedling loss. Planting depths of over 3 “ may result in severe yield loss.

Fertilizer recommendations are: 180 lb. of nitrogen, 50 lb. of phosphorous (P2O5) and 5 lb. of zinc per acre. Nitrogen is the major fertilizer requirement in non-sandy soils.

Harvest dates vary with moisture content; silage corn is harvested after Labor Day, whereas grain corn is harvested in November. Moisture is an important factor for corn being stored as grain. The grain must have a moisture content below 15% and the storage bins must be well aerated to avoid pest damage in the crop. Corn stubble left in the field is often used for grazing cattle.
 

Key Diseases
Common smut and Head smut, Ustilago zeae and Sporisorium holci-sorghi
Two smut diseases occur on corn in Wyoming, common smut, Ustilago zeae and head smut, Sporisorium holci-sorghi. Common smut appears as galls filled with a black spore mass on stems and ears. Occasionally blisters form on leaves, but these do not contain spores. Head smut affects tassels and ears, causing a leafy appearance and sterility. Plants are stunted. Both of these fungi overwinter in or on the soil. Basidiospores of U. zeae infect fast growing, meristematic tissues primarily through wounds and stomates. S. holci-sorghi infects seedlings and grows systemically to the floral tissues. Control of common smut is through proper soil fertility and careful cultivation to avoid wounding. Head smut is best controlled with resistant cultivars. These fungi survive in debris, soil, seed and transplants. Best management practices include using resistant cultivars, seed treatment with fungicides such as carboxin and crop rotation.

Goss’s wilt, Corynebacterium michiganese subsp. nebraskensis
Goss’s wilt causes water-soaked streaks, parallel to the leaf veins, to occur on the leaves. Dark, angular, water-soaked spots form next to the leaf veins. The fibrovascular bundles in systematically-infected stalks are discolored. Affected plants may be stunted. Plants can be infected, wilt and die at any stage. The bacterium overwinters in corn debris near the soil surface and in seed. Genetic resistance is available in hybrids. Growers may reduce incidence by rotation and tillage.

High Plains Disease (HPD)
HPD infects corn in the high plains of the western U.S. Most dent corn, a classification of grain corn, is resistant. Transmission is by the wheat curl mite, Aceria tosciella, which also transmits wheat streak mosaic virus. Mixed infections of HPD and wheat streak mosaic frequently occur. Symptoms on corn are severe stunting, yellowing and a strong mosaic. These symptoms are somewhat similar to those of corn lethal necrosis. However, corn plants with HPD often have distinctive longitudinal red stripes on the leaves. Affected plants typically occur next to wheat fields. Planting near volunteer wheat increases this disease. Fortunately, only a few corn hybrids seem to be affected. Warm fall temperatures favor the wheat curl mite which vectors HPD. Symptoms are similar to wheat streak mosaic, but they tend to be spottier. Yellow areas in the field appear in the spring especially near the edges adjacent to volunteer wheat. No specific control is known for HPD, but it is thought that similar controls as for wheat streak mosaic virus could be helpful. The key is to break the green bridge and prevent spread of the disease to corn or other hosts: 1) plant corn early, 2) control volunteer wheat and grassy weeds that can harbor the disease and the vector, and 3) plant resistant varieties when they become available.

Nematodes, Root Lesion, Pratylenchus spp., possibly others
In general, above ground symptoms are not readily noticable. However symptoms from severe nematode damage appear as stunted growth, chlorosis, and wilting during periods of high temperture and moisture stress. These symptoms can be similar to those induced by nutrient deficiencies. Root symptoms of nematode infection vary with the kind of nematode. Nematode damage is often most severe on stressed corn plants. Nematicides are labeled for use in Wyoming, but not generally recommended. In some cases, it may be necessary to rotate heavily infested fields to crops other than corn.

Seedling blight, Fusarium spp., Pythium spp., Diplodia spp., and Rhizoctonia solani
Pre- and post-emergence damping-off and seedling blights occur with all crops. Fusarium spp. and Pythium spp. are the most common pathogens responsible for seedling blight in corn, but Rhizoctonia solani and other fungi such Diplodia, Collectotrichum, and Peniciilium can be can be involved in some instances. They are characterized by missing or collapsed plants. When pre-emergence damping-off occurs, seedlings fail to break the soil surface. The germinating seed is rotted while still in the ground. With post-emergence damping-off, seedlings still in the cotyledon stage will rot at the soil line. The stem appears pinched and plants fall over. Seedling blights occur after true leaves emerge. The developing root system rots and plants collapse. Seedling diseases are problems when soils are cold and wet at or immediately following planting. These conditions can delay germination or stress developing seedlings, which allows pathogens to attack. Crusting caused by poor soil preparation or hard rain, or excessive trash on the surface also contributes to disease. Fungicide seed treatments such as captan, carboxin, metalaxyl, PCNB, or combinations along with avoiding cold wet soils and good soil preparation will best manage these diseases. Post emergence application of fungicides such as mancozeb directly to the soil is not common.

Stalk rots and Ear rots, Fusarium moniliforme, F. subglutinans, Gibberella zea, Macrophomina phaseolina
There are three prominent stalk rots in Wyoming. Fusarium stalk rot, Fusarium moniliforme or F. subglutinans, is the mildest. The symptoms include root rot, stunting, premature senescence, and poor yield. The interior of infected stalks appear darkened and water soaked at the early stages of disease development. Later the pith can take on a pink to red color. The fungus survives as mycelium or spores in debris and soil. Infection occurs through roots of seedlings and immature plants. Disease is favored by stresses such as high N, water stress, soil compaction, and high plant density. Use of resistant cultivars and alleviating sources of plant stress are recommended for management. There are no effective fungicides for stalk rots. Gibberella stalk rot, Gibberella zea, is very similar to Fusarium stalk rot but is much more severe. Stalks become completely hollowed out and lodging is common. Insects such as rootworms and stalk borers increase stalk rot damage by weakening plants and carrying the fungus to wounds. Charcoal rot, Macrophomina phaseolina, occurs under hot and dry conditions. It is found most often in the southeastern part of the state. The interior pith of stalks is rotted out and replaced by small black sclerotia. Lodging and yield losses can be severe. Fusarium stalk rot typically occurs in complexes with other stalk rots such as Gibberella, Diplodia, Pythium, Collectotrichum and occasionally the bacterium, Erwinia. It is important to maintain a healthy, stress-free crop to combat these diseases. Scout fields 40 to 60 days after pollination. If 10 to 15% of stalk split show signs of stalk rot, the fields must be harvested early.

Wheat streak mosaic virus
Early symptoms of wheat streak mosaic virus appear as small chlorotic spots of broken streaks at the tips of young leaves. The streaks elongate and develop parallel to the veins. Symptoms vary greatly with the plant genotype. Very susceptible lines may have poorly developed ears with little or no seed set, and general yellowing and stunting of the plants may occur. Transmission of wheat streak mosaic virus is by the minute wheat curl mite, Aceria tosichella. Yellow areas appear on the field edges adjacent to volunteer wheat during spring. The key is to break the green bridge and prevent spread of the disease to corn or other hosts: 1) plant corn early, 2) control volunteer wheat and grassy weeds that can harbor the disease and the vector, and 3) plant resistant varieties when they become available.


Additional Diseases
Barley Yellow Dwarf
Barley yellow dwarf is caused by a group of barley yellow dwarf viruses that infect many crops including barley, corn, oats, and wheat. The vectors are a group of aphids, including greenbugs, oat bird cherry aphids, corn leaf aphids, and English grain aphids The characteristic symptoms occur on barley. Plants are stunted and have a stiff, crumpled appearance. Tillering is reduced and heads do not fill. Individual leaves turn yellow from the tips in a zig-zag pattern that leaves the midvein green. The virus survives in aphids, grassy weeds, volunteer plants and fall-planted cereals. Infected plants are located at random in a field where winged adults have landed. These plants develop into foci of disease. The disease is favored by cool, wet weather in the spring and early summer. Management of barley yellow dwarf depends on reducing the green bridge between seasons or alternative hosts grown opposite the corn. Use of insecticides is seldom economical. Corn, even though it does not exhibit symptoms, is a very important reservoir for both the pathogen and some of its vectors.

Corn Leaf Rust, Puccinia sorghi
Corn leaf rust forms small, reddish brown to black, linear streaks or pustules on the leaves. Defoliation and plant stunting may occur. Cultural controls include crop rotation and resistant varieties. In years with economic levels of infection, fungicides such as chlorothalonil and mancozeb are available and must be applied early to be effective.


