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THE ECONOMIC BASIS FOR GRASSHOPPER SUPPRESSION
Grasshoppers have been of economic concern in the western United States since settlers established farming and ranching operations there in the 1800s. Settlers in northeastern Colorado suffered severe hardship in 1864 because grasshoppers ruined crops. In 1873 Fort Collins, Colorado lost many of its homesteaders who were discouraged by economic conditions and grasshopper hordes (Tresner, 1981). During the 1870s, the Rocky Mountain locust, Melanopus spretus (Walsh), destroyed crops throughout the West, and in 1877 the U.S. Entomological Commission was created to investigate the problem. Although Rocky Mountain locust problems subsided in the 1880s, interest and concern regarding grasshopper damage have not subsided.
The expansion of Colorado's urban areas into foothills, prairie, and farmland has increased concern over grasshoppers. Grasshopper numbers build to high levels in weedy areas and migrate to yards where they destroy vegetable and flower gardens and ornamental shrubs. While extreme economic loss may not be associated with this situation, the aesthetic value of homes and the happiness of homeowners are threatened.
Current emphasis of large-scale grasshopper management efforts has shifted from crop protection to rangeland protection. This shift has occurred because farmers can use numerous insecticides to effectively suppress grasshoppers, and, perhaps more important, they have the economic incentive to affect control. Other factors also work to reduce impact of grasshoppers on cropland. Insecticide applications directed toward the myriad of other insect pests associated with crops help keep grasshopper numbers low. Such practices associated with crop production as tillage, ditch burning and weed control also reduce grasshopper abundance. Ranchers, on the other hand, also have effective insecticides at their disposal, but low productivity of rangeland relative to cropland precludes their investment in expensive control procedures.
In addition to yield reduction in forage, the cost of grasshopper control is an important direct cost ranchers must incur; indirect costs include reduction in weight gain by cattle and relocation costs. Grasshoppers consume from 6% to 12% of the available forage in the western United States although in some localities they consume essentially all available forage. Grasshoppers eat approximately one-half of their body weight in green forage per day. With a grasshopper population of seven or eight per square meter in a four hectare field, grasshoppers consume as much forage as a cow (Hewitt, 1977). The U.S. Department of Agriculture suggests that treatment is justified when grasshopper numbers (adults or late instar nymphs) reach approximately nine per square meter.
Few grasshopper control recommendations consider forage condition, grasshopper species and other important variables. When forage density or biomass is low, small numbers of grasshoppers per unit area can be damaging. Thus, during dry periods, when grasshopper numbers often are highest, fewer grasshoppers can be tolerated. When the price of cattle is high, ranchers can better afford grasshopper suppression costs, and control operations are more likely instigated.
Grasshoppers differ significantly in their damage potential. Forage loss stems from both consumption and clipping without consumption (wastage). The amount of foliage consumed by a number of common species is given in table 2. Studies such as these suggest that over a season, mixed populations of grasshoppers destroy approximately 44 mg (dry weight) of foliage per grasshopper per day. Wastage by clipping may represent up to 50% of total forage reduction attributable to grasshoppers. Both consumption and wastage rates are influenced by grasshopper preference for a plant species; favored plants are more heavily damaged.
A more reliable approach to estimation of forage loss and to initiation of suppression activities is needed (Hewitt et al., 1976). Grasshopper damage potential, rangeland forage production; control effectiveness and cost must be integrated into the decision-making process. The grasshopper-feeding-day approach of Hewitt et al. (1976), which can integrate species specific consumption, mortality rates and forage availability, shows considerable promise especially when combined with control costs (see table 4). This, and the similar approach of White (1974) have proved useful experimentally. For practical, state-wide survey, however, better grasshopper identification and forage biomass estimation techniques are needed.
|Forage reduction* (mg/day)
|83.0, 42.3, 9.3
*different values represent results of separate studies.
