GRASSHOPPERS OF NEW MEXICO

GENERAL MORPHOLOGY AND BIOLOGY OF GRASSHOPPERS

Grasshoppers of New Mexico Contents

Grasshoppers include those insects in a number of families belonging to the superfamily Acridoidea in the order Orthoptera (Otte 1981). Grasshoppers are most closely related to crickets and katydids and often are mistaken for these other orthopterans. Grasshoppers may be distinguished from other orthopterans primarily on the basis of external morphology. A generalized diagram of a grasshopper is presented in Figure 1 . The most obvious and distinctive features of grasshoppers are their enlarged hind legs and their relatively short, thick antennae. The tegmina are another distinctive feature, but only in winged species.

Grasshoppers are physiologically similar to most other insects. They have an external chitinous skeleton, an open internal circulatory system, and they breath through tiny openings in the sides of their abdomen called spiracles (see Uvarov 1966 for details). Grasshoppers are poikilothermic or cold-blooded, and they rely upon thermoregulatory behavior to maintain their body temperatures (see Heinrich 1993 for a summary of thermoregulation in grasshoppers).

Grasshoppers are primarily herbivores and feed on various plants. Some species are host specific to certain plants; others feed on many different species and even families of plants. Grasshoppers locate host plants by visual and olfactory means. Further selection of plant tissue to eat is determined by smelling and tasting the plant tissue with chemical sensory receptors (sensilla) located on the mouthparts. The plant tissue is then chewed with the mandibles and consumed. Most grasshoppers rely on symbiotic bacteria in their gut to digest plant matter. Although grasshoppers are primarily herbivores, they will occasionally eat other disabled insects, including other grasshoppers, for additional protein in their diet. For example, grasshoppers are often seen on roads eating other grasshoppers that have been run over by automobiles.

Grasshoppers are typically medium to large insects. In New Mexico they range in size from the small males of Psoloessa texana Scudder (about 10 mm) to the large females of Schistocerca nitens (Thunberg) (about 70 mm). Most species are sexually dimorphic, the males being smaller than females. Size also varies geographically in many species, particularly with regard to elevation. For a given species, individuals from higher elevations are usually smaller than those from lower elevations.

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LIFE CYCLES

Grasshopper life cycles go from eggs through nymphal stages or instars to adults. Most New Mexico grasshoppers lay their eggs in the soil in late summer or autumn, and the eggs overwinter in the soil and hatch in the late spring in response to favorable temperature and moisture conditions (Hewitt 1979). Grasshopper eggs are generally laid below the soil surface in a frothy material that hardens and protects them from adverse environmental conditions. Nymphs develop through a series of 5 or 6 instars, each slightly larger than the previous one. At the end of each instar, grasshopper nymphs shed their external skeletons or skins and grow into a larger exoskeleton. The wings develop as small pads on the thorax until the final molt to the adult state, when they are pumped full of blood and become fully extended. Grasshoppers generally develop from first-instar nymphs to adults in about 4-6 weeks. They become sexually mature shortly after molting to the adult stage. The adults become reproductively active and mate in mid- to late summer, and eggs are laid in the late summer and autumn. Species of grasshoppers that exhibit this typical life cycle may be called summer species, because most of the growth and reproductive activity occurs during the summer months. Most common New Mexico grasshoppers are summer species, including Melanoplus sanguinipes (Fabricius), most other Melanoplus species, Trimerotropis species, Aulocara elliotti (Thomas), and Ageneotettix deorum (Scudder), among others.

A number of New Mexico grasshopper species do not exhibit the above seasonal pattern. Eggs of those species hatch in late summer, and the nymphs develop through several instars during late summer and autumn, becoming dormant and over-wintering as mid- or late instar nymphs. These nymphs become active in the spring to midsummer and adults lay eggs in midsummer. Grasshoppers with this type of seasonal life history are called spring species. Some common New Mexico spring species include Cibolacris parviceps (Walker), Xanthippus corallipes (Haldeman), Arphia conspersa Scudder, and Psoloessa delicatula (Scudder).

