Distribution. This native grasshopper is extremely adaptable. It is found in every state in the continental United States, and in every province in Canada. It is absent from only the northernmost, coldest regions of Canada, and from southern Florida and Texas.
Host Plants. This species feeds on a wide range of food plants, and occurs in numerous habitats. Relative to the other common crop-feeding Melanoplus spp., migratory grasshopper is more tolerant of arid, short-grass environments. It tends to prefer annual broad-leaf plants, but eat grasses. Dry plant material is an important element of the diet in addition to succulent leaf tissue (McKinlay, 1981). Many authors have noted that the population abundance of migratory grasshopper is correlated with availability of annual broadleaf plants. Among the preferred plants are dandelion, Taraxacum officinale; stinkweed, Thlaspi arvense; Johnsongrass, Sorghum halepense; Kentucky bluegrass, Poa pratensis; shepherdspurse, Capsella bursapastoris; pep-perweed, Lepidium spp.; tansymustard, Descurainia sophia; western wheatgrass, Agropyron smithii; winter mustard, Sisymbrium irio; young Russian thistle, Salsola kali; and young rabbitbrush, Chrysothamnus spp. (Pfadt, 1949; Scharff, 1954). Among the preferred weeds eaten in North Dakota alfalfa fields were awnless bromegrass, Bromus inermis; kochia, Kochia scoparia; field sowthistle, Sonchus arvensis; field bindweed, Convolvulus arvensis; and Russian thistle, Salsola kali (Mulkern, 1962). On prairie, however, the preferred host plants were Kentucky bluegrass, Poa pratensis; leadplant, Amorpha canescens; white sage, Artemisia ludoviciana; and western ragweed, Ambrosia psilostachya (Mulkern et al, 1964).
Migratory grasshopper does not normally infest vegetable crops, but prefers to inhabit weedy areas along fences, irrigation ditches, roadsides, and in pastures. However, as favored food plants become overmature, desiccated, or depleted, grasshoppers move into vegetable crops. This is especially likely during periods when grasshoppers are extremely abundant. Among the vegetable crops reported to be injured by migratory grasshopper are asparagus, bean, cabbage, carrot, cauliflower, celery, cucumber, lettuce, melon, onion, pea, radish, squash, tomato and watermelon. Field crops are more often injured, particularly alfalfa, barley, corn, oat, and wheat. However, as happens with vegetables, when grasshoppers are numerous buckwheat, clover, flax, millet, rye, young sorghum, soybean, sugarbeet, timothy, and tobacco may be damaged. Even fruits such as apple, cherry, currant, grape, peach, plum and strawberry, as well as numerous flowers and shrubs are attacked during periods of abundance.
Pfadt (1949) studied host preferences and nymphal survival on many rangeland grasses, weeds and some field crops. Not surprisingly, there was a strong positive relationship between preference and survival. Among the plants most suitable for nymphal survival were wheat, sunflower, alfalfa, corn and barley, accounting for the reputation of this species as a severe pest in central and western North America. (Alfalfa is an unusual host, however, because though it is quite suitable for large nymphs, it is but a poor source of food for the youngest of the species.) Several broadleaf weeds including dandelion; downy chess, Bromus tec-torum; tumblemustard, Sisymbrium altissimum; slimleaf scurfpea, Psoralea tenuiflora; and prickly lettuce, Lactuca scariola were quite satisfactory for survival, though not as suitable as the crop plants. Among the least suitable plants were common prairie grasses. Diet also affected fecundity, with favored food such as dandelion resulting in production of a mean value of 3.5 egg pods per female during a three-week period, whereas grasshoppers fed a mixture of prairie grasses produced only 0.3 pods per female.
Natural Enemies. Many insects parasitize or prey on Melanoplus grasshoppers. The most comprehensive listing of arthropod natural enemies was published by Rees (1973); this publication also contains keys to many of the important species.
The most important parasitoids are nymph- and adult-attacking flies (Diptera) in the families Anthomyiidae, Nemestrinidae, Sarcophagidae, and Tachinidae, though egg parasitoids (Hymenoptera: Scelionidae) also cause mortality in grasshopper populations. In Oregon, over 70% of migratory grasshoppers were parasitized by the nemestrinid, Neorhyn-chocephalus sackenii (Williston), resulting in decreased longevity and reproduction. Other Melanoplus spp. also are affected by this fly, though grasshopper populations on rangeland, not cropland, are usually affected (Prescott, 1960). In a study of migratory grasshopper parasitism conducted in Ontario, the incidence of parasitism reached about 7% by the end of September, with Blaesoxipha hunteri (Hough) and B. atlantis (Aldrich) (both Diptera: Sarcophagidae) the most effective parasitoids (Smith, 1965). Sanchez and Onsa-ger (1994), using more accurate methods to estimate parasitism of migratory grasshopper, reported generation parasitism levels of 15-41% in Montana, with anthomyids and sarcophagids accounting for 50% and 35% of the parasitism, respectively.
