Management of Grasshopper Populations
- History of Grasshopper Problems of New Mexico
- Biological and Economic Basis For Suppression of Grasshoppers
- Estimation of Grasshopper Densities
- Insecticidal Control of Grasshoppers
- Biological Control of Grasshoppers
- Cooperative Grasshopper Control Programs
Since antiquity, grasshoppers have been recognized as sporadically severe pests of crops and grazing areas. Over the centuries man has used many tactics to save plants and minimize subsequent economic, food resource, and soil erosion losses caused by these insects. Approaches to grasshopper management and degrees of success have varied considerably through time and across cultures. Here we will address only those aspects of grasshopper problems and control methodology pertinent to New Mexico or the Southwest.
History of Grasshopper Problems of New Mexico
In North America, grasshoppers were recognized as pests by certain Indian tribes. The Navajo of the desert Southwest accurately observed that grasshopper outbreaks were often associated with drought and that female grasshoppers laid their eggs in the soil (Wyman and Bailey, 1964).
While songs, prayers, and mutilation of ceremonial grasshoppers were purported to drive the pests from the Indians' corn and vegetable fields, the Navajo also used certain plant concoctions to combat grasshoppers (Wyman and Bailey, 1952). Extracts of Oxytropis lambertii *Pursh, purple locoweed, and *Phlox stansburyi (Torr.) Heller, a flowering perennial, were sprinkled on corn to protect the crop from grasshoppers. No data are available as to the efficacy of either preparation.
Perhaps because of the destructiveness of these insects, Navajo folklore credited grasshoppers as disguises for ghosts (Wyman et al., 1942), tools of retribution for witches (Kluckhohn, 1944), or causes for human maladies such as nosebleeds in children (Wyman and Bailey, 1964). While some Plains Indian tribes gathered grasshoppers for human food during famines, the Navajo seem to have considered grasshoppers with suspicion; they even regarded com and beans tainted with grasshopper frass and secretions as poisonous (Newcomb, 1940).
Among non-Indian cultures and communities established in New Mexico since the 1500s, subsistence farming was common throughout the state. Drought and grasshoppers were undoubtedly factors in crop production, livestock grazing, establishment and growth of human communities, hunting ranges, and local economies in this state just as they were in other parts of the West (Briggs, 1934). However, written records for grasshopper problems in New Mexico prior to 1850 are mostly anecdotal and even those are difficult to locate.
The comments of Jacob Fowler (with original spellings; Coues, ed., 1898) in his journal for 1821-1822 are probably representative of grasshoppers' impact on early residents and travelers in what we now know as New Mexico: "We here found the people extremely poor, and bread stuff could not be head amongst them as the said the grass hopers head eat up all their grain for the last two years ... We found them equally scarce of meet ... We must soon leave this reached place ...."
In 1868 flying swarms of the mysterious Melanoplus spretus (Uhler) got the attention of New Mexico residents and certain visiting scientists (Scudder and Cockerell, 1904). These and other M. spretus outbreaks in the Rockies during the late 1860s resulted in the establishment of the U.S. Entomological Commission in 1877 to investigate the problem (Capinera and Sechrist, 1982b). In his report to this commission, Packard stated the pest "seemed to have extended [its range] farther south than [in] any year before or since." As ex-Governor Arny informed the Commission, the grasshoppers flew at least 140 miles south of Santa Fe; this would be in the vicinity of Ft. Craig on the Rio Grande in Socorro County. This large flight of spretus so far south and at such low elevations apparently was unusual; collection records from early grasshopper specialists traveling in northern New Mexico suggest that spretus was found in Taos, Rio Arriba, Colfax, Santa Fe, and parts of Bernalillo and Valencia Counties from at least 1865 through 1877. Whether or not these southern populations died out after 1877 is a mystery. Cockerell noted that nothing had been seen of the species in New Mexico from 1893 through at least 1902 (Scudder and Cockerell, 1904); indeed, Melanoplus spretus seems to have become extinct in North America since its great period of destruction and mass movements from 1865 to 1885. It has not been collected again by entomologists working in any of the western states in nearly a century. Whether the species actually became extinct or whether it is a polymorphic form of another species, perhaps M. sanguinipes, remains to be seen (Brooks, 1958; Gurney and Brooks, 1959). Lockwood and DeBrey (1990) have postulated that this species may have become extinct due to localized destruction of the insect's critical habitat and the introduction of exotic species through agriculture and economic development of the West.
