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Grasshoppers of Colorado
SYNOPSIS OF GRASSHOPPER BIOLOGY
Grasshoppers are a diverse group of insects. This section presents an overview of grasshopper biology and stresses the features common to most species. The generalizations that follow provide insight into grasshopper development, behavior and population dynamics, but many exceptions exist. For a comprehensive review of grasshopper biology, see Uvarov (1966, 1977).
Developmental stages in grasshoppers include the egg, immature (nymph) and adult. Each individual progresses through these stages in the order presented, but the length of each stage and the time of year when a given stage occurs varies with species.
Most species pass the winter as eggs. Eggs are deposited by females during the summer and autumn. Embryological development occurs as long as weather is favorable but ceases at some point. The point of arrested development is variable, but commonly 80% or 90% of development has been completed. The grasshopper embryo remains in a state of arrested development, or diapause, until spring when a combination of temperature and moisture stimulates egg hatch, or eclosion. Usually a lengthy cold period is required before embryos will respond to favorable environmental stimuli and complete their development; this prevents eclosion from occurring during abnormally warm periods in the winter when food is not available.
The length of time required between completion of diapause and eclosion is largely a function of temperature, with higher temperatures promoting earlier eclosion. Eclosion date can be estimated from soil temperature accumulations. For example, Hewitt (1979) indicated that approximately 2,500 degree hours (number of degrees each hour that soil temperature exceeded a developmental threshold of 15.6oC) were required for hatching of Melanoplus sanguinipes. Mukerji and Gage (1978) developed a procedure based on soil temperature and moisture that explains 99% of the variance in eclosion. Much more information on grasshopper development is needed to predict eclosion and to allocate resources for grasshopper sampling and control efforts more effectively.
Eggs usually are produced in groups and in structures called egg pods. According to Onsager and Mulkern (1963) four basic types of egg pods occur, and these are distinguished primarily by characteristics of the frothy secretion associated with the eggs. The froth may be found around and among the eggs, causing the eggs to adhere to one another and soil particles to adhere to the eggs. Type I egg pods are characterized by froth surrounding the mass of eggs but not between individual eggs. Thus, the froth forms a hollow receptacle in which the eggs are deposited. Type 11 egg pods have froth surrounding eggs and between eggs. Type III egg pods have froth between eggs, but froth does not completely surround the pod. Type IV egg pods contain little froth, and it occurs only adjacent to the last-deposited eggs. Thus, egg pods may be found as hardened, soil-covered pellets (often bean-shaped), or as rather loose collections of eggs or as something in between.
Egg pods commonly contain four to 40 eggs. Females may produce four to 25 egg pods, and total egg production is commonly 100 to 200 eggs, but 500 eggs is not unusual (Criddle, 1933a). Characteristics of eggs and egg pods can provide valuable clues to the identity of grasshoppers; Onsager and Mulkern (1963) provide a key to eggs of North Dakota grasshoppers.
Eggs usually are deposited in soil, but some species prefer to oviposit among roots of plants, in wood or in cow dung. Females may be very selective in oviposition site and commonly will insert their ovipositor numerous times before locating a satisfactory site. Soil texture, temperature, vegetation conditions and moisture level commonly influence site choice. During the early part of the egg-laying season when temperatures are high, level ground or eastern slopes frequently are chosen as oviposition sites. Toward autumn as temperatures become cooler, grasshoppers frequently desposit their eggs on sunny southern and western slopes. Obviously, the oviposition site of a grasshopper species is associated with its seasonal history (Criddle, 1933a).
Egg hatch is not always synchronous because species-specific differences exist and oviposition sites may be exposed to different temperatures. Also, some species produce eggs that diapause through two winters and the intervening summer. Or, a species that normally survives the winter as a nymph may diapause throughout the winter and hatch during the following season.
Not all species overwinter as eggs. In a few species, winter is passed in the nymphal stage, and adults are present early in spring. Species that probably overwinter as nymphs in Colorado include Arphia conspersa, A. simplex, A. sulphurea, Chortophaga viridifasciata, Cibolacris parviceps, Eritettix simplex, Pardalophora apiculata, P. haldemanii, Psoloessa delicatula, P. texana, Trachyrhachys coronata, Xanthippus corallipes and X. montanus. Other grasshoppers, especially high-altitude species, may overwinter as nymphs. Xanthippus corallipes is known to pass the first winter in the egg stage and a second winter as a nymph. Adults occasionally survive the winter.
