Larry D. DeBrey
Michael J. Brewer
Jeffrey A. Lockwood
June 1993
Grasshoppers feed on a wide range of plants and other organic material (e.g., detritus, dung, dead insects, etc.). Some species of grasshoppers can reach high densities, concentrate their feeding on valued plants, and thus damage the agricultural value of both range and cropland. These grasshoppers constitute a serious insect pest problem in Wyoming and throughout the high and intermountain plains of the United States. Both ranchers and farmers historically have suffered severe hardships due to grasshopper infestation.
It is estimated that grasshoppers consume up 25 percent of the available forage in the western United States annually. When grasshopper management is not attempted in areas of grasshopper outbreaks, all available forage can be consumed. Grasshoppers are voracious feeders, consuming approximately one-half of their body weight in green forage per day. In high densities, grasshoppers can severely reduce the forage value of rangeland, resulting in reduced weight gain of cattle and lower calving reates. Grasshopper damage may force producers to buy hay, sell early, reduce stocking rates, or relocate the herd. Forage loss due to grasshopper feeding can be through direct consumption, or clipping without consumption. Clipping may be significant, accounting for up to 50 percent of forage loss to domestic stock due to grasshoppers. When clipping occurs, ground-dwelling grasshoppers eat most of the clipped pieces. Both consumption and clipping without consumption are dependent on the species preference for a food type and its feeding behavior. The damage potential of a grasshopper population also depends upon the actual number of grasshoppers in an infested area. Because of the potential economic threat caused by forage loss associated with grasshopper populations, it is important for land managers to be able to identify the types of grasshoppers (species) and estimate the numbers (density) present on their land.
Within Wyoming, there are geographic differences in abundance and types of grasshoppers found on range and cropland. Because of the large numbers of grasshopper species that occur in Wyoming (more than 100 known), this manual will only consider those with the greatest economic importance. Southeastern Wyoming has experienced significant areas of high grasshopper densities in 27 of the last 31 years, making this region the most chronically infested area in the U.S. This publication concentrates on the destructive potential of grasshoppers in this region. Management methods used to control economically damaging grasshopper infestations applicable to this and other regions are also discussed. More specific life history information on these and other grasshopper species can be found in a grasshopper species fact sheet series written by R. E. Pfadt (Field Guide to Common Western Grasshoppers).
The developmental stages of all grasshoppers include the egg, nymphal, and adult stages. Each species progresses through these stages in this order, but the length of each stage and the time of year when each stage is present varies with individual species.
Most species winter as eggs. After mating in summer, females generally deposit clusters of eggs in the soil in structures called egg pods. The size and shape of the egg pod depend on the species of grasshopper. The eggs develop, in summer, to a certain embryonic stage, and then cease to develop. The eggs are said to be in diapause. Diapause is associated with reduced growth that allows the embryos within eggs to survive the winter. After an egg enters diapause, it will remain in this state until spring. When the soil becomes warm in spring, the embryo completes development and increased moisture stimulates the grasshoppers to hatch. Usually, a long period of cold weather is required before the embryos respond to warm spring weather. This prevents grasshoppers from hatching during unusually warm periods in the winter. Generally, grasshopper eggs will remain in diapause until the ground temperature reaches 50o to 55o Farenheit. The rate of egg development increases as daily temperature increase. The time required for egg development also depends on the species of grasshopper. Hatching will occur early in a warm spring, and a cold spring will delay it. Therefore, the hatching time for a species may vary as much as four weeks from one year to the next.
Newly hatched grasshoppers look like miniature adults except they lack wings and the sex organs are not developed. As grasshoppers grow, they periodically shed their outer skin (molt). Depending on the sex and the species, they molt four to six times, with five molts being the most common. This period of growth is called the nymphal period. The immature grasshopper between molts is referred to as an instar. The first instar occurs between hatching and the first molt; the second instar occurs between the first and second molts, and so on. When the last instar molts, the grasshopper becomes an adult. Instars can be distinguished by the development of the wing pads (Fig. 1). The rate at which nymphal development occurs is strongly influenced by temperature and food quality. The more favorable the conditions, the faster the development. Most species reach the adult stage in 30 to 50 days after hatching.