In the 20th century, the first documented grasshopper plagues in the new state of New Mexico occurred in 1927. A staff writer with the New Mexico Agricultural College noted that a mechanical hopper dozer was used to collect the pests from alfalfa near Socorro (Anon., 1927). Results were not reported.
By the early 1930s, numbers of pest grasshoppers were increasing steadily during a prolonged drought in several northern counties of the state (Anon., 1932; Anon., 1934a); counts in San Juan County ranged from 374/square yard near Blanco to as many as 1136/square yard at La Plata (Anon., 1934b). Although the New Mexico legislature appropriated $2000 for grasshopper control in 1933, the amount was inadequate to control the devastated, drought-stricken state (Anon., 1934b). The sheer enormity of the infested area in New Mexico and adjacent states as well as the relatively high cost of bait and its application stymied early state and local efforts to deal with grasshopper problems.
In early 1934, Congress made an appropriation of over $2.3 million to assist in national grasshopper control efforts. By August of that year, New Mexico qualified to receive 460 tons of sodium arsenate-bran bait, which was promptly dispensed and apparently helped abate the problem (Anon., 1934b). In 1936, the U.S. Department of Agriculture approved a project for the study of rangeland grasshoppers, designating Fred Morton as leader and basing him at the Grasshopper Laboratory of the Bureau of Entomology and Plant Quarantine, Bozeman, MT (Pfadt and Hardy, 1987). The program marked the beginning of many cooperative research and regulatory efforts by the federal government and various state agencies.
After World War II, arsenical baits were largely replaced by various chlorinated hydrocarbon insecticides. Aldrin was recommended for rangeland treatments (Moore, 1951), while toxaphene was promoted for protection of forage crops such as alfalfa (Anon., 1949). Further, these newer materials were formulated for application as liquids.
By the early 1950s, another outbreak of grasshoppers was building primarily in northern New Mexico (Moore, 1951). By this time, sodium arsenate bait had been replaced by aldrin, chlordane, and toxaphene. Recommendations were made to treat grasshopper hatching sites, field margins, and other weedy, infested areas before grasshopper nymphs matured and dispersed by flight. Cooperative spraying by all growers in infested areas was promoted as a means to control developing pest problems more effectively.
By the mid-1950s, the USDA's Bureau of Entomology and Plant Quarantine, the predecessor of the Animal and Plant Health Inspection Service (APHIS), and cooperating state agencies began extensive systematic surveys of grasshopper populations throughout New Mexico and other western states with substantial areas of rangeland. Areas with exceptionally large populations of grasshoppers were targeted for chemical control. Lee Seaton, a USDA entomologist in New Mexico from 1955 to 1964, began making lists of grasshopper identifications for all counties in the state, an effort continued today by APHIS and its cooperators, the Cooperative Extension Service at New Mexico State University and the New Mexico Department of Agriculture.
In 1967 New Mexico grasshopper populations exploded again. Over 5.5 million acres supported 8 or more grasshoppers per square yard (Durkin, 1967). An additional 8-10 million acres were infested with 3-7 grasshoppers per square yard. Nineteen of the state's 33 counties had severely infested areas (Durkin, 1967). While some of the northern counties of the state were heavily infested, some of the largest areas devastated by the pests occurred further south in Torrance and Lincoln counties. Over 600,000 acres in Lincoln and Lea Counties were sprayed in cooperative programs during that year.
Since 1967, statewide grasshopper surveys have shown dramatic fluctuations in the numbers of grasshoppers in different parts of the state. Areas categorized as heavily infested (8 grasshoppers/square yard or more) varied widely in size and location. From 1967 through 1989, 10 cooperative treatment programs have been conducted in various but not necessarily contiguous parts of heavily infested areas. A high of approximately 1.5 million acres was treated in cooperative programs organized in 10 counties in 1979. No cooperative grasshopper control programs were conducted anywhere in New Mexico from 1986 to 1988.
Biological and Economic Basis For Suppression of Grasshoppers
All stages of nymphs and adult grasshoppers have chewing mouth parts and digestive systems adapted to processing large amounts of forage. Grasshoppers are predominately herbivores, although they will consume other arthropods (even other grasshoppers) under some circumstances.
Certain species may eat nearly one-half of their body weight in green forage daily (Capinera and Sechrist, 1982b). However, while foraging, grasshoppers may clip and make at least that much plant matter unavailable to other herbivores. Laboratory estimates for reduction (mg/day) by adults of selected rangeland species range from 14.2 for Ageneotettix deorum to 143 for Aulocara elliotti *and 150 for *Melanoplus foedus Scudder (Hewitt, 1977). After an extensive review of research in the U.S. and Canada, Hewitt and Onsager (1982) estimated the seasonal loss of (range) forage to be about 12 pounds of air dried forage per acre per grasshopper annually. If the average density is eight grasshoppers per square yard over a 10-acre area, these pests can consume and clip enough forage to feed a cow for one year (Hewitt, 1977). Under most circumstances USDA suggests that treatment of stockable rangeland is justified when numbers of adult grasshoppers or late instar nymphs reach nine per square yard (Anon., 1987b).