Other variations on life cycles include high-elevation populations that have two-year life cycles, and species with eggs that may persist in the soil for several years until hatching conditions are favorable. Some species such as Trimerotropis pallidipennis (Burmeister) in southern New Mexico may also have two generations in one summer.

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NATURAL HISTORY

Grasshoppers occur in a wide variety of New Mexico habitats, from low-elevation, hot, dry deserts to high-elevation, cool, moist meadows and woodlands. In general, most grasshopper species occur in semi-arid environments, and it is in the warm semi-arid grasslands and shrub lands that grasshopper species diversity and population densities are greatest (Otte 1976). Grasshoppers are relatively large, active insects and require structurally open habitats where they are physically free to move about, and where sunlight levels are high so they can maintain high metabolic rates. High levels of sunlight are especially important for eggs and developing nymphs.

Habitat specificity varies considerably among different species of grasshoppers (Joern 1979a, 1979b). Some species such as Melanoplus sanguinipes and Trimerotropis pallidipennis are typically common and are found in a wide variety of habitats over wide geographic and altitude ranges. Other species are much more restricted or specific to particular types of habitats. For example, Melanoplus magdalenus Hebard is restricted to certain high-elevation meadows in west-central New Mexico mountains. Cibolacris samalayucae Tinkham is restricted to certain sand dune areas, and Anconia hebardi Rehn to certain salt flats in south-central New Mexico, west Texas, and north-central Mexico. Most species of grasshoppers occur in certain general types of habitats such as desert-shrub, desert grassland, mountain meadows, or woodlands.

Habitat specificity is often correlated with diet specificity: grasshoppers tend to feed on particular plants that occur in their preferred habitats. For example, species that occur in desert grasslands tend to specialize on grasses. Species that live on the ground in gravelly desert areas tend to be mixed feeders on various forbs and grasses. Those that typically live on shrubs tend to be specific as to the types of shrubs. Bootettix argentatus Bruner is host-specific to creosote bush (Larrea tridentata [D.C.] Coville) and is found only where creosotebush occurs. Likewise, Aeoloplides species live only on plants in the family Chenopodiaceae.

Grasshoppers that occur in particular habitats and feed on particular plants have adapted morphologically and behaviorally to live in their microhabitats. Uvarov (1977) proposed a series of grasshopper life forms that correspond to morphological features of grasshoppers that live in particular microhabitats. Four general life-form categories are applicable to most New Mexico grasshoppers: 1) terricoles that live on the bare ground surface, 2) herbicoles that live on forbs, 3) graminicoles that live on grasses, and 4) arbicoles that live on shrubs and small trees. Many species do not fit entirely into one life-form type, but share characteristics of two. Because life-form attributes reflect habitat and feeding preferences, life-form morphology may be used as a tool to determine feeding habitats of grasshoppers from given areas.

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BEHAVIOR

Grasshoppers, and Orthoptera in general, are well known for their behavior, particularly the sounds that they produce. Sound production and display flights in grasshoppers are behaviors associated with mating and territoriality. Grasshoppers do not sing or chirp like crickets and katydids because they do not have a file and scraper mechanism on their tegmina. Grasshoppers produce sound by two mechanisms called stridulation and crepitation. Stridulation occurs when a grasshopper rubs the insides of the hind femora against the abdomen or tegmina to produce a scraping or chirping sound; most grasshoppers produce sound by this method. The loud sounds produced by Syrbula and Bootettix are good examples of stridulation. Crepitation is sound produced by rapidly flexing the hind wings in flight producing crackling and snapping sounds. Crepitation is limited to the band-winged (Oedipodinae) and a few slant-faced (Gomphocerinae) grasshoppers. The crackling sounds produced by Arphia and Trimerotropis in flight are good examples of crepitation. Circotettix rabula Rehn and Hebard is one of the loudest crepitating grasshoppers in New Mexico, producing loud snapping and rattling sounds. Stridulation and crepitation sounds differ among different species, and like bird calls, can be used to identify different species of grasshoppers.