Figure 1. Grasshopper instars, first through fifth.
Newly molted adults have fully functional wings but are not yet ready to reproduce. Females have a one to two week pre-productive period in which they gain weight and mature eggs before mating. After mating, the female digs a small hole with her ovipositor into the soil and deposits eggs, which she surrounds with a case (pod). Egg pods, depending on species, contain between 1 and 90 eggs, and a female may produce 4 to 25 egg pods. Maximum egg production commonly ranges from 100 to 200 eggs, though as many as 500 may be laid. Eggs are usually deposited in the soil, but some species will lay their eggs in roots, cracks in wood, and cow dung. Site selection may be influenced by soil texture, temperature, vegetation, and moisture level. Specific selection criteria depend on the species of grasshoppers. During the early part of the egg-laying season when soil temperatures are high, level ground and eastern slopes are commonly chosen for egg laying. As temperatures cool, sunny southern and western slopes are chosen.
Not all grasshoppers overwinter as eggs. There are some species that overwinter in the nymphal stage. In these species, adults are present much earlier in the spring than those species that overwinter as eggs. In southeastern Wyoming, two species, Psoloessa delicatula, the brownspotted grasshopper, and Eritettix simplex, the velvetstriped grasshopper, overwinter as nymphs. Another common grasshopper, Melanoplus confusus, the pasture grasshopper, hatches very early in the spring and reaches the adult stage with the other early species in spring. When abundant, these grasshopper species can be of economic importance by damaging rangeland, though this is rare. These three species (plus other non-damaging species), are the first noticeable grasshoppers in the spring and should not be misinterpreted as an early hatch of grasshoppers that overwinter as eggs.
From year to year, grasshopper populations fluctuate widely between endemic (low) and epidemic or outbreak (high) densities. Even within a season, populations at any site may shift from high to low densities. These fluctuations are often due to weather changes. Despite these within-season, site-specific variations, some multiple-year trends are common. Low population densities tend to be more stable than high population densities. However, the relatively rare multiyear outbreak populations are noteworthy because of their destructiveness. If possible, prevention of these outbreak populations should be considered because outbreaks can last three to six years and have been known to last up to 20 years.
Grasshopper populations commonly require about five years to shift from low-density to high-density populations. It is generally accepted that population densities double each year until the fifth year when densities triple or quadruple. However, recent research indicates that outbreaks may occur within one year. The different species at a site may increase or decrease in abundance simultaneously, although a species that is best adapted for a site may make up more than 50 percent of the population. Occasionally, two or three species may become co-dominants and compete for available food and space.
Mixed-feeding (grass and forbs) species of grasshoppers are the most abundant on rangelands. However, species vary in density and dominance depending on the topography, soil and vegetation type, and site management. Because of the influence of weather, parasites, disease, insecticide use, and land management, species type and density may change in time, often quite rapidly.
Grasshoppers are relatively large insects with distinct features. With practice, identification of grasshoppers to the taxonomic level necessary to distinguish pest from harmless species is possible. With few exceptions, the grasshoppers in Wyoming belong in the three taxonomic groups described below. There are seasonal abundance differences between these groups (Table 1). Common names of these grasshoppers are listed in Table 2. In addition to these groupings, specific species identifications and biology for selected pest grasshoppers in Wyoming can be found in Pfadt's species fact sheets.
|
(Gomphocerinae) |
(Melanoplinae) |
(Oedipodinae) |
|
|---|---|---|---|
| Early hatching group, hatching in mid-spring |
|
|
|
|
|
|
||
|
|
|
||
|
|
|
||
| Intermediate hatching group, hatching in late spring |
|
|
|
|
|
|||
| Late hatching group, hatching begins in early summer |
|
|
|
|
|---|---|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Spurthroated Grasshoppers (Melanoplinae)
All the members of this group, both nymphs and adults, have a distinct conical or cylindrical spur located between the front legs (Fig. 2). The front of the face points straight downward, perpendicular to the long axis of the body (Fig. 3). Nymphs are often green, slender, and active. The adults of pest species are all strong fliers and capable of dispersing long distances. Many members of this group can be economically damaging to either range or cropland. Melanoplus bivittatus, M. sanguinipes, and M. infantilis are all quite common in Wyoming and are major pest species.