Among the most important predators are sphecid wasps (Hymenoptera: Sphecidae). Adult sphecids capture and paralyze nymphal and adult grasshoppers, bury them within cells in the soil, and deposit an egg on the surface of the grasshopper. Upon hatching, the larva devours the paralyzed grasshopper. Predatory beetles (Coleoptera) attack the egg, nymphal and adult stages of grasshoppers, and include ground beetles (Carabidae), tiger beetles (Cicindelidae), soldier beetles (Cantharidae), and blister beetles (Meloi-dae). Blister beetles are most important, though because the grasshopper egg pod is the stage destroyed, and the predatory activities are hidden below-ground, their effect is often not appreciated. Parker and Wakeland (1957) summarized the results of several studies on egg pod predation in western states; during the period 1938-1940, for example, an average of 8.8% of egg pods were destroyed by blister beetles. Flies also are important predators, particularly robber (Asilidae) and bee flies (Bombyliidae). Robber fly larvae and Gryllus spp. field crickets (Orthoptera: Gryllidae) occasionally attack egg pods, and robber fly adults routinely attack nymphs and adults of grasshoppers, though other insects also are taken. Robber flies undoubtedly are important predators under rangeland conditions, but predation rates of grasshoppers in cropping systems has not been determined. Also, the propensity of robber flies to capture other predators such as sarcophagids significantly decreases their value (Rees and Onsager, 1982). Bee fly larvae are predatory on grasshopper eggs and on other insects. In western studies, an average of 6.2% of egg pods were destroyed by bee fly larvae (Parker and Wakeland, 1957).
Birds are known to be important predators of grasshoppers. They are among the most important sources of food for many avian species due to their large size and abundance. The great abundance of grasshoppers in the spring coincides with the period when most birds are nesting. Birds forage freely on them in open areas such as grasslands, with some species consuming 65-150 grasshoppers per day (McEwen, 1987). Although avian predators significantly decrease the abundance of grasshoppers in grasslands, it is less certain that they forage freely in crops.
Microbial pathogens can be quite important mortality agents, especially when weather conditions are suboptimal for grasshoppers, or when grasshoppers are very abundant. The principal microbial pathogens of grasshoppers are fungi, viruses, protozoans, and nematodes and nematomorphs.
The fungus Entomophthora grylli causes "summit disease," a behavior wherein grasshoppers ascend vegetation, cling to the uppermost point, and perish. In some areas, particularly near bodies of water, many dead grasshoppers can be found attached to plants. Melanoplus spp. are susceptible to one pathotype of the fungus, and significant grasshopper population decreases have been linked to the incidence of this fungal disease. Infection normally occurs when nymphs contact spores that are sheltered in the soil. Spores produced in grasshoppers dying due to this disease remain in cadavers or soil for protracted periods of time. Attempts to manipulate this pathogen have met with mixed results (Carruthers et al., 1997). This is the only common naturally-occurring fungus of grasshoppers.
Several viruses called entomopoxviruses affect grasshoppers. One such virus, Melanoplus sanguinipes entomopoxvirus, affects the crop-feeding Melanoplus spp. and American grasshopper, Schistocerca americana (Drury). The virus disease spreads naturally by cadaver feeding. Infected grasshoppers are pale colored and lethargic, have prolonged developmental periods, and often perish. These diseases are quite rare in the field (Streett et al., 1997).
Several types of protozoa are associated with grasshoppers, including amoebae, eugregarines, and neo-gregarines, but the most important are microsporida. Species of Nosema are most common, and Nosema locustae has been developed as a microbial insecticide.
Nosema spp. affect feeding, development, reproduction and survival, and are transmitted by ingestion (Johnson, 1997). However, they infrequently appear at high levels in natural populations.
Nematodes are important mortality factors of grasshoppers in South America, New Zealand and Australia, but in North America only Mermis nigrescens, Agamermis decaudata, and Hexamermis spp. affect them. Mermis nigrescens is most important, and is unique in that nematodes crawl from the soil onto vegetation to deposit desiccation-resistant eggs. The eggs hatch when consumed by grasshoppers, and the resulting larvae kill the grasshoppers, return to the soil, and continue the life cycle. These nematodes apparently do not thrive in arid areas, as the adults only emerge during wet periods, and are more common in irrigated croplands than dry rangeland. Sometimes they parasitize up to 70% of grasshoppers in an area. Nematomorphs, commonly called horsehair worms, resemble nematodes but tend to be much longer. They are rare, possibly because part of their life cycle must occur in water, but they attract considerable attention because of their large size (Baker and Capinera, 1997).