GRASSHOPPER SUPPRESSION METHODS
Sampling Grasshopper Abundance
The decision to initiate grasshopper suppression efforts should not be made unless impending crop or forage loss is evident. If expected losses do not equal or exceed control costs, suppression efforts probably are not warranted. Crop or forage loss estimates are made primarily on the basis of grasshopper abundance although other factors should be considered, as discussed previously. Abundance of adults and nymphs is commonly determined; egg pod sampling is extremely labor-intensive and rarely is done.
Grasshopper abundance is determined on a density per unit-area basis. Sweep net samples generally do not provide useful population estimates because relating catches to area is difficult and sweep net catches are especially influenced by plant height and density and by meteorological conditions. An exception occurs when plant biomass is very high as in alfalfa fields; in this case, sweep net samples are used to relate grasshopper density to catches in other alfalfa fields, or to catches at other times, but not to other crops.
Estimates of grasshopper numbers usually are presented as number per square yard, although counts may be made on a square foot basis, depending on grasshopper density. In conducting a survey, the surveyor selects the area to be checked from a distance. The margins of the square yard or square foot are estimated visually, and grasshoppers are counted as they leave the area in response to the approach of the surveyor. This method is not exceptionally accurate because boundaries are only estimated. However, on rangeland this method is preferable to sweep net sampling. Also, if the surveyor takes the time to examine the foliage carefully to avoid underestimating grasshopper numbers, the technique is acceptable. As with any sampling technique, weather conditions will influence counts. Details on grasshopper survey procedures are presented by the USDA (undated) and Hantsbarger (1979).
Grasshopper adult surveys are conducted by the Colorado Department of Agriculture with assistance of the USDA in late summer and early autumn. Based on adult counts, grasshopper population estimates are made for the subsequent summer. Grasshopper adult counts of eight or more per square yard are considered threatening. Suppression programs should not be initiated without confirmation of high grasshopper densities in spring nymphal surveys.
Predators and parasites often affect the potential number of grasshoppers in an area, as do low numbers of eggs produced by the adults observed; consequently, abundance can decrease precipitously between adult and nymphal surveys. Nymphal surveys are conducted in the same manner as adult surveys. Care must be taken to ascertain that egg hatch has occurred prior to the survey. Considering the wide variety of grasshopper species often present in an area, this is a formidable task. Also, young grasshoppers are readily confused with leafhoppers unless they are examined closely. Care must be taken so that grasshopper counts are not inflated.
An improvement on grasshopper density estimates can be obtained by establishing definite boundaries for the square yard or square foot grasshopper counts. Richards and Waloff (1954) and Onsager and Henry (1977) recommend prior placement of hoops to mark boundaries. Grasshoppers are flushed from the hoops, and counted in the aforementioned manner. A wand is sometimes employed to aid in flushing grasshoppers. The size of the hoop can be varied with grasshopper density. The hoop count technique requires two visits to a site; hoops usually are positioned on the day preceding counts. Thus, twice as much labor is required although accuracy is enhanced.
Grasshopper sampling should include species determination in addition to total grasshopper abundance. Grasshopper damage potential differs significantly, and host plant associations often determine damage status. For example, Aeoloplides turnbulli, one of the most common grasshoppers in Colorado, feeds primarily on Russian thistle and belevedere summercyprus. Such species may be more beneficial than detrimental where grass production is important. The keys provided in this manual will greatly enhance species determination during adult surveys. However, nymph determinations remain difficult. Adult counts made in the summer preceding nymphal surveys should provide surveyors with reasonable data on species composition in an area.
Chemical Suppression of Grasshoppers
Chemical suppression of grasshopper populations involves application of insecticides to bait or foliage. In either case, mortality results primarily from grasshoppers ingesting food contaminated with insecticide. Each approach has advantages and disadvantages.