Key Disease Management Strategies
Cultural Controls
Deep tillage, seed treatment, crop rotation, stress management and resistant hybrids reduce corn disease problems. Corn stalk rot, as well as other diseases, favor stressed plants. Cool weather at planting, excessive nitrogen, low potassium, over planting, moisture stress and insect damage encourage disease and pest problems. Although diseases commonly affect corn crops, most disease problems are managed with cultural controls rather than treated with fungicides.


Nematicides – (Not generally recommended)

Pesticide: dichloropropene (Telone II)
Target Pests: Nematodes
Recommended rate: 1124.57 - 2249.03 fl oz ai/A (9-18 gal of product/A)
Comments: Liquid fumigant. Pre plant application and aerate.

Pesticide: dichloropropene + chloropicrin (Telone C-17)
Target Pests: Nematodes
Recommended rate: dichloropropene- 1083.67-2167.42; chloropicrin- 228.36-456.74 fl oz ai/A (10.8-21.6 gal of product/A)
Comments: Pre plant application and aerate

Pesticide: metam sodium (Vapam)
Target Pests: Nematodes
Recommended rate: 2018.39-4036.79 fl oz ai/A (37.5-75 gal of product/A)
Comments: Pre plant application and aerate


Fungicides –

Pesticide: captan (Captan 30DD)
Target Pests: Damping off, seedling blights
Recommended rate: rate per 100 lb seed, 0.44 fl oz ai/A (1.5 fl oz of product/A)
Pesticide: captan (Captan 400)
Target Pests: Damping off, seedling blight
Recommended rate: rate per 100 lb seed, 0.48 - 0.91 fl oz ai/A (1.25 - 2.38 fl oz of product/A)
Comments: Seed Treatment

Pesticide: carboxin (Vitavax 34)
Target Pests: Damping off, seedling blights, head smut
Recommended rate: rate per 100 lb seed, 0.68 - 1.36 oz ai/A (2-4 oz of product/A)

Pesticide: chlorothalonil (Equus 720, Equus 720 SST)
Target Pests: Rust, leaf blight
Recommended rate: 6.48 - 17.28 fl oz ai/A (0.75 - 2 pt of product/A)
Comments: Do not store treated seed in silo or allow livestock to forage. Preharvest interval 14 days.

Pesticide: mancozeb ( Manzate 80 WP)
Target Pests: Damping off, seedling blights, rusts
Recommended rate: 19.2 oz ai/A (1.5 lb of product/A)
Comments: Preharvest Interval 40 days. Do not feed forage to livestock.

Pesticide: metalaxyl (Apron Flowable)
Target Pests: Damping off
Recommended rate: rate per 100 lb seed, 0.03 - 0.46 fl oz ai (0.11 -1.64 fl oz product)
Comments: Seed Treatment

Pesticide: metalaxyl + captan + diazinon + lindane (Agrox premiere with Apron)
Target Pests: Damping off, seedling blights
Recommended rate: metalaxyl- 0.02 ai; captan- 0.26 ai; diazinon- 0.28 ai; lindane- 0.45 oz ai/ 50 lb seed (1.8 oz of product per 50 pounds of seed)
Comments: Planter Box Seed Treatment. Do not store excess treated seed beyond planting time.

Pesticide: PCNB (RTU-PCNB)
Target Pests: Damping off, seedling blights
Recommended rate: rate per 100 lb seed, 0.72 fl oz ai/A (3 fl oz)
Comments: Seed Treatment

Pesticide: thiram (42S Thiram)
Target Pests: Damping off, seedling blight
Recommended rate: 0.63 fl oz ai per bushel (1.5 fl oz per bushel)


Key Insects
Banks Grass Mite, Oligonychus pratensis
Banks grass mites are important pests of field corn. Mites are microscopic arthropods that feed on the sap of host plants. They have eight legs (six in the first stage). Banks grass mites are extremely small, 0.001", and yellow to cream colored. Heavy populations of Banks grass mite can kill small plants and reduce kernel size in larger plants. They produce heavy webbing to protect colonies consisting of eggs, larvae and adults. Damaged leaves first become yellow, then brown and finally necrotic. The overwintering form of the mite is bright orange. With the onset of winter, the mites move to wheat plant crowns where they feed until spring when they travel to corn plants either by walking short distances or by “ballooning” - becoming windborne on web spun silk threads over longer distances. Small pearly white eggs are laid which eventually give rise to pale to bright green male and female adults. Banks grass mites are most likely to infest water- stressed plants, so irrigation can be an important cultural control mechanism. Factors that encourage Banks grass mite infestations include host drought stress, high temperatures, low rainfall, low humidity, absence of two-spotted spider mites (Tetranychus urticae), lack of natural enemies, insecticide use for other target pests, and adequate moisture for alternate hosts during the previous growing season. Most Banks grass mite problems occur in the drier growing areas and are always associated with grasses. For example, many problems in corn begin when wheat in adjacent fields dries down. Banks grass mite has significant resistance problems in neighboring states. Threshold-based miticide treatments such as dimethoate can be prescribed if the level of infestation increases. Insecticide treatments should be avoided after tasseling to avoid spider mite outbreaks. Preserving naturally occurring spider mite predators by avoiding broad spectrum insecticides is critical to proper management of spider mite problems in corn.

European corn borer, Ostrinia nubilalis
The European corn borer is one of the most destructive pests of corn in the U.S. The larvae are creamy white to flesh-colored with small, dark spots on each abdominal segment. Heads are dark brown. Full grown larvae are 0.75 - 1" long. Female moths are buff-colored with wavy, olive-brown bands on the wings and a 1" wingspan. Male moths are slightly smaller and darker than females. The European corn borer usually goes through two generations each year. On rare occasions there may be a partial third generation. Corn borers require 3 - 4 weeks to develop from egg to adult. Borers overwinter as full grown larvae in corn stalks, cobs and plant debris in corn fields. Most of the overwintered larvae will go through the pupal stage and develop into the spring moths by the middle of June. On warm, calm evenings in June, adults fly from weedy or grassy margins into corn fields and lay eggs, usually when the corn is in the early whorl stage. Eggs are usually laid near the midrib on the underside of corn leaves in clusters of 15 - 25, overlapping like fish scales. Eggs hatch in about 5 - 7 days in mid to late June. Young larvae feed first on the leaf near where they hatch. As the larvae grow, they move to the whorl or leaf sheath area and feed. When leaves emerge, the “shot-hole” feeding signs in the leaves can be seen. Larvae may also feed in the leaf midrib. Most of the mature larvae bore into the stalks, feed, and finish development there. Fully grown first generation larvae develop into pupae from which the summer moth emerges in mid to late July, thus beginning the second generation. Female summer moths prefer to lay eggs in tasseling corn or green silk corn. Corn planted unusually late is more severely damaged. Eggs are laid on the underside of the leaves between the ear zone and the tassel. Corn borer damage results from leaf feeding in the first generation resulting in loss of leaf tissue, interference in the movement of plant nutrients and midrib breakage. If extensive, this injury can cause a substantial reduction in yield. Damage occurring from stalk tunneling in all generations results in destruction of food-conduction channels. This weakens the plant, causing stalks breakage, smaller ear size and weight, and reduced yield. Tunneling also makes the plant vulnerable to organisms that cause stalk rot. Any ear damage in the second generation, tunneling in the shank, and feeding on silks, kernels and cobs results in yield loss, quality impairments, dropped ears and broken shanks. European corn borer is a significant pest in eastern Wyoming, mostly in sprinkler irrigated corn. Treatment should be considered when 25% of plants are infested with egg masses or larvae. After pollination, this percentage should be raised to at least 50 % infestation to justify chemical treatment. Bt (Bacillus thuringiensis) corn has pretty good resistance to first generation of European corn borer.