Identification of grasshopper species is critical for making pest-control decisions, especially since some species are actually beneficial (feeding on noxious weeds) and many others are economically neutral. Similarly, the species of plants at risk to grasshoppers, the condition of the range, and the potential for forage production later in the growing season are also important factors to consider before making a decision to commit money and resources to grasshopper control.
Herbivores, including grasshoppers, have an impact on the energetics of ecosystems, especially in regard to recycling nutrients through the system (Chew, 1974; Kitchell et al., 1979; Petrusewicz and Gordzinski, 1975; Lightfoot and Whitford, 1990). While the clipping and consumption of forage mentioned above is considered damaging to livestock grazing, on the other hand, it can be considered as a means of returning nutrients to the soil, providing food for insectivorous species, and even promoting some plant growth.
The consequences of grasshopper damage to plants vary considerably across modern social and economic lines. For urban and suburban residents, grasshoppers can be numerous, annoying, essentially uncontrollable pests that reduce the aesthetic appeal of flower gardens and other ornamentals. In particularly bad years, grasshoppers may destroy home vegetable or flower gardens or make plant protection so difficult, expensive, and time consuming as to discourage gardening as a hobby. In their vendettas against grasshoppers, gardeners may misidentify grasshopper problems, use inappropriate grasshopper control materials or ineffective techniques at inappropriate times, or be frustrated by neighbors who do little or nothing to combat the problem.
However, in nearly all of these cases, urban and suburban gardeners are hobbyists who obtain their food, fiber, and income from other sources. For farmers and ranchers who depend almost totally on their land's productivity to stay in business, grasshoppers can be as economically devastating today as they ever were. However, control options considered by the farmer or rancher today are highly dependent on the balance between costs of the various options and the return on that investment.
To a very limited extent, farmers can reduce crop risk to grasshoppers by controlling weeds in and around their fields. They may be able to treat nearby breeding or hatching areas with insecticides or to create temporary insecticidal barriers around fields to combat crawling young grasshoppers. Even then, the success of such a program may vary widely from one year to the next or one season to the next depending upon pest species composition, time of year, pest density, location, weather patterns, and other situations. Frequently, once pest grasshoppers become winged adults, farmers are often not able to kill the pests either efficiently enough or fast enough to save their crops.
In many respects, ranchers today have fewer effective and economical options that they can exercise for grasshopper management. The relatively low productivity and dollar value of western rangeland and the comparatively large acreage needed to support an animal are key factors for consideration in the management decision making process. When forage production is low because of drought or overgrazing, small numbers of grasshoppers per grazing unit can be intolerable. Further, if livestock prices are also low, the rancher often is not able to afford pest treatments and suffers additional direct and indirect consequences as a result. At the least, the rancher may have to gather and move livestock to new pastures or provide supplemental feed. These actions probably will result in reduced weight gains by the affected livestock from stress and/or simple lack of food; supplemental feeding may severely deplete winter food stores while also costing the rancher additional money to distribute the feed.
In more severe situations, ranchers may unintentionally overgraze their pastures while hoping that range conditions will change; In the process, they may exhaust winter feed resources. Livestock may have to be gathered and sold quickly, often at substantial losses. Further, heavily damaged pastures are often subject to soil erosion or invasion and establishment of undesirable weeds and increases in "weedy" grasshopper populations.
If these problems are not substantial enough, ranchers who consider applying insecticides for grasshopper control cannot be assured that this option will provide the immediate relief that they and their livestock need; further, there is no guarantee that this relief, if realized, will last longer than the year of application.
In recent years, range scientists have attempted to make the decision-making process more quantitative and more reliable for ranchers and other land managers. Grasshopper damage potential, mortality rates, chemical control effectiveness, forage production, control costs, grasshopper identification, and specific consumption rates are all parts of the modern formulae (White, 1974; Hewitt et al., 1974; Torell et al., 1987). With some of these modem formulae, what initially may appear to be a grasshopper plague based on simple density estimates may not be economically or ecologically feasible or justifiable to treat (Torell et al., 1987).