Weather. Migratory grasshopper is greatly influenced by weather. Through most of its range longevity and reproduction are limited by shortage of warm weather. Thus, abnormally warm and dry periods of about three years stimulate increase in their numbers. Warm weather during spring and autumn is particularly important. Cool and cloudy weather in the spring inhibits feeding by young nymphs, and results in high mortality. Also, adults have the potential to be long-lived and highly fecund, but their reproductive effort is normally terminated prematurely by the onset of cold weather. When summers are hot or prolonged, development proceeds faster or longer, resulting in greater egg production. In southern areas grasshoppers are less limited by shortage of warm weather, but are more affected by shortages of food. Therefore, occurrence of precipitation early in the season to provide luxurious foliage, especially broadleaf weed vegetation, is an important prerequisite for population increase (Capinera and Horton, 1989).
Life Cycle and Description. In most areas of its range, migratory grasshopper produces a single generation, and overwinters in the egg stage. However, in southern portions of its range two generations may occur annually. Eggs hatch relatively early, usually beginning in early June but about a week after twostriped grasshopper, Melanoplus bivittatus, begins to hatch. Hatching is protracted and may require up to six weeks in an area, resulting in asynchronous development of the population. Early hatching individuals mature early in the summer and have adequate time for reproduction whereas late-hatching individuals are handicapped by the onset of cold weather.
though if migratory grasshoppers are cultured at low temperature six instars is most common.
The nymphs are tan or gray, occasionally greenish, throughout nymphal development. They bear a curved black stripe that extends from behind each eye across the pronotum. The lower edge of the stripe is bordered in white. The outer face of the hind femur is marked with an interrupted black stripe. Body length in instars 1-5 is 4-6, 6-8, 8-11, 11-16, and 1623 mm, respectively. The number of antennal segments is 12-13, 15-17, 18-20, 21-22, and 22-24 in the corresponding instars. A detailed description of the nymphal stages is found in Shotwell (1930).
Adult. The adult is a medium-sized species, measuring 20-26 mm long in males and 20-29 mm in females. They are grayish-brown, and often tinged with reddish-brown. A broad black stripe extends back from the eye and about two-thirds of the length of the pronotum. The front wings are grayish-brown or brown, usually with a row of brown spots centrally. The hind wings are colorless. The hind femora are not distinctly marked. The hind tibiae are greenish-blue or red. The cerci of males are broad and flat, and turn dorsally at the tip. The subgenital plate is elongate, and bears a notch and grove apically. Females have a pre-oviposition period of 2-4 weeks. Adults normally live 60-90 days, though with good food and weather, and living under low density conditions, longevity may be extended considerably. They can mate repeatedly. (See color figure 165.)
Nymphs and adults are affected by daily change in temperature. Activity levels at the soil surface, including feeding, are at their maximum when the air temperature is 18-25°C. This often results in a peak in feeding in late morning, followed by cessation of feeding at mid-day due to excessively hot temperature, and then perhaps a secondary peak in feeding in the afternoon as temperatures cool. Mass flights by adults take place only if air temperature is high, often about 29° C, but high densities and light wind also are required. When it is hot, grasshoppers tend to climb upward to escape the soil, which is usually considerably warmer than the air temperature. However, they also tend to roost on tall vegetation at night, as this allows them to be warmed by sunlight early in the morning, thus extending their period of activity.
Because of its importance as a field crop pest over wide areas of central North America, there is an extensive literature on M. sanguinipes. Important aspects of biology were given by Parker (1930,1939), Shotwell (1941), Pfadt (1949), Parker et al. (1955), Pfadt and Smith (1972), Onsager and Hewitt (1982), and many others. A very good synopsis was presented by Pfadt
(1994b), who also pictured all stages of development. Melanoplus sanguinipes was included in many grasshopper keys, including those by Blatchley (1920), Dakin and Hays (1970), Helfer (1972), Capinera and Sechrist (1982), and Richman et al. (1993). Melanoplus sanguinipes was also included in a key to grasshopper eggs by Onsager and Mulkern (1963). A synopsis of migratory grasshopper, including keys to related Canadian Orthoptera, was given by Vickery and Kevan (1985). Rearing of Melanoplus species was described by Henry (1985).
Migratory grasshopper is a defoliator, often completely removing leafy vegetation and leaving only stem tissue. Sometimes other tissue is eaten; heads of wheat may be clipped, for example. Migratory grasshopper thrives on rangeland that has a high density of broadleaf weeds, so it often moves from grazing land to nearby irrigated crops. In this behavior it differs from some other species, particularly differential grasshopper, Melanoplus differential-is (Thomas), and twostriped grasshopper, M. bivittatus (Say), which favor the taller undisturbed vegetation usually associated with fences and irrigation ditches, and usually do not develop high numbers on grazing land. Migratory grasshopper is often quite dispersive, and of course this behavior is the basis for its common name. When they are developing at high densities, the weather is abnormally warm, and a light wind is present, swarms of grasshoppers may disperse tens or even hundreds of kilometers and descend without warning to cause immense damage. With the availability of modern insecticides and aircraft for application, such potential disasters can be dealt with quickly and efficiently. When such insecticide and effective application technologies are not available, or where environmentally sensitive land or crops are concerned, grasshopper swarms can be disastrous. This species is the most important grasshopper pest in western North America.