Figure 2. Spurthroated grasshopper showing spur between front legs (arrow).
Figure 3. Spurthroated grasshopper showing face perpendicular to the long axis of
the body.
Slantfaced Grasshoppers (Gomphocerinae)
In this group the front of the face usually slants back toward the body (Fig. 4). There is no spur between the front legs. Nymphs are often brown, slender, and active. The lower wing on adults is colorless. Many members of this group can be economically damaging and are capable of dispersing long distances. Ageneotettix deorum, Amphitornus coloradus, Aulocara elliotti, and Cordillacris occipitalis, are quite common in southeastern Wyoming.
Figure 4. Slantfaced grasshopper showing face slanting inward toward the main body.
Bandwinged Grasshopper (Oedipodinae)
In this group, the front of the face points straight downward, perpendicular to the long axis of the body, but there is no spur between the front legs. Nymphs are robust and sluggish. The lower wings of adults are often brightly colored. Because certain species of bandwinged grasshoppers feed almost exclusively on grasses and sedges, they potentially could cause economic damage. However, the adults are usually present in low densities, and populations do not measurably damage rangelands. Metator pardalinus and Camnula pellucida at times occur in conjunction with another pest species that cause economic damage. They are common in northern and eastern Wyoming.
List of Species Fact Sheets from Pfadt's Field Guide to Common Western Grasshopper
Biological Control
Predators and parasites of grasshoppers are numerous and at times play an important role in maintaining grasshopper populations at low densities. Several species of flies and wasps are parasites of grasshopper nymphs and eggs.
Predators, such as birds, rodents, flies, and beetles, are often more effective in reducing grasshopper populations than parasites. A study in Platte County reported that robber fly predators may account for 11 to 15 percent mortality in certain grasshopper populations. In combination with climatic factors adverse to population growth, predators and parasites may be responsible for limiting grasshopper populations to low densities. Unfortunately, under weather conditions favorable to grasshopper populations, population increases may overwhelm these natural controls and grasshoppers may reach high densities.
The predominant naturally occurring biological control of grasshoppers under most conditions is the fungi Entomophus grylli. It infects grasshoppers through contact with the body. Grasshoppers infected with this fungus climb to the top of grass stems or brush limbs and die with their heads pointing upwards. The legs stiffen at death and cadavers remain clasing the stalks until wind or rain dislodges them. This disease has at times decimated populations, but high humidity is required. Because sparse vegetation reduces humidity, E. grylli has little or no effect on grasshopper populations when the rangeland is overgrazed. E. grylli is being tested as a commercial biological control agent but is not currently available. Another fungal pathogen, Beuvaria bassiana, appears to have considerable potential as a biological agent and is being developed for commercial use.
A biological control that has been tested extensively and is commercial available is the protozoan Nosema locustae (Nosema). Nosema is applied as a bran bait in commercial formulations, consisting of 1 billion spores per pound of bran. A commonly used application rate is 1 pound of bran per acre. Nosema enters the grasshoppers' fat bodies (somewhat equivalent to our liver) and multiplies rapidly. This infection causes "diarrhea" and dehydration and eventually death. Many species of grasshoppers are cannibalistic and eat the diseased dead. The spores are ingested, which helps spread the pathogen through the population. Nosema may overwinter on egg pods laid by infected females, in overwintering nymphs, and in the soil. However, Nosema is slow acting and may not reduce grasshopper populations to non-economic numbers the year of application. Also, the timing of application is important. Early instars are killed at such low levels of infection that few spores are produced to infect other grasshoppers. Older nymphs and adults are more resistant to the infection and mortality can be low. The optimum time to apply Nosema is when the grasshoppers are in their third instar. Missing this optimal application time may lead to unsatisfactory results. Also, because Nosema is applied on bran bait, those species of grasshoppers that do not eat bran are not directly infected, although there may be some infection through cannibalism. For example, a pest species of southeastern Wyoming rangelands, Amphitornus coloradus (striped grasshopper), is not susceptible to Nosema applied on bait. The use of Nosema for grasshopper control has had mixed results. It has been quite successful in some areas, but has had little effect in others. An important benefit of Nosema is that it is target specific. It infects only grasshoppers and crickets and does not harm beneficial, terrestrial, or aquatic insects and other nontarget organisms.