Southwestern Corn Borer, Diatraea grandiosella
The preferred host of the southwestern corn borer is corn, though a few species of wild and cultivated grasses serve as alternate hosts. They are similar to the European corn borer, but are more difficult to control. Larvae of this species are the most destructive stage in the life cycle, and their appearance varies depending on whether they are summer form or winter form larvae. Summer form larvae are off white with distinct black spots. Winter form larvae are those that overwinter and pass through diapause, or suspended development. They are uniformly light yellow and completely devoid of pigments. Full grown larvae are 1.25" long. Southwestern corn borers complete a generation in 6 - 8 weeks, and pass through at least two, if not three, generations per year. Adults are dull white moths that emerge from stubble, mate and deposit their eggs on upper and lower corn leaf surfaces. Eggs are laid singly or in groups of 2 - 5. Egg color initially is greenish white, but within 24 - 36 hours three distinct red lines running the length of the eggs appear. Eggs hatch in 4 - 7 days. First generation larvae appear in June and begin to feed on the corn whorl. With time, they move, tunneling into the stalk and destroying bud leaves. Severe infestations result in “dead heart”, completely killing the plant. Plants that survive feeding by first generation larvae are generally stunted. Feeding by the second and third generation larvae is the most damaging to corn yield. Second generation larvae emerge in mid to late summer and begin feeding between corn husk layers on the primary ears. After two weeks of feeding, they tunnel into the stalk. Most of the damage caused by this generation is due to stalk breakage because of the extensive tunneling. Third generation and second generation, depending on the time of year, larvae prepare for diapause by migrating to the base of the stalk below ground level where they prepare an overwintering cell. This involves girdling the plant a few centimeters above the ground and leads to lodging. Southwestern corn borers can survive in the dormant state from September to May. Population buildups have been linked to conservation tillage, which leaves the overwintering habitat undisturbed, and an increase in crop irrigation which has extended corn acreage throughout the range of the southwestern corn borer. Cultural practices which minimize the impact of corn borer feeding includes early planting and fall or spring discing or plowing. Resistant varieties of corn have improved southwestern corn borer management strategies such as Bt (Bacillus thuringiensis) corn.

Two-spotted Spider Mite, Tetranychus urticae
Spider mites can be serious pests of corn, especially during hot, dry years. Two-spotted spider mites generally appear on corn after flowering and rapid population growth occurs after pollen shed. Adult females are pale to dark green with conspicuous dark spots on each side. Food particles showing through the transparent body wall causes these darkened areas. Eggs are spherical, clear and colorless when first laid, becoming opaque and turning ivory just before hatching. Newly hatched larvae are initially colorless and become green after feeding. Individuals from both nymphal stages are green to pale yellow. Adult females usually overwinter on the host plant or in nearby debris. Overwintering females are pale orange to straw-colored and active, though they do not produce eggs during the winter. Two-spotted spider mites usually begin colonies on the undersides of plant leaves. Webbing is visible to the naked eye and all stages of mite development occur in and beneath the webbing. Many female mites may be present on each infested leaf. Small colonies usually begin near a leaf vein, and with favorable weather, spread rapidly to nearby leaves and plants. Development from egg to adult requires 5 - 15 days. Feeding usually takes place on lower leaf surfaces, though upper leaf surfaces are occasionally infested. Light to moderate infestations on lower leaf surfaces can be detected by a marked stippling on upper leaf surfaces. As feeding continues and the mite population increases, leaf color becomes lighter as a result of chlorophyll removal from leaf tissue. Heavily damaged leaves turn tan or bronze, become desiccated and usually die. Optimum temperatures for two-spotted spider mites are 86 - 90°F and less than 50% relative humidity. They occur in more humid growing areas such as along river bottoms. Threshold-based miticide treatments such as propargite or bifenthrin can be prescribed if the level of infestation increases because of a reduction in natural predators. Insecticide treatments should be avoided after tasseling to avoid spider mite outbreaks because of reduction of predation.

Western Bean Cutworm, Richia albicosta
The Western bean cutworm, originally a pest of bean fields, has become a serious pest of field corn. Western bean cutworm larvae are dark brown with faint diamond-shaped markings on their backs. As they grow, the larvae change to a lighter color and by maturity they are gray to pinkish brown. At maturity, three short dark stripes running lengthwise on the first segment behind the head appear. The overall color of the adult moth is dark brown. The body is light brown and 0.75" long. Wings spread 1.5" and are marked with creamy white stripes on the front leading edge. There is only one generation per year. Cutworms overwinter as prepupae. In May and early June they develop into pupae. The moths emerge between July 14 and August 1. Eggs are laid in masses ranging from 4 - 200 eggs per mass shortly after the moths emerge. When first laid, the eggs are white. As they mature, they change to brown and then immediately prior to hatching they are purple to black in color. Eggs hatch in 5 - 7 days. Following hatch, cutworm larvae move to one of two places on the corn plant, depending on the stage of development of the corn. If the corn has not tasseled, young cutworms migrate to the whorl and feed on pollen in the developing tassel. If the corn has tasseled, they migrate to the ear and feed on the silk. As the cutworms mature, they concentrate their feeding on the developing kernels. They reach the kernels either through the silk or through holes cut in the husk. The majority of cutworms feed until mid-September. When the larvae mature, they enter the soil, construct earthen cells at a depth of 3 - 9", and develop into the pre-pupal stage to overwinter. Young western bean cutworm larvae may cause poor pollination of corn by feeding on the silks. Older cutworm larvae feeding can destroy as much as 25 - 30% of the kernels on an ear. Kernal destruction by two or more cutworms per ear may reduce the yield of shelled corn by as much as 30 - 40%. Usually the reduction of yield in corn intended for silage is not as severe. However, due to kernel loss, silage quality is compromised. If more than 8% of the plants have egg masses and the crop is 95% tasseled, insecticides such as bifenthrin, chlorpyrifos, esfenvalerate, lambda-cyhalothein, methyl parathion, or permethrin may be necessary.

Western Corn Rootworm, Diabrotica virgifera virgifera
Corn rootworms (a complex of four species in the genus Diabrotica) are among the most serious pest on corn in the U.S. Western corn rootworms cause the majority of the damage to field corn grown on medium to heavy textured soils throughout the state. Adult western corn rootworms have black or black and yellow wing covers. Larvae are white with a brown head and tail plate. Western corn rootworms overwinter in the soil as eggs which hatch in late spring. Egg hatch is dependent on soil temperature and usually begins in mid-May, while eggs in cooler areas will not hatch until early June. After hatching, the small rootworm larvae move to nearby corn roots and begin feeding on root hairs and small roots. Larger rootworms feed on and tunnel into primary roots. By early July, most rootworm larvae have finished feeding and have metamorphosed into adult beetles after completing a week long pupal stage. Adult beetles emerge from the soil to feed on corn leaves and weed blossoms, though they prefer to feed on corn silks and pollen. Beetles clip off fresh, green silks at the ear tip. Once the plants have completed pollen shed, adults migrate to later planted fields or other pollen producing crops. Beetles mate in July and August. Females, which begin laying eggs when they are about two weeks old, deposit their eggs almost exclusively in corn fields in the top four inches of soil. Western and northern corn rootworms complete one generation per year. The larval stage of corn rootworms cause the most economical damage. Under heavy rootworm pressure, corn root systems can be completely destroyed. However, damage usually is not that severe. Yield losses due to rootworm feeding are difficult to determine, as many corn varieties can compensate for some root damage. In general, economic losses occur after one or more primary roots are chewed to within 1.5" of the plant. Rootworm damage can also result in extensive “gooseneck” lodging that makes the crop difficult to harvest mechanically. Adult rootworm silk feeding is thought to interfere with pollination and grain production. Studies under Colorado conditions have failed to show significant yield loss from as many as 20 beetles feeding on a single ear, so insecticide treatments to prevent such damage are generally not recommended. Usually, insecticides such as carbofuran, chloropyrifos, or phorate are applied before corn emerges. Corn growers have another solution in 2003: genetically engineered Bt (Bacillus thuringiensis) corn targeted specifically to corn rootworms. Registration of Bt corn specifically for rootworms is conditional and only approved for use until 2006 at this time.


Additional Insects
Dingy Cutworm, Feltia ducens
Dingy cutworms eat leaves on young corn plants. The larvae found in corn during late April and early May are usually dingys. They only have one generation a year and rarely cut corn making identification important when leaf feeding is present in a field. Insecticides such as bifenthrin, carbaryl, chlorpyrifos, esfenvalerate, lambda-cyhalothein, methyl parathion, or permethrin are effective if applied in a timely manner and directed at the soil around the base of the crop plants. Insecticides such as permethrin, chloropyrifos, or carbaryl are considered if 1 in 20 (5%) plants are damaged and cutworms are present.

Pale Western Cutworm, Agrotis orthogonia
Pale western cutworm is one of several species of caterpillars that attack field corn. The pale western cutworm is a major pest of grains. Adult moths emerge from the soil in late summer and fall. Eggs are deposited in loose soil and usually hatch in late winter. Under some conditions, eggs hatch in the fall and the insect overwinters as a partially grown caterpillar. Hatch may be delayed for up to several months if moisture and temperature conditions are unfavorable. Larvae prefer loose, sandy or dusty soil and are found most easily in the driest parts of the field, such as hilltops. After feeding is complete, pale western cutworm larvae move to pupal chambers constructed several inches below the soil surface. Adults begin to emerge in August, but most egg laying occurs in September. Pale western cutworm is a subterranean cutworm, feeding on stems at the crown. In corn, the cutworms may cut off small seedlings below ground. Outbreaks are associated with dry conditions in the previous spring. Pale western cutworms seem to feed under dry conditions, so yield relationships are difficult to define. Insecticides such as bifenthrin, carbaryl, chlorpyrifos, esfenvalerate, lambda-cyhalothein, methyl parathion, or permethrin are effective if applied in a timely manner and directed at the soil around the base of the crop plants. Insecticides such as carbaryl are considered if one plant in 20 is injured, and cutworms are present.