Following egg hatch, nymphs undergo a series of molts accompanied by changes in appearance and size. Nymphal instars vary in number among and within species but usually number five or six. Females sometimes have one more instar than do males. A natal molt also occurs immediately after eclosion, but this is not included in computations. Molting usually takes less than 30 minutes to complete, resulting in escape from the old skin, or integument. The rate at which development occurs is governed principally by temperature and food quality; favorable conditions increase development rate. Most species attain the adult stage in 30 to 50 days.
Changes in morphology, which accompany grasshopper growth, include an increase in number of antennal segments, growth of wings and appearance of external adult genitalia. Antennal segments increase in number from 12 to about 23 in regular increments in some Melanoplus spp. Wings increase in size (rudimentary wings are called wing pads), develop venation and change orientation as the nymphs develop toward adulthood. Genitalia, especially male cerci, change in appearance and are useful for identification of Melanoplus spp. Wing structure is most useful for instar identification, and a key is provided in the section on grasshopper identification. Extra instars, when they occur, appear after the third instar (Shotwell 1941).
Adults appear following the final molt. In some species considerable sexual dimorphism occurs while in others both sexes are very similar. Adults may be long-lived. Late developing species commonly survive until the first severe frost while adults of early developing species may die in mid-summer. Occurrence of adults and other stages of development of common Colorado grasshopper species are shown in table 1. Many of these data were collected in Wyoming, but conditions are similar in Colorado. Table 1 shows division of species into early, intermediate and late-hatching groups, but considerable variation exists in appearance of developmental stages.
Daily and seasonal shifts in grasshopper behavior are related to effects of weather, the need to propagate and availability of food. Grasshoppers sometimes exhibit complex behaviors; among the most important are those related to mating, flight and feeding.
Grasshopper dispersion is not uniform. Most species tend to occur in aggregations. Thus, long-range movement may not be necessary to bring potential mates together although in the case of species occurring at very low densities considerable searching may be required. Visual, tactile, acoustic and olfactory stimuli play roles in courtship behavior. Males commonly produce signals that attract and excite females; other males also may be attracted. Common mating displays include flashing of brightly colored wings and production of whirring or snapping noises. Pair formation and copulation also may result from chance encounter. In Melanoplus spp. a male usually approaches a female stealthily and pounces on her. The male of all species invariably mounts the female from behind, lowers the tip of his abdomen below hers and attaches the genitalia. This is followed by sperm transfer, and the female remains unreceptive to further copulation attempts until she has absorbed the sperm packet. Mating displays and genital differences insure effective reproductive isolation (Otte, 1981).
Movement of grasshoppers may occur if nymphs hatch in areas that lack suitable food. Also, since large numbers of eggs may be deposited in the same location, food can be depleted quickly, which necessitates movement. Drought often makes forage unpalatable, and both nymphs and adults walk or fly to more succulent plant material (fig. 368). These examples of dissemination reflect movement by individuals in response to organic needs - the necessity to find food or oviposition sites. However, not all movements are made on an individual basis, nor are they made for feeding or oviposition.
Swarms of grasshoppers are dreaded by agriculturalists. Some species of grasshoppers, such as Melanoplus sanguinipes and Dissosteira longipennis, may on occasion become gregarious and form swarms. Large numbers may take flight simultaneously, fly long or short distances and descend and destroy agricultural crops. Swarms usually take flight during periods of hot, dry weather and are blown in the direction of the prevailing wind.
The Rocky Mountain locust, Melanoplus spretus, caused extensive damage to the central and north central states in the 1800s. Swarms were so abundant and inflicted such severe damage that many settlers left their homes convinced that agriculture would never be possible in the middle west. Yet M. spretus no longer occurs, and it is now believed to be only a swarming phase of M. sanguinipes.
Swarms of M. sanguinipes have been reported to travel 66 miles per day and eventually settle 200 to 300 miles from their point of origin (Parker et al., 1955). Grasshopper densities following swarm descent increased from one to five grasshoppers per square yard to 32 or more per square yard or even to several hundred per square yard. Swarms may land and take flight repeatedly and may cause immense damage, depending upon the length of their stay. Flights terminate when temperature decreases or the sun is obscured by clouds. Females eventually mature reproductively, settle and begin egg deposition. M. sanguinipes swarms apparently favored cropland and barren land over rangeland for oviposition, but eggs were distributed widely. Nymphs hatching from eggs deposited by swarming adults demonstrated greater gregarious tendencies and more often formed bands compared to nymphs of the same species developing from eggs deposited by non-swarming adults. An interesting and thorough description of a swarming species, D. longipennis, is given by Wakeland (1958).