Poisoned baits were first used widely for grasshopper suppression about 1913. Baits
were the dominant form of insecticide application by 1936 and were used extensively
until the late 1940s. Traditionally, baits consisted of a carrier, usually such food
as wheat bran flakes, rolled barley or apple pomace; a toxicant, formerly an arsenical
insecticide but now usually carbaryl; and a moistener, either water or oil. Molasses
frequently was added to make the bait more attractive, and ground corncob or sawdust
frequently was used as a diluent in place of up to one-half of the bran. Wheat bran
flakes can be used alone with only a toxicant added (usually 1% to 2%) with good results.
Bait can be distributed by ground or air; in either case, the volume of material that must be applied poses signficant problems. Procurement, mixing, storage and delivery of the vast quantities of bait required for treatment of large acreages make bait application a relatively expensive suppression tactic (see fig. 371 and fig. 372). Bait usually is applied at a rate of approximately 1.7 kg bait per hectare. Increasing the volume of bait or concentration of insecticide applied may increase degree of control, but usually it is not cost efficient (Mukerji et al., 1981). Only a portion of a grasshopper population is susceptible to bait treatment because (1) some species do not consume baits, (2) some members of the population are molting and, therefore, do not feed and (3) some individuals do not encounter bait or do not ingest enough bait to be killed (Onsager et al., 1980a). Mortality among some common rangeland grasshopper species exposed to insecticide treated bran bait is given in table 3. Grasshoppers vary signficantly in acceptance of bait although most major pest species tested to date are susceptible to control through this suppressive tactic. In general, species in the subfamilies Gomphocerinae and Catantopinae are more susceptible than those in the subfamily Oedopodinae (Onsager et al., 1980b). Grasshopper species differ in susceptibility to insecticides (McDonald, 1967) in addition to differing in bait ingestion. The disadvantages associated with bait use are off set somewhat by the selectivity provided by baits. Such beneficial insects as biocontrol agents and pollinators, wildlife, and other animal life are less affected by this method of insecticide application. Also, Mukerji et al. (1981) reported that much less insecticide was required to achieve a satisfactory level of grasshopper suppression using bran bait compared to liquid formulation. For relatively safe application of insecticides on such small acreages as roadsides and fence rows or in vacant lots in suburban areas, bait applications are preferable.
Aerial application of liquid insecticide has largely replaced bait application for large-scale grasshopper suppression programs. Starting in the late 1940s and early 1950s, such chlorinated hydrocarbons as toxophene and aldrin were widely used. They have been replaced by such organophosphates as malathion, and such carbamates as carbaryl. Although the chlorinated hydrocarbons have long residual action and provide a greater reduction in grasshopper populations, their residual nature allows the buildup of toxic residues in the food chain. The newer insecticides generally provide very acceptable levels of grasshopper suppression without buildup of toxic residues. Such modern insecticides as malathion are applied without dilution and at very low rates, a technique referred to as ultra-low volume (ULV). Others, such as carbaryl, usually are applied with oil and water to increase adhesion and to provide dilution, respectively. Malathion ULV applications are less expensive than other materials currently available, but ranchers often select more expensive materials such as carbaryl because they seem to obtain slightly better grasshopper suppression. Whether the increased cost can be justified is debatable.
Onsager (1978) recently studied the characteristics of a carbaryl formulation (Sevin-4-oil) and reported 85% to 98% mortality of grasshoppers treated at 0.56 and 1.12 kg AI per hectare. Malathion ULV provided an equivalent level of control at the same rate of application although under certain weather conditions (cool and wet) malathion was less effective. Higher rates of insecticide application gave more rapid mortality, but total mortality was not affected. Insecticide residue caused grasshopper mortality for at least 21 days although other studies (e.g., Lloyd et al., 1974) have suggested less persistence. An economic analysis of this insecticide comparison is summarized in table 4. A much better return of investment was obtained by initiating control during early instars; control of adults resulted in a net loss. In addition to saving more forage by surveying grasshoppers when they are young, it is important to recognize that large, mature grasshoppers are less susceptible to insecticides. Also, although the lowest rate of insecticide application took longer to realize maximum control, it yielded the highest percentage of return on investment.