Corn Earworm, Helicoverpa zea
The corn earworm may appear in various colors, including yellow, pink, green, and black. Their bodies have alternating light and dark stripes and are covered with small spines. Regardless of body color, all have yellow-brown head capsules, except when newly hatched. Fully developed larvae are 1.5 - 2" long. Adult moths are buff to grayish-brown with a wingspread of about 1.5". Most adult moths migrate north from the southern states in the spring. Female moths lay a single off-white colored egg. Corn earworms are migratory, causing infestation timing and generation numbers to vary. First generation earworms often feed in corn whorls, producing “shot-holes” and damaging developing tassels. Second generation moths seek out green corn silks to deposit their eggs. The eggs hatch into young larvae in 2 -10 days and begin feeding on the corn silk, sometimes clipping it. Later, the larvae bore through the silk channel to the ear tip and begin feeding on the kernels. Larger larvae are cannibalistic, so usually only one larva reaches maturity in an ear of corn. Mature earworm larvae crawl down the stalk, burrow into the ground and pupate in an earthen cell. Adult moths emerge from the pupal cells 10 - 25 days later. Larvae begin feeding on silks and later tunnel into ears where they feed on developing kernels, usually beginning at the ear tip. Fecal pellets (frass) accumulate along feeding channels. Not only do larvae cause direct loss by feeding on kernels but also cause openings in the husks that allow entry of disease organisms and bird feeding. Non-chemical control includes predacious insects and parasitic wasps. Insecticides are rarely economically practical in field corn, but Bt (Bacillus thuringiensis) hybrids offer some control.

Grasshoppers
Grasshoppers follow roughly a 22-year cycle, with the last major outbreak occurring in the late 1970s and early 1980s. Grasshoppers vary considerably depending on the species. The major species are four species of Melanoplus and Camnula pellucida. Grasshopper nymphs have a similar appearance to adults but are smaller in size. Grasshoppers lay eggs in undisturbed areas, usually in late summer and early fall. Small nymphs or “hoppers” hatch the following spring. Winged adults will appear 5 - 6 weeks after hatch. A few Wyoming grasshopper species have eggs that hatch in late summer and overwinter as nymphs. Winged adults of these species usually appear early in the following summer, often causing undue alarm about unusually early grasshopper activity. Some of these species are important on rangeland, but none are considered a threat to field crops. The usual pattern of grasshopper damage in field crops is for early development to occur in weedy areas of roadsides, fence rows, irrigation ditches, and other non-crop areas. As these food sources are exhausted or begin to dry down, the grasshoppers leave in search of other food - often an irrigated crop. Here they will first feed in the field margins and subsequently spread throughout the field. Most field crop damage is caused by the differential, red-legged, two-striped, and migratory grasshoppers. Chemical treatment may be justified if there are more than 20 nymphs per square yard in field margins. Foliar applied insecticides such as bifenthrin, chlorpyrifos, lambda-cyhalothein, malathion, methyl parathion, and permethrin are effective if corn is infested at economically damaging levels.

Seedcorn Maggots, Delia platura
The adult seedcorn maggot is a hairy, gray fly about 0.2" long that lays eggs in areas high in decaying organic matter. The maggot is yellowish white, legless and about 0.25" long. The first adult activity in the spring occurs in early May during corn planting. Offspring of these adults mature to form a second generation of adults in late May or early June when beans are planted. These adults are also attracted to areas of high organic matter that have just been tilled. Maggots will feed on the germinating seed and hollow out or otherwise damage cotyledons. Later feeding by the maggots may be evident as burrowing within the stem. This feeding may increase the incidence of secondary rotting within the stem. Scout for seedcorn maggot where plants have failed to emerge. Problems occur where high organic matter attracts female flies and cool, damp soil conditions delay seed germination and prolong the period vulnerable to maggot attack. Scout where there is poor emergence, gaps or skips. If there is extensive damage consider options such as replanting. Insecticidal treatments such as bifenthrin, carbofuran, chlorethoxyfos, phorate, tefluthrin, or terbufos are used only if a soil insecticide is not used for other pest problems.

Wireworms, Family: Elateridae
Wireworms are the larval stage of a family of beetles commonly called click beetles. Adults are brown or black and elongate, tapering toward each end but more so towards the rear. Earlier larval stages are very small and white, later stages have a characteristic hard shell appearance and a shiny yellow to reddish-brown color with six slender legs. Mature larvae range from 0.5 - 1" in length, depending on the species. Wireworms usually overwinter in the adult stage. Females deposit eggs in the soil. Wireworm damage usually is detected after the opportunity for making preventive pest management choices is past, so scouting for this pest prior to planting is necessary. Typical recommendations are scouting for wireworms at least one week before planting and using bait traps. Wireworm cultural controls include rotating to non-host crops except soybeans as well as tilling fallowed fields. If stand growth is significantly reduced early in the season, replanting may be an option. If wireworms have been a problem in the past, seed treatments such as lindane or treatment at planting with chloropyrifos can manage this pest. For seed treatments to be effective, all seed must be coated completely with lindane. Wireworms are more effectively controlled before planting or with cultural controls.


Key Insect Management Strategies
Cultural Controls
The most reliable way to control corn rootworm is through crop rotations; the field needs to be rotated out of corn for at least a year for rootworm management. Accurate timing and application of irrigation water and preventive soil applied insecticides improve rootworm control. Bt (Bacillus thuringiensis ) corn and resistant hybrids provide good control of European corn borer and southwestern corn borer with some control of corn earworm. In 2003, Bt corn targeting corn rootworms has become available and will be until 2006 at this time. Avoiding plant stress and avoiding rotation with wheat or planting near wheat are effective spider mite controls.

If necessary, threshold-based insecticide applications of spinosad, methomyl, esfenvalerate, or chloropyrifos can be applied in refuge areas of corn earworm. Western bean cutworms, cutworms and grasshoppers can be controlled with threshold-based insecticide applications of bifenthrin, chloropyrifos, esfenvalerate, and permethrin.


Insecticides–

Pesticide: bacillus thuringiensis (Dipel DF)
Target Pests: Army cutworm, Corn earworm (Headworm), European corn borer
Recommended rate: 8.64 - 17.28 oz ai/A, depending on the insect (1 - 2 lb of product/A)
Comments: Genetically engineered Bt (Bacillus thuringiensis) corn is currently specific to European corn borer and southwestern corn borer

Pesticide: bifenthrin (Capture 2EC)
Target Pests: Armyworm, Banks grass mite, Corn earworm, European corn borer, Fall armyworm, Grasshoppers, Seedcorn maggot, Southwestern corn borer, Two spotted spider mite, Western bean cutworm, corn rootworm adults
Recommended rate: 0.75 - 1 fl oz ai/A (3 - 4 fl oz of product/A- Foliar)
Comments: Preharvest interval 30 days. Do not allow livestock to graze treated area within 30 days of treatment.
Use Data: 1997 = rate used 0.10 lb ai/A
1% of total acres treated

Pesticide: carbaryl (Sevin XLR Plus)
Target Pests: Armyworms, Corn earworm, Fall armyworm, Pale western cutworm, corn rootworm adults
Recommended rate: 14.11 - 28.22 fl oz ai/A (1-2 qt of product/A)
Comments: Preharvest Interval 48 days. Do not treat within 14 days of silage or grazing.