Factors responsible for the transition of M. sanguinipes from a solitary, non-swarming state to gregarious, swarming populations are not understood fully. Certain physical and behavioral characteristics of swarming and non-swarming grasshoppers can be induced from environmental conditions and density (Brett, 1947). Solitary and gregarious "phase" development has been studied for a number of years in African locusts, and evidence exists for the importance of weather, food, density and conditioning in development of swarms (Uvarov, 1966, 1977). Similar mechanisms probably induce swarm development in North American species. Grasshopper swarms have not been a problem since about 1940 probably because of widespread use of effective insecticides. Brett (1947) suggested that extensive alfalfa plantings might be responsible for keeping M. sanguinipes from attaining swarm densities. This species is highly attracted to alfalfa, but growth and development are impaired by consumption of this forage.
Feeding behavior in grasshoppers is diverse but somewhat predictable. Polyphagous species, such as many melanoplines, have extensive host plants. Such oligophagous species as Hypochlora alba feed only on a few plants, in this case Artemisia spp. Some species are strictly graminivorous, others are forbivorous and many are mixed feeders. Plant debris and insect material are fed upon; cannibalism is common. Most Gomphorcerinae are grass feeders as are many Oedopodinae. Catantopinae, especially Melanoplus spp., tend to be mixed feeders but prefer forbs. Broad food preferences are correlated somewhat with mouth structures. The incisor teeth of mandibles are relatively blunt in graminivorous species but are pointed in forbivorous species; intermediates between each type are common (Isely, 1944). Specific food preferences for common Colorado grasshoppers are provided in this section.
Visual and olfactory stimuli are important in host recognition, but even foliage with proper color and odor may be rejected after tasting. Foliage may be completely devoured or only nibbled. Partial feeding is common, if plants are not preferred hosts, and grass commonly is fed upon only at the base of a stalk or leaf; the distal portion may be severed and not consumed.
Food preference is determined by a number of factors. Such physical factors as leaf toughness or hairiness and water content frequently are believed to influence feeding behavior, but chemical differences are more important. Sugars, phospholipids, organic nitrogen compounds, tannins and others influence host preference. Although some exceptions exist, grasshoppers generally prefer plants that are most suitable for their growth and survival. Feeding generally is limited to temperatures between 15o and 30oC. Relatively little time is spent in feeding, approximately 15% (Uvarov, 1977).
Grasshopper populations are characterized by violent oscillations between endemic (low) and epidemic or outbreak (high) densities. Populations can establish an equilibrium state at either density and remain there for as long as 20 years. Populations at any site may shift from one equilibrium state to another, and this almost always is due to weather changes. The endemic equilibrium state tends to be more stable than the epidemic state. In addition to weather-related direct effects on population transition to the endemic state, weather-induced disease epizootics sometimes are important. Agricultural practices may modulate natural transitions by, for example, providing green forage during drought. Insecticides may protect crops from grasshoppers but do not actually affect population patterns (Turnock, 1977).
Grasshopper populations commonly require about five years to move from endemic (less than 1 per square yard) to epidemic (24 to 32 per square yard) densitites, but they return to endemic densities in one or two years. Pfadt (1977) suggests that population densities double each year until the fifth when densities triple or quadruple. Most species at a site increase or decrease in abundance simultaneously. Certain species are well adapted for a site and remain numerically dominant over long periods of time.
It probably is unrealistic to expect one factor to totally account for trends in grasshopper abundance, but weather appears to be the driving force behind increases in grasshopper number. Weather directly affects survival of young nymphs; first-instar grasshoppers are easily killed by heavy rainfall (Criddle, 1933a). Weather indirectly affects survival of young grasshoppers because young insects are especially susceptible to nitrogen shortage in food (White, 1976). Favorable weather in autumn allows a prolonged oviposition period, resulting in more eggs per female. Epizootics of fungal disease also are related to precipitation. Abundance of horsehair worms, which inhibit reproduction when they parasitize grasshoppers, is correlated with presence of water.