Application of insecticide-treated bait is an effective alternative to foliar treatments for Melanoplus spp, because these grasshoppers spend considerable time on the soil where they come into contact with baits. Bait formulations are bulky and more difficult to apply than liquid products, so they are not often used, but have the advantage of limiting exposure of crops to insecticide residue and of minimizing mortality of beneficial insects such as predators and parasitoids due to insecticide exposure. Also, the total amount of insecticide active ingredient that is necessary to obtain control is usually considerably less when applied by bait, because the grasshoppers actively seek out and ingest the bait. Finally, for relatively expensive products that must be ingested to be effective, such as microbial insecticides, baits are the most effective delivery system.
The attractant used most commonly for grasshopper bait is flaky wheat bran, though other products such as rolled oats are sometimes suggested. No additives, other than insecticide (usually 5% active ingredient), are necessary because the wheat bran is quite attractive to Melanoplus grasshoppers. Other additives such as sawdust, water, vegetable or mineral oil, molasses, amyl acetate, salt, or sugar have been suggested, but provide little or no additional benefit over dry bran. The bait should be broadcast widely to maximize the likelihood of grasshopper contact, and be applied while they are in the late instars as the adults ingest less bait. Shotwell (1942) and Cowan and Shipman (1940) provided excellent information on formulation of grasshopper baits; Mukerji et al. (1981) gave an interesting perspective of bait use on rangeland.
Cultural Practices. Elimination of weeds within, and adjacent to, crops is the most important cultural practice, and can have material benefit in preventing damage to crop borders. However, during periods of weather when grasshoppers become numerous they may move long distances and invade crops.
Tillage is an effective practice for destruction of eggs, particularly in migratory grasshopper, which is especially likely to deposit eggs among crop plants. Deep tillage and burial are required, as shallow tillage has little effect. All the crop-feeding Melanoplus species deposit some eggs in crop fields, especially during periods of abundance, but it is fence row, irrigation ditch, field edge, and roadside areas that tend to be the favorite oviposition sites, so tillage is not entirely satisfactory unless other steps are taken to eliminate grasshopper egg pods from these areas that cannot be tilled. Although providing suppressive effects, deep tillage is not consistent with the soil and water management practices in many areas, so it may not be a good option.
Row covers, netting, and similar physical barriers can provide protection against grasshoppers. This approach obviously is limited to small plantings, and can interfere with pollination. Also, grasshoppers are capable of chewing through all except metal screening, so this approach can not guarantee complete protection.
Biological Control. The opportunities for biological control are limited. Historically, poultry were found to consume many grasshoppers and could provide considerable relief if the grasshopper-infested garden was small or moderate in size and the birds were plentiful. This remains a viable option for some people, and turkeys are usually considered most suitable among poultry. The birds may also inflict some direct damage to plants, however, so introduction of poultry is probably most viable when grasshoppers are plentiful and threatening.
The microsporidian pathogen Nosema locustae is well-studied as a microbial control agent of Melanoplus spp. (Ewen and Mukerji, 1980; Johnson and Pavlikova, 1986; Johnson and Henry, 1987; Bomar et al, 1993), and is available commercially. It is fairly stable, and easily disseminated to grasshoppers on bait. However, its usefulness is severely limited by the long period of time that is required to induce mortality and reduction in feeding and fecundity. Also, the level of mortality induced by consumption of Nosema is quite low (Johnson and Dolinski, 1997), often imperceptible. It is best used over very large areas, not just on individual farms, and should be applied at least one year in advance of the development of potentially damaging populations. More rapid suppression of grasshoppers is attainable by applying very high levels of Nosema (Capinera and Hibbard, 1987), but this is not usually considered to be an economic approach, and commercial products are not prepared in this manner.
Fungi have also been investigated for grasshopper suppression, and a grasshopper strain of Beauveria bassiana has been effective in some trials (Moore and Erlandson, 1988; Johnson and Goettel, 1993; Jaronski and Goettel, 1997). Behavioral thermoregulation by grasshoppers, wherein they bask in the sun and raise their body temperatures, is potentially a limiting factor for use of fungi. Basking grasshoppers easily attain temperature in excess of 35°C; such high temperature decreases or even prevents disease development in infected grasshoppers (Inglis et al., 1996). Inconsistent quality control in production of fungi also limits use of these organisms for grasshopper control.
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