Daily grasshopper activities are generally similar, but differ somewhat among different life forms in different habitats and center around thermoregulation, feeding, and mating. Most grasshoppers are active during daylight hours, and activity depends upon the amount of sunlight and temperature. Grasshoppers generally become active in the early morning and seek sunlight to increase their body temperatures. By late morning or midday they move about actively, feeding and engaging in mating activities. On hot days, activity usually decreases from mid- to late afternoon, and grasshoppers often seek shade or perch on plants to avoid excess heat. It is likely that digestion occurs during this time. At dusk, grasshoppers generally seek resting places for the night, usually on vegetation.

In warm areas or at warm times of the year, many species of grasshoppers are active at night. Some species such as Bootettix will even continue to stridulate after dark. On warm summer nights one can find grasshoppers wandering around on vegetation, often feeding. Few species fly at night, but some do. In southern New Mexico, Trimerotropis pallidipennis is commonly seen flying around lights at night.

Most grasshoppers probably do not move great distances from where they hatched. Dispersal is common and accounts for most grasshopper movement over time. Grasshoppers are generally solitary except for mating behavior and move about independently. Rarely do grasshoppers actually become gregarious and migrate; however, swarming and migration occur in some species when population densities are particularly high. Migration is well known in species such as the desert locust (Schistocerca gregaria) and the migratory locust (Locusta migratoria) of Asia and Africa. In North America the only species of grasshopper that truly swarm and migrate are Melanoplus sanguinipes and rarely, Dissosteira longipennis (Thomas). These migration events occur only under unusually high population densities (Parker et al. 1955, Wakeland 1958).

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POPULATION DYNAMICS

Grasshoppers are well known for their variable population densities, and it is the occasional large increases in population densities that concern most people with agricultural interests. It is important to understand that not all species of grasshoppers have widely fluctuating population densities. Although all grasshopper populations fluctuate in density over time, some fluctuate much more and reach higher overall densities than others (Pfadt 1977). Melanoplus sanguinipes, several other species of Melanoplus, Camnula pellucida (Scudder), Hesperotettix viridis, and Trimerotropis pallidipennis are New Mexico species that tend to have highly variable population densities from year to year and occasionally reach high densities in some areas. Most other species tend to remain at relatively lower densities, even though their populations may fluctuate considerably.

Grasshopper population dynamics, habitat specificity, and dispersal characteristics are interrelated, as they are for other animals and plants. Some organisms such as weedy ephemeral plants have high potential rates of population increase, good dispersal capabilities, and tend to occur over broad geographic ranges in a variety of habitats. Other organisms, such as many rare plant and animal species, have low rates of potential population increase, poor dispersal capabilities, and tend to be limited to certain habitats. Most organisms have characteristics somewhere between these two extremes, but may tend toward one or the other. Grasshoppers range in population characteristics from "weedy" species to "rare" species. The weedy species tend to be agricultural pests and exhibit population outbreaks. These species tend to have wide geographic distributions, live in a variety of habitats, and feed on many types of food plants. Melanoplus sanguinipes, M. packardii, M. occidentalis (Thomas), and Trimerotropis pallidipennis are examples of such weedy species in New Mexico. Many common rangeland species such as Aulocara elliotti, Ageneotettix deorum, Cordillacris occipitalis (Thomas), and Psoloessa texana are often abundant but tend to have fairly specific habitat requirements and relatively more stable populations than the "weedy" species. Most New Mexico grasshoppers species are uncommon, with limited geographic distributions and habitat requirements.