Pesticide: carbofuran (Furadan 4F)
Target Pests: European corn borer, grasshoppers, Seedcorn maggot, Southwestern corn borer, Western and Northern corn rootworm
Recommended rate: 1.1 fl oz ai/1000 ft row ; 10.56 - 14.08 fl oz ai/A(2.5 fl oz of product per 1000 ft row ; 24-32 fl oz of product/A)
Comments: Do not apply to seed corn. Pre Harvest Interval of 30 days. 2 pt/A will suppress Banks grass mites and foliar applications can be used when grasshopper populations reach economic thresholds. Extremely hazardous to bees. Do not allow livestock to graze within 30 days of last application.
Use Data: 1992 = rate used 0.88 lb ai/A
5% of total acres treated
1997 = rate used 0.90 lb ai/A
2% of total acres treated

Pesticide: chlorethoxyfos (Fortress 5G)
Target Pests: Seedcorn maggot, rootworms
Recommended rate: 015 - 0.23 oz ai/1000 row ft (3 - 4.5 oz product per 1000 row ft)
Comments: Management of resistance is necessary
Use Data: 1997 = rate used 0.50 lb ai/A
1% of total acres treated

Pesticide: chlorpyrifos (Lorsban 4E)
Target Pests: Armyworm, European corn borer, Fall armyworm, Grasshoppers, Pale western cutworm, Southwestern corn borer, Western bean cutworm,
rootworm (chemigation)
Recommended rate: 14.37 - 28.74 fl oz ai/A (2 - 4 pt product/A)
Conservation Tillage: 7.18 - 14.37 fl oz ai/A (1-2 pt product/A)
Comments: Preharvest Interval 35 days
Pesticide: chlorpyrifos (Lorsban 15G)
Target Pests: European corn borer, Pale western cutworm, Western and Northern corn rootworm
Recommended rate: 0.53 - 1.2 oz ai/1000 row ft (3.5 - 8 oz product per 1000 row ft)
Comments: Preharvest Interval 35 days. Do not allow livestock to graze or feed treated silage to meat or dairy animals within 14 days of last application. Do not feed traeted corn fodder to meat or dairy animals within 35 days of last application.
Use Data: 1992 = rate used 0.76 lb ai/A
3% of total acres treated
1997 = rate used 0.76 lb ai/A
1% of total acres treated

Pesticide: cyfluthrin + tebupirimfos (Aztec 2.1%G)
Target Pests: Pale western cutworm, Seedcorn maggot, Western and Northern corn rootworm, wireworms
Recommended rate: 0.13 oz ai/A (6.7 oz product/1000 row ft)
Comments: REI of three days in Colorado due to low rainfall

Pesticide: dimethoate (Dimethoate 4EC)
Target Pests: Banks grass mite, Grasshoppers, Corn rootworm adults, Aphids
Recommended rate: 4.7-7.1 fl oz ai/A (0.66-1.0 pt product/A)
Comments: Preharvest Interval 14 days. Extremely hazardous to bees.
Use Data: 1992 = rate used 0.42 lb ai/A
11% of total acres treated
1997 = rate used 0.60 lb ai/A
5% of total acres treated

Pesticide: esfenvalerate (Asana 0.66 XL)
Target Pests: Armyworm, Corn leaf aphid, Corn earworm, European corn borer, Fall armyworm, Grasshoppers, Pale western cutworm, Southwestern corn borer, Western bean cutworm
Recommended rate: 0.24 - 0.81 oz ai/A (2.9-9.6 oz product/A)
Comments: Preharvest interval 21 days
Use Data: 1992 = rate used 0.03 lb ai/A
7% of total acres treated
1997 = rate used 0.06 lb ai/A
5% of total acres treated

Pesticide: lambda-cyhalothein (Warrior)
Target Pests: Armyworm, Corn leaf aphid, Corn earworm, European corn borer, Fall armyworm, Grasshopper, Pale western cutworm, Southwestern corn borer, Western bean cutworm
Recommended rate: 0.15-0.44 fl oz ai/A (1.28-3.84 fl oz product/A)
Comments: Preharvest Interval 21 days.
Use Data: 1997 = rate used 0.024 lb ai/A
11% of total acres treated

Pesticide: malathion (Malathion 5)
Target Pests: Aphids, Grasshoppers
Recommended rate: 13.63-18.18 fl oz ai/A (1.5-2 pt product/A)
Comments: Pre Harvest Interval 5 days

Pesticide: methomyl (Lannate)
Target Pests: Corn leaf aphid, Corn earworm, Fall armyworm, Corn rootworm adults
Recommended rate: 3.48-6.96 fl oz ai/A (0.75-1.5 pt product/A)
Comments: Pre Harvest Interval 3-21 days

Pesticide: methyl parathion (Penncap-M)
Target Pests: European corn borer, grasshoppers, Southwestern corn borer, Western bean cutworm
Recommended rate: 3.39-13.57 fl oz ai/A (1-4 pt product/A)
Comments: Preharvest Interval 12 days; Restricted Use Pesticide. Extremely hazardous to bees.
Use Data: 1992 = rate used 0.53 lb ai/A
1% of total acres treated
1997 = rate used 0.60 lb ai/A
6% of total acres treated

Pesticide: permethrin (Ambush 2E)
Target Pests: Corn Earworm, European corn borer, Fall armyworm, Grasshoppers, Pale western cutworm, Southwestern corn borer, Western bean cutworm
Recommended rate: 0.82-3.28 oz ai/A (3.2-12.8 oz product/A)
Pesticide: permethrin (Pounce 1.5G)
Target Pests: Corn Earworm, European corn borer, Fall armyworm, Grasshoppers, Pale western cutworm, Southwestern corn borer
Recommended rate: 1.61-3.19 oz ai/A (6.7-13.3 lb product/A)
Pesticide: permethrin (Pounce 3.2E)
Target Pests: Corn Earworm, European corn borer, Fall armyworm, Grasshoppers, Pale western cutworm, Southwestern corn borer, Western bean cutworm
Recommended rate: 0.77-3.07 oz ai/A ( 2-8 oz product/A)
Comments: Preharvest Interval 30 days
Use Data: 1992 = rate used 0.08 lb ai/A
14% of total acres treated
1997 = rate used 0.17 lb ai/A
11% of total acres treated

Pesticide: phorate (Thimet 15G)
Target Pests: Seedcorn maggot, Western and Northern corn rootworm
Recommended rate: 1.2 oz ai/A (8 oz product/1000 row ft)
Pesticide: phorate (Thimet 20G)
Target Pests: Seedcorn maggot, Western and Northern corn rootworm
Recommended rate: 1.2 oz ai/A (6 oz product/1000 row ft)
Comments: Preharvest Interval 30 days
Use Data: 1992 = rate used 1.14 lb ai/A
6% of total acres treated
1997 = rate used 1.2 lb ai/A
1% of total acres treated

Pesticide: propargite (Comite II)
Target Pests: Banks grass mite, Two spotted spider mite
Recommended rate: 25.06 - 37.68 fl oz ai/A (2.25 - 3.38 pt product/A)
Use Data: 1992 = rate used 1.54 lb ai/A
6% of total acres treated
1997 = rate used 1.10 lb ai/A
5% of total acres treated
Comments: Preharvest Interval 30 days

Pesticide: tefluthrin (Force 3G)
Target Pests: Seedcorn maggot, Western corn rootworm
Recommended rate: 0.12-0.15 oz ai/1000 row ft (4-5 oz of product/1000 row ft)
Use Data: 1992 = rate used 0.14 lb ai/A
4% of total acres treated
1997 = rate used 0.08 lb ai/A
1% of total acres treated
2001 = rate used 0.05 lb ai/A
4% of total acres treated

Pesticide: terbufos (Counter 15G)
Target Pests: Nematodes, Seedcorn maggot, Western and Northern corn rootworm
Recommended rate: 0.9-1.2 oz ai/1000 row ft (6-8 oz product/1000 row ft)
Pesticide: terbufos (Counter CR)
Target Pests: Nematodes, Seedcorn maggot, Corn rootworms
Recommended rate: 0.9-1.2 oz ai/1000 row ft (4.5-6 oz product/1000 row ft)
Use Data: 1992 = rate used 1.30 lb ai/A
39% of total acres used
1997 = rate used 1.26 lb ai/A
17% of total acres treated
2001 = rate used 1.02 lb ai/A
21% of total acres treated


Key Weeds
Canada Thistle, Cirsium arvense
Canada thistle, a member of the Sunflower family, was introduced from Europe. It is a creeping perennial which reproduces by seed and fleshy, horizontal roots. Canada thistle is on the Wyoming noxious weed. Stems are erect, hollow, smooth and slightly hairy, 1 - 5' tall, simple, and branched at the top. The leaves are set close on the stem, slightly clasping, and dark green. Leaf shape varies widely from oblong to lance-shaped. There are numerous sharp spines on the outer edges of the leaves, branches and main stem. The flowers are small and compact, about 0.75" or less in diameter, and light pink to rose-purple in color, occasionally white. The seed are oblong, flattened, dark brown, and approximately 0.125" long. Canada thistle emerges in April or May in most parts of Wyoming. Infestations are found in cultivated fields, riparian areas, pastures, rangeland, forests, lawns, gardens, roadsides, and waste areas. Because of its seeding habits, vigorous growth, and extensive underground root system, control or eradication is difficult. It is distributed across Wyoming in elevations ranging from 4,000 - 9,500'.