Chemical Control
Chemical control of grasshoppers currently involves the use of aerially applied malathion, carbaryl (trade name Sevin, both as a liquid and bran bait), or acephate (trade name Orthene). Carbaryl baits are occasionally applied from the ground. The principle source of mortality to grasshoppers is through ingestion, though these chemicals also kill by body contact. These chemcials interfere with the insect nervous system resulting in death.
These insecticides are fast acting. When applied at recommended rates under optimal conditions, they can reduce grasshopper populations with 24 to 72 hours. Control rates of 80-85 percent are common. These chemicals are applied at low volumes when used as sprays, and malathion is used at ultra-low volumes (8 fluid ounces per acre). Therefore, optimal conditions include low moisture (to prevent dilution of the insecticide), cool temperatures (so heat rising from the ground does not prevent the chemical from reaching the ground), and little or preferably no wind (to prevent drift from the target area). These conditions usually restrict aerial application to early morning, from sunrise until ground temperature reaches approximately 75o to 80o Farenheit.
A disadvantage of using these chemicals, as liquids, is that they are less target specific and kill insects that are beneficial such as the ants, honey bees, parasites, and predators. These chemicals are also toxic to aquatic insects such as mayflies, caddisflies, and stoneflies. Reducing these insects may adversely affect other animals that depend on them as food. All of these chemicals may have some toxic effect on birds and mammals (including humans) if improperly used. Therefore, the application rates and other warnings listed on the label should be read and followed in order to avoid direct hazards to vertebrates.
See Reduced Agent Area Treatments for a new strategy in the chemical control of grasshoppers.
Growth Regulators
A chitin inhibitor for grasshopper control is currently being field tested, but is unavailable commercially for range grasshopper control (sold as Dimilin for registered sites). Chitin is the material that provides structure for the insect's exoskeleton (the outer shell). Dimilin inhibits hardening of the exoskeleton after a molt, causing the insect to die. It may interfere with egg production because the egg shell is also made of chitin.
Dimilin can be applied aerially, mixed with water or oil, or by using bran as bait. Like Nosema and Sevin, Dimilin is only effective on those grasshoppers attracted to bran baits. An important benefit to Dimilin application, if it becomes commercially available, is that it is not toxic to adult insects (pollinating bees and predators) and is non-toxic to birds and mammals. However, Dimilin is persistent in the environment and toxic to immature aquatic insects. In tests in Platte County, Dimilin was found to be effective as Sevin. Dimilin works fairly rapidly, with the majority of the kill occurring within seven days of application.
There are about 100 known species of grasshoppers in Wyoming. Of these, there are 15 species in Wyoming that are major pest species of either range or cropland (Table 2). Anabrus simplex, the Mormon cricket, deserves mention because of its destructive capability when crickets move in mass from range to croplands. Mormon crickets emerge in spring earlier than most grasshoppers and are not typically part of a pest complex. Historically, the Wyoming counties that have grasshopper problems are Big Horn, Washakie, Hot Springs, Sheridan, Johnson, Campbell, Crook, Weston, Converse, southern Natrona, Niobrara, Goshen, and Platte counties. Because of elevation and vegetational restrictions (pine forests or deserts) and other factors not yet clearly understood, the other Wyoming counties have been fortunate in that outbreaks of grasshopper pest species are rare.
Sweep Netting
It is important to do some sweep netting to verify that the "hoppers" are indeed grasshoppers and that they are pest species. Sweep net samples will also assist in determining the stage of development of the grasshoppers.
In the early to midspring it is hard to distinguish a first instar grasshopper from a leafhopper. Leafhoppers are small, active insects that readily jump or fly. They are triangular shaped, with the wings help over their body in an arch, and variously colored (Fig. 5). They are perhaps the most abundant rangeland insect. When hopping, leafhoppers are hard to distinguish from a jumping early instar grasshopper. Several sweeps with a net will help to determine if the jumping insects are grasshoppers, leafhoppers, or some other insect.