The wisdom of striving for nearly complete destruction of grasshopper populations is questionable. In the short-term, obtaining 99% or 100% mortality might seem desirable, but if this occurs, natural mortality factors, including predators and parasites of grasshoppers, are deprived of food and also perish. Free from naturally occurring biological control, grasshoppers may be able to assume damaging numbers quickly. From a long-term perspective, selecting a suppression tactic that inflicts a lower level of mortality among grasshoppers and also preserves biological control agents may be more beneficial. Biological control agents presumably would act to suppress further grasshopper outbreaks or at least delay the resurgence of grasshopper populations.
Birds are important grasshopper predators and serve to exemplify some of the problems associated with insecticide use. Such materials as toxaphene and diazinon induce high levels of bird mortality following application to rangeland. Others, such as propoxur and azinphosmethyl, cause reduction in bird abundance without direct evidence of bird mortality; emigration from treated sites is implicated. Malathion and carbaryl, both widely employed for rangeland grasshopper suppression, seem to have no direct effect on wildlife populations although emigration has occurred under some circumstances. Insecticides used for grasshopper control should degrade rapidly in the environment (McEwen et al., 1972).
|Rate (kg AI/ha)
|Forage Saved (kg dry weight/ha)
|Forage Saved (Value in $)
|Net Return ($/ha)
*Costs of treatment per hectare were $3.95 for application plus $1.42, $2.84 and $5.68, respectively, for the 0.28, 0.56 and 1.12 kg dosages of carbaryl and $1.58 for malathion. Forage was valued at $44 per metric ton. Grasshopper density was 10 to 15 per m2.
Chemical suppression programs are expensive; nevertheless, they are routinely conducted. Part of their appeal stems from apparent long-term reduction in grasshopper abundance. Hence, control costs can be amortized over several years. Chemical control undoubtedly provides temporary relief, but whether chemical control programs provide long-term suppression is open to question. To determine long-term effectiveness, populations must be followed over a several year period in treated and nearby untreated areas. Only rarely has any long-term population monitoring of treated sites with appropriate controls been conducted. Blickenstaff et al. (1974) reviewed several control programs, conducted principally in Wyoming. It appears that chemical control programs frequently do not provide long-term suppression. Failure of chemical control programs to provide for long-term suppression is due to a number of factors, including (1) reinvasion of treated areas from bordering untreated areas, (2) natural declines in grasshopper populations in untreated areas, (3) occurrence of diapausing eggs and (4) rapid reproduction in residual populations, which survive because of accidental or deliberate skips in treated acreage (Blickenstaff et al., 1974). Pfadt (1977) also conducted control studies in Wyoming and reported long-term suppression (five to six years) in some instances but only short-term suppression in others.
Cooperative Grasshopper Control Programs
Grasshopper population outbreaks can involve such large acreages that effective suppression is beyond the capabilities of a single landowner. Landowners can organize and form cooperative grasshopper control districts. Ranchland, but not cropland, grasshopper suppression programs are eligible for state and federal funding to defray costs. Costs of grasshopper suppression are divided equally among ranchers, the state and the federal government.
Procedures for formation of cooperative grasshopper control programs are provided by Sullivan et al. (1981). Important aspects include: (1) ranchers must form a pest control district; (2) cooperative control districts must be solid, contiguous areas; (3) degree of infestation must be determined by survey and must consist of an economically threatening population (usually more than eight grasshoppers per square yard); (4) at least 66 2/3% of the landowners in the proposed district must vote in favor of formation of the district, and the landowners voting must own at least 66 2/3% of the land in the proposed district; (5) operational aspects of grasshopper control programs will be managed by Colorado Department of Agriculture and USDA, APHIS personnel.