Field Bindweed, Convolvulus arvensis
Field bindweed, a member of the Morningglory family, is a creeping perennial introduced from Europe. It reproduces by seed and horizontal roots. Field bindweed is on the Wyoming noxious weed list. The stems are smooth, slender, slightly angled, 1 - 4' long and spread thickly over the ground or wind around erect plants and other objects. Leaves are alternate, 1 - 2" long, with great variation in shape. They are somewhat arrow-shaped with spreading, pointed, or blunt lobes at the base. Flowers are bell- or trumpet-shaped, white, pink, or variegated, and about 0.75 - 1" in diameter. Flowering is from June to September. Field bindweed is one of the most competitive perennial weeds. A two or three year food supply is stored in the extensive underground root system. This makes it difficult to kill by cultivation because roots will live as long as their food reserve lasts. Seed remain viable in the soil for up to 40 years. It is widespread in cultivated and uncultivated areas, pastures, lawns, gardens, roadsides, and waste areas throughout Wyoming from 4,000 - 8,000' in elevation.

Foxtail, Setaria spp.
Foxtail, members of the Grass family, are native to Eurasia, but common throughout most of North America. All foxtails bear seed in long, cylindrical panicles with crowded spikelets. These plants are responsible for reductions in yield, increased seed cleaning costs, and expensive control measures. Flowering and seed production occur from July to September. A primary control method for foxtail is the application of pre-emergence grass herbicides such as alachlor, glyphosate, or S-metolachlor, which provide early season control and reduce early season competition with corn.

Kochia, Kochia scoparia
Kochia, a member of the Goosefoot family, is native of Eurasia. It is an annual, reproducing solely by seed. Flowering season is from July to October. Stems are erect, round, slender, pale green, branched, and 1 - 6' tall. Leaves are narrow, bright green, hairy, numerous and are attached directly to the stem. Upper leaves are more narrow than lower leaves. Flowers are inconspicuous in the upper leaf axils. Seed are about 0.063" long, wedge-shaped, dull brown, and slightly ribbed. Kochia can be found throughout Wyoming up to 8,500' in elevation. It has become a major problem on roadsides, waste areas, and non-cultivated fields. In the fall, plants become red, later turning brown and breaking away from the root, causing them to tumble over the ground scattering large amounts of seed. Many kochia populations in Wyoming are resistant to sulfonylurea, imidazilinone, triazine and benzoic acid herbicides, representing three very different modes of action. Therefore, care should be taken when using herbicides to control kochia. Some alternative herbicides include: fluroxypyr, bromoxynil, MCPA ester, isoxaflutole, pyridate, flufenacet, fomesafen, glyphosate, lactofen and clomazone. It is important to rotate herbicide modes of action to prevent future cases of herbicide resistance.

Pigweeds, Amaranthus spp.
Pgweeds, members of the Amaranth family, are weed pests in several cropping systems. A. hybridus, smooth pigweed, was the first triazine resistant plant documented. A. lividus, is a prostrate species with a notch in the tip of the leaf. A. spinosus, spiny amaranth, has sharp, stron spines on the stem. A. graecizans, breaks off at the ground line and being round shaped is blown around by the wind. Redroot pigweed is ALS herbicide resistant.

Purslane, Portulacca oleracea
Purslane, a member of the Purslane family, is a fleshy, prostate annual with smooth reddish or flesh colored stems. Branches radiate from a central rooting point, reach lengths in excess of 12" and form dense vegetative mats. Smooth, shiny, succulent leaves are somewhat teardrop-shaped, wider at the tip than at the base. Five petalled yellowed flowers are borne singly in leaf axils, and open only in the sunshine. Extensive seed production throughout the growing season and its ability to resprout following cultivation make this plant especially difficult to control.

Quackgrass, Elytrigia repens
Quackgrass, a member of the Grass family, is an aggressive perennial reproducing by seed or spreading by a shallow mass of long, slender, branching rhizomes. Quackgrass is on the Wyoming noxious weed list. Rhizomes are usually yellowish-white, sharp-pointed, somewhat fleshy. These rhizomes are effectively spread by tillage, increasing the distribution of the population in a field. They are able to penetrate hard soil or even tubers and roots of other plants. . Stems are erect and usually 1 - 3' tall. Leaf blades are 0.25 - 0.5" wide, flat pointed and have small auricles at the junction of blade and sheath. Leaf sheaths and upper leaf blade surfaces are thinly covered with soft hairs. Spikelets are arranged in two long rows, flatwise to the stem. Florets are either awnless or have short, straight awns. Tillage is an effective control by depleting food reserves and bringing rhizomes to the surface.

Sandbur, Cenchrus pauciflorus
Sandbur, a member of the Grass family, is a warm season annual with tufted stems. It grows 8"- 3' tall, occasionally erect, but usually spreading horizontally and forming dense mats. Leaf sheaths are flattened, very loose, and smooth with hairy margins. Burs are thickly set with stiff, sharp, spreading spines. They usually contain two light brown, oval to oblong seed.


Additional Weeds
Barnyardgrass, Echinochloa crusgalli
Barnyardgrass, a member of the Grass family, is an annual that prefers wet sites. It is not usually a problem in well-drained cultivated fields but can grow heavily around irrigation pipe leaks and other wet spots in the field. It is a vigorous, warm season annual grass reaching 1 - 5' in height. Many stem bases are reddish to dark purple. Leaf blades are flat, broad, smooth, and without a ligule or auricle at the junction of sheath and blade. Seed are the only source of reproduction. It flourishes in warm conditions.

Black Nightshade, Solanum ptycanthum
Black nightshade, a member of the Nightshade family, is an annual that grows 6 - 24" tall with glabrous, appressed-hairy stems. Black nightshade plants vary greatly in form and color. Seed leaves of black nightshade are elongate-oval and pointed; the first true leaves are spade-shaped with smooth edges. Lower leaf surfaces are often purple. Berries turn from green to black when mature and the calyx covers only a small part of the fruit surface. Petioles, stems and leaves have some hairs but are not densely hairy or sticky. Leaves are ovate, smooth to wavy-edged and have a tapered tip. Flowers are white to pale blue, 0.25 - 0.4" wide borne in clusters. Flowering season is from May to October. Seed are the only source of reproduction.

Bristle Foxtail, Setaria verticillata
Bristle foxtail, a member of the Grass family, is an annual, 1 - 3' tall. Leaf blades are flat, 0.2 - 0.5" wide, smooth or with a few hairs near the base on the upper surface. Leaves have open sheaths, lack auricles, and have hair-fringed membranes. The inflorescence is a spike-like panicle that appears somewhat segmented or attached to the rachis after seed drop. Bristle foxtail closely resembles green foxtail when in the vegetative growth stage. The primary difference between this and other Setaria species is the minute barbs on the stiff bristles of S. verticillata are oriented downward, rather than outward, causing seed heads to cling more effectively to clothing and animals.

Common Cocklebur, Xanthium strumarium
Common cocklebur, a member of the Sunflower family, is a native weed. It is an annual, 2 - 4' tall, and stems are erect, branched, ridged, spotted, and very rough. Leaves are alternate, triangular or heartshaped, rough on both sides, and long-petioled. Flowers are small and in axils of the upper leaves. Male and female flowers are separate. Flowering is from July to September. Fruits are 1" long, woody, with hooked prickles and two curved spines at the tip and two seed. The dark brown seed are flattened and have barbed hooks on the tips, allowing them to be easily distributed by wildlife. Seed are the only source of reproduction. It is a prolific seed producer and competes well with soybeans. New infestations appear around field edges or roadsides. Common cocklebur is especially abundant in areas where receding water has exposed previously submerged land. The burs are irritating both to humans and to animals.

Common Ragweed, Ambrosia artemisiifolia
Common ragweed, a member of the Sunflower family, is a native annual that reaches 4' tall. Stems and leaves are blue-green and are covered with fine hairs. Leaves are pinnately divided and are both alternate and opposite. Leaf undersides are gray due to the fine hairs. Flowers are on terminal branches. Male and female flowers are separate and in clusters. Staminate flowers are above pistillate flowers. Seed are awl-shaped with a spiny projection on one side. Flowering is in late summer, and seed mature by October.

Puncturevine, Tribulus terrestris
Puncturevine, a member of the Caltrop family, is an annual native of Europe. It forms mats on the soil surface with its dense, trailing stems that are 0.5 - 5' long. Leaves are opposite, hairy, divided into 4 - 8 pairs of leaflets, each of which are 0.25 - 0.5" long and oval. Flowers are yellow, 0.3 - 0.5" wide with 5 petals, borne in leaf axils. Flowering season is from May to October. Seed are the only source of reproduction. It is commonly found in waste places, roadsides, and pastures, and is most abundant in disturbed sandy and gravelly soils.