Estimation of Grasshopper Densities
Ranchers, pest management consultants, and specialists with various federal and state agencies survey millions of acres of rangeland annually for grasshoppers. Because of the vast areas covered by these surveys and the relatively low productivity offered by rangeland, accurate yet efficient sampling methods are essential, yet seldom implemented. Adult grasshoppers and larger nymphs are commonly used for density estimates and prognoses of future damage, especially to rangeland. Numbers or sizes of egg pods are rarely used for this purpose because sampling this underground stage is difficult, time-consuming, and requires special equipment; also, the taxonomy of grasshopper egg masses would have to be determined.
The ideal sampling method would be inexpensive and quick, offering the observer accurate estimates of both nymphs and adults of whatever grasshopper species happened to be in the vicinity. Unfortunately, researchers interested in developing such a method run into a variety of obstacles. Collection records such as those presented in this manual indicate that numerous species of grasshoppers are likely to occur in any given part of the state. Field observations of various species indicate considerable differences in mobility and wariness, both among species and development stages. Prevailing weather conditions have been previously mentioned as sources of error for grasshopper counts. Foliage type, density, and height are also confounding factors (see Southwood, 1978 for a review). Finally, there can be considerable variability among human observers with respect to following instructions, efficiency, and accuracy of observations.
Thus, no ideal sampling technique has been developed. Further, it appears that ease of use and time necessary to do the work are inversely related to accuracy and precision of the population estimate (Thompson, 1987).
Grasshopper densities are generally presented as numbers of all species (lumped) per square yard (most common estimate), per square meter, or per square foot. The technique most commonly used for these estimates requires the surveyor to count the number of grasshoppers fleeing a visually estimated one-square-foot area approximately 10-15 feet ahead (USDA, undated).
The surveyor repeats this procedure 18 times per sample location, adds the 18 observations, and divides by two to determine the numbers of grasshoppers per square yard. Although this technique remains in common use, its inaccuracy and inconsistency has been well documented (Onsager, 1977).
More accurate density estimates can be obtained by placing square-foot or square-yard hoops on rangeland prior to conducting the actual counts (Richards and Waloff, 1954, Onsager and Henry, 1977). Unfortunately, the price of accuracy requires at least twice as much travel, time, and labor to produce data because study sites must be visited twice: once to set the devices and again to pick them up and collect data; consequently, this procedure is used infrequently and primarily for research applications. Techniques developed for special research projects requiring even more accurate estimation of grasshopper densities include quick traps (Turnbull and Nichols, 1966), night cages (Anderson and Wright, 1952), drop cages (Hills, 1933), and net samplers (Smalley, 1960).
Various methods have been attempted to determine grasshopper species composition. If density and population diversity are relatively low, the field surveyor who uses the 18-station walking count method may be able to identify some key species as they jump away (Joern and Pruess, 1986; Pfadt, 1982, 1984). Sweep-net sampling may be the most widely used alternative for this purpose. Its advantage is that specimens can be preserved for identification under more controlled conditions or for further study by experts; the disadvantage is that some species may be abundant at a site but are rarely, if ever, captured, in the sweep net. Capinera and Sechrist (1982a) described a "flush-capture" technique that would improve on the sweep-net technique because variance would be controlled. However, like the more accurate estimators of density, the "flush-capture" technique has logistical drawbacks.
Because operating resources are limited in New Mexico, standard sampling procedures for grasshoppers have employed the 18-station walking technique described above (USDA, undated) with subsequent plotting of survey results (grasshoppers/square yard) on detailed county or regional maps. Estimates of grasshopper densities in New Mexico are routinely made after most economic species have hatched (early summer) and again after most of the same species are adults (early fall). This is a cooperative survey among personnel of the U.S. Department of Agriculture and the New Mexico Department of Agriculture.
For the nymph survey in particular, initial survey locations may be 5-10 or more miles apart, depending upon the terrain, purposes of the survey, grasshopper densities, and signs of damage by these pests. Results of the nymph survey, grasshopper damage estimates, grasshopper identifications, development stages, range conditions, and current and long-range weather forecasts are all considered when a grasshopper control program may be initiated in the near future. Time is usually of the essence; if nymphal surveys suggest the possibility of a control program, survey personnel from state and federal agencies remain in close contact with each other and with affected landowners. Field experiments (Onsager, 1978) and modeling trials (Hardman and Mukerji, 1982; Onsager, 1984) strongly suggest that treatments provide maximum prevention of forage destruction if applied when most grasshoppers in an infestation are in the 3rd or 4th nymphal instar, relatively early in the year.