For cooperative programs to be successful, ranchers must organize early. There is considerable delay in obtaining insecticides and aircraft, and needs must be anticipated. Availability of trained personnel is limited, and programs are conducted on a first-come-first-served basis. Characteristically, ranchers delay district formation until a signficant loss has already occurred, and grasshoppers are maturing. Not only are adult grasshoppers more difficult than immatures to kill, but once the adult stage is reached, oviposition begins. If significant egg deposition occurs before grasshoppers are killed, long-term suppression is unlikely.
Biological Suppression of Grasshoppers
Although grasshoppers are preyed upon by a wide variety of predators and parasites and are infected by a number of pathogens, these currently do not offer much potential for manipulation with subsequent suppression of grasshopper numbers. Naturally occurring biological control probably reduces the rate of population buildup and may even prevent buildup or cause population decrease in some localities. Until further information becomes available on the constraints and attributes of potential biological control agents, the biological control potential of grasshoppers must be viewed rather pessimistically; however, an important exception is use of the protozoan Nosema.
Considerable research has been conducted on Nosema locustae Canning. This pathogen occurs naturally and has a wide host range although some species of hosts are particularly susceptible and may be important in promoting disease development in less susceptible species. The natural incidence of infection generally is less than 10% (and often less than 1%) although epizootics occasionally occur. The pattern of occurrence is a characteristic lag-type host parasite density relationship (Henry, 1981).
For infection to occur, grasshoppers usually must ingest Nosema spores although spores also are found on or in the eggs. Fatbody, neural and other tissues are infected. Infection slows grasshopper development, decreases activity, increases cannibalism, reduces fecundity and sometimes causes mortality. The deleterious effects of Nosema are enhanced by early infection and ingestion of a large number of spores. An optimal scenario might involve application of 1.12 to 1.68 kg wheat bran per hectare containing 1.6 x 109 to 2.3 x 109 spores, distributed while M. sanguinipes is predominantly in instar 3; grasshopper mortality should be 50% to 60% within four to six weeks, followed by 35% to 40% infection among survivors (Henry and Oma, 1981). Thus, mortality is not realized quickly, and high levels of mortality are not obtained. Over time (a minimum of one year), grasshopper populations could be reduced to low levels. While not suitable for short-term control, Nosema locustae should be useful for long-term management.
Nosema is compatible with low concentration of malathion. The insecticide kills rapidly (within 24 to 48 hours) while Nosema provides long-term control. Application of bait treated with insecticide and pathogen may prove useful for long-term population suppression where there also is protential for immediate damage (Mussgung and Henry, 1979; Onsager et al., 1981).
Mass production of Nosema species is relatively easy, but it requires production of grasshoppers. Spores can be stored frozen for several months, but they degrade rapidly at field temperatures. Production and storage is discussed briefly by Henry and Oma (1981). Estimated costs of spore production are $0.21 per hectare, which makes Nosema application economically feasible. Several commercial firms currently market Nosema locustae, but product quality is variable. Homeowners in suburban areas will be disappointed by Nosema performance, if they treat small acreages. The combination of low immediate mortality and extensive immigration of healthy grasshoppers into treated areas will mask effectiveness of the product. Other Nosema species also are promising. N. acridophagus Henry and N. cuneatum Henry are more pathogenic than N. locustae. Development of these products as biological insecticides has been hampered by culture problems; mortality of grasshoppers occurred so rapidly that spore production was poor. Recent discovery of an alternate host, corn earworm, Heliothis zea (Boddie), in which spores could be produced effectively, suggests promising developments in the near future (Henry et al., 1979). N. acridophagus and N. cuneatum may provide effective, rapid knockdown of grasshoppers comparable to insecticides and may serve as an alternative to N. locustae, a substitute for insecticides or a replacement for insecticides in N. locustae-insecticide mixtures.
Poultry se numbers of grasshoppers. The turkey industry in Colorado reportedly was initiated, at least in part, because of the abundant supply of grasshoppers. A turkey supposedly will consume up to 100 grasshoppers per hour (Tresner, 1981).