Russian thistle, Salsola iberica and S. collina
Russian thistle, a member of the Goosefoot family, was introduced from Russia. It is an annual and reproduces by seed. It is a round, bushy, branched plant growing 1 - 3.5' high. The branches are slender, succulent when young, and woody when mature. The first leaves to develop are alternate, dark green, soft, slender, and 1 - 2.5" long. These senesce early and new leaves form which are alternate, short, stiff, spiny, and not over 0.5" long, with two sharp-pointed bracts at the base. The flowers are small, inconspicuous, green-white or pink, and are usually solitary in the leaf axils. Seed are conical and 0.063" in diameter. Russian thistle grows in dry plains, cultivated fields, roadsides, and waste areas, primarily in grain-growing areas of the state. At maturity, the plant breaks off at the base. Its round shape allows it to tumble, scattering seed for long distances. It is widespread over Wyoming in elevations up to 8,500'.

Sunflower, Helianthus annuus
Sunflower, a member of the Sunflower family, is a native weed. It is an annual, 1 - 10' tall. Stems are erect, simple to branched and rough. Leaves are alternate, simple, rough, and hairy. Ray flowers are yellow to orange-yellow and disk flowers are brown. Flowering is from July to September. Seed are the only source of reproduction.

Tansy mustard, Descurania pinnata
Tansy mustard, a member of the Mustard family, is a native winter annual, 4 - 32" tall. The plant is covered with fine hairs. The stem is erect, branched and 4 - 30" high. The flowers are small, pale yellow, and occur in small clusters at the tips of elongating racemes. Leaves are alternate and pinnately dissected, 2 - 4" long. Tansy mustard has stellate pubescence and racemic inflorescences. Petals are yellow, yellowish-green to cream. Pods are linear with two rows of seed in each seed pod. Tansy mustard spreads by seed from early to late summer. It is dispersed throughout Wyoming up to 8,000' in elevation.

Toothed spurge, Euphorbia dentata
Toothed spurge, a member of the Spurge family, is native to the Great Plains region. It spreads by seed and grows under a wide range of environmental conditions. Toothed spurge is an annual, growing up to 3' tall. Leaves are up to 3" long, ovate to linear, coarsely toothed, mostly opposite, hairy and often dotted with a few purplish-red spots. Stems are many branched and generally curve upwards. Both stems and leaves exude a milky latex when broken. The inconspicuous flowers develop in late summer, followed by 3 sided, turban-shaped, 0.25", smooth, green fruits. Seed are rough, bumpy, oval and gray.

Wild Buckwheat, Polygonum convolvulus
Wild buckwheat, a member of the Buckwheat family, is an annual weed with arrowhead shaped leaves. It has trailing stems that wind around other plants and is often mistaken for field bindweed. Its leaves are heart shaped, alternate and more pointed than those of field bindweed. The leaves have an inconspicious papery sheath that encircles the stem at the base of each petiole. Stems can be 8 - 40" long. In contrast to field bindweed, wild buckwheat has small, green flowers in the leaf axils. Seed are triangular, black and slightly roughened. Seed are the sole source of reproduction.

Wild Proso Millet, Panicum miliuceum
Wild proso millet, a member of the Grass family, is an annual, reproducing by seed. Although proso millet is grown in Wyoming as a crop, the wild species is very different. Seed germination is in the spring and throughout the summer when soil temperatures reach 68° F. Seed germinate and emerge from the soil as deep as 4" below the surface, but are equally able to germinate closer to the soil surface. Once in the soil, wild proso millet seed can remain viable for five or more years. Wild proso millet plants range in height from 1' in dry areas to 4 - 6' on irrigated sites. Stems of wild proso millet have an abundant hairs located at right angles to the stem. As plants grow larger, they lodge and put down additional roots wherever stem joints touch the ground. Plants can begin flowering in July and continue throughout the summer. The inflorescence produced by wild proso millet is large and bushy. Seed matures in late August through September. Once mature, it readily shatters when the plant is disturbed. Plants can produce 400 - 12,000 seed per square foot, depending upon the degree of plant competition. One wild proso millet plant per square foot in corn may produce 400 seed per square foot.

Witchgrass, Pancium capillare
Witchgrass, a member of the Grass family, is a native weed. It is an annual with an upright or spreading growth habit, up to 3' tall from a bent or twisted base. The entire plant is soft and hairy. Leaf blades are up to 0.75" wide, usually with prominent midribs. Seed are the only source of reproduction. It flourishes in warm conditions. Witchgrass is not a serious problem on rangelands that are in good condition because it to does not compete well with established perennial grasses. It does, however, compete well with poorly established crops. It is abundant where the soil is somewhat sandy, on roadsides and in waste places.


Key Weed Management Strategies
Cultural Controls
Purslane becomes a weed problem in thin stands. It is less of a problem when corn plants are in the silking stage because it becomes over-shaded by the corn plants. Round-up ready corn is available for weed control.


Herbicides–

Pesticide: 2,4-D (Various formulations)
Target Pests: Bindweed, Kochia, Pigweed
Recommended rate: 0.25-0.75 lb ai/A (Rate varies with formulation. See label.)
Use Data: 1992 = rate used- 0.29 lb ai/A
22% of total acres treated
1997 = rate used- 0.47 lb ai/A
22% of total acres treated
2001 = rate used- 0.24 lb ai/A
6% of total acres treated

Pesticide: 2,4-D + 2,4-DB (Weedone LV4)
Target Pests: Various Broadleaf weeds
Recommended rate: 10.75-43.00 fl oz ai/A (1-4 pt product/A)
Comments: Rates vary depending on time of application

Pesticide: acetochlor (Harness)
Target Pests: Downy brome, Foxtails, Kochia, Sandbur
Recommended rate: 14.96-32.91 fl oz ai/A (1.25-2.75 pt product/A)
Pesticide: acteochlor (Harness 20G)
Target Pests: Downy brome, Foxtails, Kochia, Sandbur
Recommended rate: 19.2-38.4 oz ai/A (6-12 lb product/A)
Use Data: 1997 = rate used- 0.31 lb ai/A
1% of total acres treated
2001 = rate used- 0.20 lb ai/A
1% of total acres treated

Pesticide: acetochlor + atrazine (Harness Xtra)
Target Pests: Downy brome, Foxtails, Kochia, Sandbur
Recommended rate: acetochlor: 26.7-34.1 fl oz ai/A; atrazine: 10.5-13.5 fl oz ai/A (1.8-2.3 qt of product/A)

Pesticide: alachlor (Lasso)
Target Pests: Foxtails, Johnsongrass, Kochia, Sunflower
Recommended rate: 36-58 fl oz ai/A (2.5-4 qt product/A)
Pesticide: alachlor (Micro-Tech)
Target Pests: Foxtails, Johnsongrass, Kochia, Sunflower
Recommended rate: 33-53 fl oz ai/A (2.5-4 qt product/A)
Use Data: 1992 = rate used- 2.46 lb ai/A
22% of total acres treated
1997 = rate used- 2.40 lb ai/A
4% of total acres treated

Pesticide: atrazine (Atrazine 4L)
Target Pests: Pigweed, Kochia, Russian thistle, Sunflower
Recommended rate: 19.5-26.1 fl oz ai/A (3-4 pt product/A)
Pesticide: atrazine (AAtrex 90)
Target Pests: Pigweed, Kochia, Russian thistle, Sunflower
Recommended rate: 15 oz ai/A (2.2 lb product/A)
Use Data: 1992 = rate used- 1.08 lb ai/A
43% of total acres treated
1997 = rate used- 1.23 lb ai/A
33% of total acres treated
2001 = rate used- 0.82 lb ai/A
56% of total acres treated

Pesticide: bentazon (Basagran)
Target Pests: certain broadleaf weeds and sedges
Recommended rate: 6.4-14.1 fl oz ai/A (1-2 pt product/A)
Comments: Rates vary depending on weed species
Use Data: 1997 = rate used- 0.64 lb ai/A
1% of total acres treated
2001 = rate used- 1.00 lb ai/A
1% of total acres treated

Pesticide: bromoxynil (Buctril)
Target Pests: Canada Thistle*
Recommended rate: 2.0-10.6 fl oz ai/A (1-2 pt product/A)
Comments: *used in mixtures with other herbicides for control
Use Data: 1997 = rate used- 0.33 lb ai/A
4% of total acres treated
2001 = rate used- 0.42 lb ai/A
1% of total acres treated

Pesticide: carfentrazone (Aim)
Target Pests: Lambsquarters, Pigweeds, Nightshade
Recommended rate: 0.12 oz ai/A (0.33 oz product/A)