Population increases follow abnormally warm and dry periods, and grasshopper numbers usually decrease with the onset of moist, cool conditions. Edwards (1960) analyzed weather data and grasshopper abundance in Canada for the period 1930 to 1958 and reported a strong correlation between high mean monthly temperatures during July to September for three years and subsequent grasshopper numbers. A weaker but significant correlation was found between two years of low precipitation during April to August and subsequent grasshopper abundance. Gage and Mukerji (1977) conducted another evaluation for the period 1943 to 1974 and reached similar conclusions. A common but erroneous assumption is that severe winter weather will affect grasshopper survival. However, diapausing grasshopper eggs seem to be relatively impervious to weather conditions.
Trends in grasshopper abundance in Colorado for the period 1933 to 1981 are given in fig. 369. Major epidemics occurred in 1936-8, 1957-8 and 1980-2 at approximately 22-year intervals. Smaller epidemics occurred midway between major grasshopper outbreaks. Major droughts in the western United States tend to occur at 22-year intervals, followed by minor ones at 11-year intervals. Grasshopper populations apparently respond to the favorable conditions provided by high temperatures and low rainfall.
Major periods of drought are correlated with sunspot cycles. Decrease in sunspot numbers is followed by precipitation decrease (Mitchell et al., 1979), which in turn is followed by grasshopper population increases. Sunspot cycles have continued, relatively unabated, since 1700 (Eddy, 1977). Therefore, weather and grasshopper cycles probably will continue also.
In addition to numerical change between seasons, grasshopper numbers vary considerably within a season. Typical grasshopper density trends in northern Colorado are shown in fig. 370. Grasshopper numbers decrease through the winter due to mortality among overwintering nymphs, and abundance reaches a minimum in April or May. Many species begin eclosion in May and June, so grasshopper numbers increase dramatically, peaking in June or July. Grasshopper abundance decreases through the summer; the rate of decline is dependent upon a number of mortality factors. Two peaks in adult numbers occur, reflecting maturity of those that overwinter as nymphs and eggs, respectively.
Melanoplus sanguinipes, and probably many other species as well, have the potential to increase 82-fold between generations (Pfadt and Smith, 1972). Due to various forms of environmental resistance, populations rarely increase more than 10-fold and usually much less (Pfadt, 1977). Capacity to increase in abundance is primarily a function of reproductive potential, but actual population increase reflects level of reproduction and level of mortality.
Grasshopper reproduction varies considerably, depending on quantity and quality of food. For example, Barnes (1955) found that bermuda grass and alfalfa were poor hosts for Melanoplus sanguinipes, relative to goosefoot and mustard. Similarly, MacFarlane and Thorsteinson (1977) demonstrated that survival of Melanoplus bivittatus was high on some varieties of faba bean but low on others. Age and nutrient content of foliage are important, and even the temperature at which foliage is grown can have an effect. Plant hormone concentrations also will influence grasshopper growth (Smith and Northcott, 1951; Smith, 1960; Visscher et al., 1979; Visscher, 1980). Nitrogen (protein) levels in foliage seem to be especially important for insect survival, growth and reproduction (Mattson, 1980), and White (1976) suggested that increase in nitrogen concentration in plants, which is associated with low rainfall, might trigger grasshopper outbreaks by making relatively unsuitable food more nutritious.
Predators and parasites of grasshoppers are numerous and account for significant grasshopper mortality. Various species of flies and wasps parasitize grasshopper nymphs and eggs. The most important predators are flies, beetles, birds and rodents. Rees (1 973) and Lavigne and Pfadt (1966) provide excellent compilations of predators and parasites of grasshoppers. Unfortunately, these biocontrol agents cannot be counted upon to keep grasshopper numbers in check. They probably provide much of the ecological resistance that keeps grasshopper population growth below the potential rate of increase. A Wyoming study, for example, estimated that 11 % to 15% mortality in a grasshopper population was due to predation by robber f lies (Lavigne and Pfadt, 1966). Biological control agents also may be responsible for limiting population growth to two-fold increases during the early years of grasshopper outbreaks. When favorable weather ensues, however, grasshopper populations seem to be able to overwhelm most biological control agents and escape from endemic levels to damaging epidemic levels.
The predominant naturally occurring disease of grasshoppers in North America is a fungus, Entomophthora grylli Faes. Grasshoppers dying from this disease characteristically climb to the top of grass stems or twigs of bushes and die there. The legs stiffen at death, and the cadavers remain clasping the stalks (MacLeod et al., 1980). This disease has on some occasions decimated grasshopper numbers, but high humidity usually is required (Pickford and Reigert, 1964).