Figure 5. Do not confuse leafhoppers (this picture) and young grasshoppers (Figure
1).
Sweep net samples also allow determination of the species in the infestation. There are six major pest (epidemic) species in southeastern Wyoming: Ageneotettix deorum, Amphitornus coloradus, Aulocara elliotti, Cordillacris occipitalis, Melanoplus sanguinipes, and Melanoplus bivittatus. The first four are major pest species of rangeland, Melanoplus bivittatus is a cropland pest, and Melanoplus sanguinipes attacks both rangeland and crops.
During an outbreak there may be greater than 16 species present (species complex), but typically it is one of these species or a combination of a few that dominate the outbreak. Detailed information for each of these species can be found in Pfadt's species fact sheets. Species identification is useful because (1) species vary in their reproductive potential and in their capacity to cause damage, (2) some species feed on noxious weeds and may be beneficial rather than harmful, (3) highly migratory species often pose a serious threat to distant crops or rangelands, and (4) it is important in deciding which treatment, if any, should be used (e.g. a bran bait application will have little or no effect on A. coloradus). Primarily, ready identification of the pest species known to occur in your area is essential.
Sweep netting is also the most accurate way to determine what instars of grasshoppers are present. This information is useful in determining if a grasshopper control program is needed. If the majority of the grasshoppers are first and second instars, treatment is not immediately necessary. Early instar grasshoppers are sensitive to changes in the weather and susceptible to predation, and such naturally occurring mortality factors may result in a population crash. However, high densities of early instars are a good indication that the potential for economic loss exists and populations should be monitored over the season. A rule of thumb is that one-half of all early instar grasshoppers will become adults. If you are counting 15 early instar grasshoppers per yard square, it is quite possible that only seven or eight will become adults.
If the majority of the grasshoppers are adults, more than likely the females are laying eggs and the benefit of control is substantially reduced. If the grasshopper populations are at high densities, treatment at the adult stage might save a small amount of forage consumed during the insects' life and possible reduce the epidemic potential for the following year by ending egg production. However, these benefits rarely offset the cost of treatment. Ideally, if control measures are initiated when the grasshoppers are between third and fifth instars, forage is saved and the potential for population recovery the following year is virtually eliminated.
The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) has estabilished an action threshold for pest grasshoppers of eight per square yard. Densities at or above this threshold are an indication that management action should be considered. However, a grasshopper population of eight per square yard does not necessarily mean that a control program should be initiated. Management should be based on the cost of the control versus the amount of expected loss (forage and weight gain), the life stages of the grasshoppers, and the species complex. An estimate of the density of grasshoppers at a site is part of the process, along with species determination, to determine if the population should be controlled. Table 3 outlines a method to obtain an estimate of the grasshopper density per square yard.
With a little practice, the counts will become reasonably accurate. It is feasible to count 0-5 grasshoppers per square foot with good accuracy. At higher densities accuracy of the estimate is recuded but is less important. At this point the action potential has ben greatly exceeded. For example, anything greater than 5 grasshoppers per square foot is greater than 45 grasshoppers per square yard. It is essential to remember that these grasshoppers must be composed of pest species to obtain economic benefits from implementing an expensive control strategy such as insecticides.
Overall, information on which pest species are present, the control strategies that are most effective in their control, and the density of the grasshoppers will assist the land manager in determing the economic importance of grasshoppers on a particular site and whether initiating control measures will be beneficial.
We thank Alice Minney for the illustrations. We also thank Robert Pfadt, Richard Olson, and Scott Schell for their critical reviews and suggestions.
Capinera, J.L. and T.S. Sechrist. 1982. Grasshoppers (Acrididae) of Colorado. Identification, biology and management. Bulletin No. 584S. Colorado State Experiment Station. Fort Collins, Colorado.
Hawkins, B. 1987. Final environmental impact statement on the rangeland grasshopper cooperative management program. APHIS FEIS 87-1. USDA-APHIS. Washington D. C. 20250
Pfadt, R. E. 1988. Field Guide to Common Western Grasshoppers. Bulletin 912 USDA APHIS-Wyoming Agricultural Experiment Station, University of Wyoming.