Pesticide: dicamba (Clarity)
Target Pests: Canada thistle, Kochia, Pigweed
Recommended rate: 4.5-9 fl oz ai/A (8-16 fl oz product/A)
Comments: Maximum of 24 fl oz product per season
Use Data: 1992 = rate used- 0.50 lb ai/A
41% of total acres treated
1997 = rate used- 0.36 lb ai/A
50% of total acres treated
2001 = rate used- 0.13 lb ai/A
45% of total acres treated

Pesticide: dicamba + atrazine (Marksman)
Target Pests: Various grass and broadleaf weeds
Recommended rate: dicamba: 97 fl oz ai/A for coarse, 104 fl oz ai/A for fine; atrazine: 161 fl oz ai/A for coarse, 172 fl oz ai/A for fine (1.5L product/A for coarse soils, 1.6L product/A for medium to fine soils)
Comments: commonly used in mixture to control resistant weeds

Pesticide: dimethenamid (Frontier 6.0)
Target Pests: Foxtails, Kochia, Nightshade
Recommended rate: 15-24 oz ai/A (1.48-2.37 lb product/A)
Use Data: 1992 = rate used- 0.84 lb ai/A
1% of total acres treated

Pesticide: dimethinamid + atrazine (Guardsman Max)
Target Pests: Various grass and broadleaf weeds
Recommended rate: dimethinamid: 8.7-13.4 fl oz ai/A; atrazine: 16.9-25.9 fl oz ai/A (3.0-4.6 pt product/A)
Comments: Use an alternate program with non-triazine products

Pesticide: EPTC (Eradicane 6.7E)
Target Pests: Sandbur, wild proso millet
Recommended rate: 64-96 oz ai/A (4.75-7.33 pt product/A)
Use Data: 1992 = rate used- 4.53 lb ai/A
4% of total acres treated
1997 = rate used- 4.20 lb ai/A
1% of total acres treated

Pesticide: glufosinate-ammonia (Liberty)
Target Pests: Various weeds
Recommended rate: 5.0-6.2 oz ai/A (28-34 oz product/A)
Comments: For use on Libertylink® seed

Pesticide: glyphosate (many)
Target Pests: All weedy species
Recommended rate: 16 fl oz ai/A (1.0 qt product/A)
Use Data: 1997 = rate used- 0.58 lb ai/A
17% of total acres treated
2001 = rate used- 0.58 lb ai/A
29% of total acres treated

Pesticide: imazethapyr (Pursuit AS)
Target Pests: Certain broadleaf weeds
Recommended rate: 0.9 fl oz ai/A (0.25 pt product/A)
Pesticide: imazethapyr (Pursuit DG)
Target Pests: Certain broadleaf weeds
Recommended rate: 1.0 oz ai/A (1.44 oz product/A)
Comments: Apply only to Pursuit resistant or Pursuit tolerant hybrids. These hybrids have the designation “Imi-Corn”.
Use Data: 1997 = rate used- 0.08 lb ai/A
2% of total acres treated

Pesticide: metribuzin (Sencor 75 DF)
Target Pests: Various weeds
Recommended rate: 1.2-1.8 oz ai/A (1.6-2.0 oz product/A)
Comments: Triazine herbicide that can be applied at lower rates as well as mixed with alternate herbicides
Use Data: 1997 = rate used- 0.11 lb ai/A
1% of total acres treated

Pesticide: nicosulfuron (Accent)
Target Pests: Barnyardgrass, Foxtails, Pigweed, Shattercane, Sandbur, Wild proso millet, Witchgrass
Recommended rate: 0.50 oz ai/A (0.67 oz product/A)
Use Data: 1992 = rate used- 0.04 lb ai/A
6 % of total acres treated
1997 = rate used- 0.03 lb ai/A
5 % of total acres treated
2001 = rate used- 0.01 lb ai/A
31% of total acres treated

Pesticide: nicosulfuron + rimsulfuron + atrazine (Basis Gold)
Target Pests: Barnyardgrass, Foxtails, Pigweed, Shattercane, Sandbur, Wild proso millet, Velvetleaf , Lambsquarters, Sunflowers, Russian Thistle
Recommended rate: nicosulfuron: 0.18 oz ai/A; rimsulfuron: 0.18 oz ai/A; atrazine: 11.54 oz ai/A (14 oz product/A)

Pesticide: pendimethalin (Prowl)
Target Pests: Barnyardgrass, Foxtails
Recommended rate: 1.1.5 lb ai/A (2-3.5 pt product/A)
Use Data: 1992 = rate used- 1.06 lb ai/A
10% of total acres treated
1997 = rate used- 1.20 lb ai/A
3% of total acres treated

Pesticide: primsulfuron-methyl (Beacon; Beacon 75 WG)
Target Pests: Shattercane, Johnsongrass, Sorghum almum
Recommended rate: 0.57 oz ai/A (0.76 oz product/A)
Use Data: 1992 = rate used- 0.04 lb ai/A
1% of total acres treated
1997 = rate used- 0.04 lb ai/A
1% of total acres treated

Pesticide: pyridate (Tough 5EC)
Target Pests: Many broadleaf weeds
Recommended rate: 6.7-13.4 fl oz ai/A (12-24 fl oz product/A)
Use Data: 1997 = rate used- 0.90 lb ai/A
4% of total acres treated
2001 = rate used- 0.37 lb ai/A
12% of total acres treated

Pesticide: rimsulfuron + thiensulfuron-methyl (Basis)
Target Pests: Barnyardgrass, Foxtails, Pigweed, Shattercane, Sandbur, Wild proso millet, Velvetleaf , Lambsquarters, Sunflowers, Russian Thistle
Recommended rate: rimsulfuron: 0.16 oz ai/A; thiensulfuron-methyl: 0.08 oz ai/A (0.33 oz product/A)
rimsulfuron use
Use Data: 1997 = rate used- 0.31 lb ai/A
1% of total acres treated
2001 = rate used- 0.20 lb ai/A
1% of total acres treated

Pesticide: S-metolachlor (Dual)
Target Pests: Foxtails, Sandbur, Velvetleaf, Shattercane
Recommended rate: 5.9-13.1 fl oz ai/A (0.67-1.0 pt product/A)
Pesticide: S-metolachlor (Dual II)
Target Pests: Foxtails, Sandbur, Velvetleaf, Shattercane
Recommended rate: 17.0 fl oz ai/A (0.75 qt product/A)
Use Data: 1992 = rate used- 1.08 lb ai/A
43% of total acres treated
1997 = rate used- 1.23 lb ai/A
33% of total acres treated
2001 = rate used- 0.82 lb ai/A
56% of total acres treated
Comments: S-metolachlor was metolachlor

Pesticide:sethoxydim (Poast Plus)
Target Pests: grass control
Recommended rate: 4.6 fl oz ai/A (2.25 pt product/A)
Comments: Poast Protected™ field corn only
Use Data: 1997 = rate used- 0.20 lb ai/A
1% of total acres treated


Critical Pest Management Issues
Insecticides aimed at corn insect pests may also kill the natural enemies of spider mites. Most foliar applied insecticides will provoke spider mite outbreaks, especially when applied after tasseling. Banks grass mite has significant resistance problems in neighboring states making alternative controls very limited. Soil applied insecticides will not affect spider mite control. Insecticides used for corn rootworm control may also harm honeybees.

ALS herbicide resistance can be selected for very quickly, even after two applications. More commonly, population pressure due to consecutive applications or continuous production of the same crop year after year using only ALS herbicides selects for resistance. In many situations, resistance is selected for along right-of-ways and then resistant biotypes move into agricultural fields. It is important to rotate herbicide modes of action to prevent future cases of herbicide resistance.

Weeds, diseases, and insects can all develop resistance over time to pesticides. When the same pesticide is used consecutively over a period of time, the target pest can become resistant to that pesticide and render the pesticide obsolete.


Wyoming contact:

Sources

1998-2001 National Agricultural Statistics Service (www.nass.usda.gov/co).

Bauder, T. And R. Waskom. Best Management Practices for Colorado Corn. Colorado State University Cooperative Extension.

Colorado Department of Agriculture and National Agricultural Statistics Service. Colorado Agricultural Statistics Publication, 1998-2001.

Colorado State University Cooperative Extension. Colorado Pesticide Guide, Field Crops, 1998.

Colorado State University Cooperative Extension. Colorado Weed Management Guide, 2002.

High Plains Integrated Pest Management Guide for CO, Western NE & WY, 1998.

Montana-Utah-Wyoming Cooperative Extension Services 2001-2002 Weed Management Handbook

NebGuide (www.nebguide.com)

Wyoming Agricultural Statistics 1999-2003 (http://www.nass.usda.gov/wy/)