Disturbances of natural habitats usually result in changes in grasshopper species composition and thus the general population dynamics of the grasshoppers inhabiting the area. Disturbances that alter grasshopper assemblages include fire (Evans 1984, 1988), livestock grazing (Pfadt 1982, Joern 1982), and human-caused perturbations such as landscape modifications for agriculture and construction (Scoggan and Brusven 1973, Lightfoot 1986). Weedy or early colonizing grasshopper species tend to disperse into disturbed areas, replacing species that were present prior to disturbance. Disturbed sites usually have lower species diversity and are dominated by abundant colonizer species. As discussed above, Melanoplus sanguinipes is a weedy or colonizer species. Habitat disturbances such as over-grazing of rangelands are linked to increased population densities of pest species such as Melanoplus sanguinipes; Pfadt (1982) found that Melanoplus sanguinipes dominated other grasshoppers in heavily grazed areas, and densities of this species were highest in the most heavily grazed areas.

Factors affecting grasshopper population dynamics may be divided into abiotic and biotic factors. Abiotic factors appear to be the most important in regulating annual and seasonal population sizes (Dempster 1963, Edwards 1960, Hewitt 1979). Temperature and moisture directly affect egg survival, development, and hatching and the subsequent survival and development of nymphs. Temperature and moisture have been used fairly successfully as principal driving variables for grasshopper population simulation models (Rodell 1977, Gryllenberg 1974). Egg hatching is determined largely by accumulated degree days and moisture conditions (Hewlett 1979, Mukerji and Gage 1978). Once eggs have hatched, the small developing nymphs require adequate green, nutritious, plant material for food and warm temperatures to maintain foraging activity and digestion. If it is too dry, food quality will be low, and if it is too wet and cool, nymphs will not feed and starve. The best conditions for typical summer grasshopper species in New Mexico appear to be a mild autumn for egg laying, a cool wet winter for good spring plant growth, and a dry, warm spring for grasshopper nymphal development.

Populations subject to reaching very high densities, such as those of Melanoplus sanguinipes, usually do so over a period of several years. In a given area, densities of grasshoppers tend to increase over 2-5 years before peaking in an "outbreak" year (Pfadt 1977). Whether or not an increasing cycle continues, or how high the population densities finally peak, depends largely on weather conditions. If favorable conditions prevail, then populations may continue to increase to high densities. If weather conditions are poor during one or more years, the increase may be delayed or arrested.

Biotic factors that affect grasshopper populations include predators and parasites, though the importance of natural enemies in regulating grasshopper populations is generally less than the effects of weather. Birds, robber flies (Asilidae), and hunting wasps (Sphecidae) are probably the most significant predators of grasshoppers in New Mexico. Numerous animals feed on grasshopper eggs in the soil, and these egg predators and parasites may have a significant impact on grasshopper populations. Important egg predators include rodents, bee fly larvae (Bombyliidae), blister beetle larvae (Meloidae), and ground beetles (Carabidae).

Important grasshopper parasites include larvae of various flies (Sarcophagidae), a protozoan (Nosema locustae Canning), and a fungus (Entomophthora grylli Faes). These parasites appear to function in a grasshopper density-dependent manner: the higher the grasshopper density, the greater the spread and infection rate of the protozoans and fungi. Both Nosema and Entomophthora are somewhat dependent upon weather conditions, being favored by wet weather. These protozoan and fungal consumers of grasshoppers offer some potential for grasshopper biocontrol agents, because they can reduce targeted grasshopper populations in some cases; they also can be propagated in the lab, formulated appropriately, and distributed on rangelands.

Grasshoppers of New Mexico Contents

DISTRIBUTION OF GRASSHOPPER SPECIES IN NEW MEXICO

New Mexico encompasses a variety of landscapes and habitats within several major biogeographic provinces. Lowe and Brown (1982) identified biogeographic provinces in the Southwest based largely on distributions of natural vegetation, but also including animal distributions. These biogeographic provinces tend to have distinctive plant and animal associations that are historically derived from different source regions and now represent complexes of natural communities existing under different climatic regimes. We have adapted Lowe and Brown's (1982) biogeographic provinces to a simplified version of the USDA-SCS Potential Natural Vegetation map of New Mexico (USDA 1978) to yield a map with boundaries for the biogeographic provinces within the state (Fig. 2). A physiographic map (Fig. 3) shows the locations of mountain ranges and river valleys in the state.

The grasshopper fauna of New Mexico exhibits large-scale geographic patterns relative to these biogeographic provinces. Examination of individual grasshopper species distribution maps in this text will reveal repeated species patterns occurring in particular parts of New Mexico. Also, some species are widespread throughout the state, while others have narrow geographic distributions. The four biogeographic provinces in Figure 2 tend to have fairly distinctive associated grasshopper faunas. We believe that it is useful to list some of the common grasshopper species that are typically found in each of these provinces, as an aide to familiarize readers with the common grasshopper species they may encounter in these biogeographically different parts of the state.

Great Basin Province. This region covers most of the low-elevation areas in the northwest and north-central portions of New Mexico. Typical habitats consist mostly of valleys, hills, and mesas vegetated with shrubs (sagebrush, saltbush, rabbitbrush), some grasses, and considerable open areas of bare claylike soils. Common grasshoppers of this region typical to the Great Basin to the west include: Trimerotropis sparsa (Thomas), T. gracilis (Thomas), T. inconspicua Bruner, and Aeoloplides spp.

Chihuahuan Province. This region covers most of the southern and central portions of New Mexico. Habitats consist mostly of broad flat lowlands, valleys, hills, and mesas vegetated with grasses and shrubs including creosotebush, mesquite, tarbush, snakeweed, and saltbush. There is considerable open bare soil, as well as numerous sand dune and salt flat areas. Common grasshoppers include: Trimerotropis pallidipennis, T. californica Bruner, Derotmema laticinctum Scudder, Cibolacris parviceps, Bootettix argentatus, Conozoa texana (Bruner), and Melanoplus aridus (Scudder).

Plains Grasslands Province. This region covers most of the eastern half of New Mexico. Habitats consist mostly of open, flat expanses of grassland (primarily grama and buffalo grasses). The southern portion of the Plains Grasslands includes large expanses of shinnery oak sand dunes. Common grasshoppers of the Plains Grasslands include: Hadrotettix trifasciatus (Say), Trimerotropis melanoptera McNeill, Aulocara elliotti, Ageneotettix deorum, Cordillacris crenulata (Bruner), Melanoplus occidentalis, M. sanguinipes, M. lakinus (Scudder), and Arphia pseudonietana (Thomas).

Forest and Woodland Province. These regions cover higher elevations throughout New Mexico and include a variety of tree associations from pinyon-juniper at lower elevations to high-altitude mixed conifers. Open grassy meadows and rock outcroppings are important habitats for many species of grasshoppers within the forests and woodlands.

The forests and woodlands of New Mexico have two different historical source areas, which are reflected in present-day species compositions. The forests of the southwestern quarter of New Mexico are derived from the Sierra Madre region in northwestern Mexico and are equivalent to the madrean and interior Arizonan provinces of Lowe and Brown (1982). The forests of northern and central New Mexico (including the Sacramento Mountains) are derived from the Rocky Mountain flora to the north and are equivalent to the petran province of Lowe and Brown (1982). These differences in source areas are also reflected in the grasshopper fauna associated with these forests.

The Sierra Madrean forests and woodlands of southwestern New Mexico are dominated by oaks and pines at lower elevations, and pines and other conifers at higher elevations. Common grasshoppers include: Trimerotropis modesta Bruner, T. cyaneipennis Bruner, Barytettix humphreysii (Thomas), and Heliastus benjamini Caudell.

The Rocky Mountain forests and woodlands are dominated by juniper and pine at low elevations, and mixed conifers at high elevations. Common grasshoppers include: Circotettix rabula, Trimerotropis cincta (Thomas), and T. fratercula McNeill.



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