Results of the adult survey are used to predict areas where economically significant populations (8 or more grasshoppers per square yard) may produce high numbers of nymphs the following year. Adult grasshoppers are rarely considered for treatment programs. Not only have they already done their maximum damage to range, but also they probably have already oviposited and are senescent. An annual pictorial summary of results color coded for sub economic (3-7 grasshoppers/square yard) and economic populations (8 or more grasshoppers/ square yard) is compiled each winter with data from all cooperators by USDA state offices. Copies of these maps are usually available in early spring through the NMSU Cooperative Extension Service, the New Mexico Department of Agriculture, or the U.S. Department of Agriculture. Surveyors can consult these annual maps to make any necessary adjustments in their plans for nymphal surveys the following spring.
"Sentinel survey sites" have also been established to track population dynamics of grasshoppers in selected sites in New Mexico. In 1985, 91 of these sites were permanently established by USDA-APHIS-PPQ in parts of counties known to have persistent problems with large numbers of economically significant grasshoppers; this effort was part of a larger one for western and midwestern states to develop some long-term, detailed observations of grasshopper species interactions and developmental dynamics. Not only are counts of the various species or types of grasshoppers taken during each of these site visits, but also specimens for identification. These sites are visited four times during the season, with first visits occurring very early in the spring in the more southern locations with lower elevations, and later in the year for those that are farther north or higher. Three visits primarily involve nymphal grasshoppers, while the fourth is timed for assessing the damage potential of adult grasshoppers.
Insecticidal Control of Grasshoppers
Synthetic insecticidal baits or foliar treatments have been used for grasshopper control in the western U.S. since the early 1900s. Insecticide ingredients and formulations used for grasshoppers have necessarily changed over time, reflecting changes in availability, economics, application methods, general safety, efficacy, environmental concerns, and survey methodology.
In Colorado and elsewhere in the western U.S., baits were the most widely used formulations from about 1913 until the late 1940s (Capinera and Sechrist, 1982b). An inexpensive agricultural byproduct such as wheat bran, rolled barley, or fruit pomace was used as the food attractant and carrier. Arsenical insecticides were used as toxicants through the 1930s and early 1940s. To reduce costs, cheap dry diluents such as corncob grits or sawdust were milled with the toxicant and carrier. Small amounts of water or an oil helped bind the toxicant to the dry mixture; addition of a small amount of molasses to the bait usually increased acceptance by the grasshoppers. More recently carbaryl has been used as the toxicant in baits, but the formulation process remains much the same.
Baits have been applied by various kinds of ground rigs and, since the 1970s, by airplane. Because application rates are usually 1-1.5 pounds of formulation per acre, large-scale control programs present logistical problems in obtaining the required volume of formulated material, transporting it to the treatment site, storage, and application. Other disadvantages in using baits include: they may not be equally attractive to all grasshopper species in a local pest complex; some individuals do not find the bait or consume only a sub-lethal dose; and some individuals may be molting instead of eating when the active ingredient in the bait is most effective (Onsager et al. 1980). Grasshoppers vary significantly not only in bait acceptance but also in susceptibility to commonly used insecticides (McDonald, 1967). On the other hand, because baits are relatively selective for grasshoppers, they minimize the hazard to various natural enemies of grasshoppers and non-target organisms. Thus, baits may be a preferred formulation for environmentally sensitive areas such as roadsides or urban lots. Mukerji et al. (1981) reported that less active ingredient was needed to provide a satisfactory level of grasshopper control with bait versus a liquid formulation.
Use of liquid insecticide formulations began in the late 1940s with chlorinated hydrocarbons such as aldrin and toxaphene. Although both materials were effective and economical, and their residuals were comparatively long lasting, concern over their appearance in the food chain caused them to be replaced with short-term residual materials such as malathion and acephate (both organophosphates) and carbaryl (a carbamate).
Malathion is most commonly used undiluted and at very low rates -- a specific formulation and application technique known as "ULV" or "ultra-low volume." It usually is the least expensive insecticide labeled for grasshoppers on a variety of crops and rangeland. Malathion is especially effective on grasshoppers at temperatures above 90oF when conditions are dry (Onsager, 1978).
Carbaryl has been formulated as a bait (described above) and as a liquid applied in a mixture of oil and water. The oil enhances toxicant adhesion to foliage, while the water aids in product dilution. This toxicant has proven more effective on grasshoppers when weather conditions were cooler or wetter than recommended on the malathion label (Onsager, 1978). Depending upon the study, effective carbaryl residues have persisted as long as 21 days post treatment (Lloyd et al., 1974, Onsager, 1978), although 15 days is considered normal (Anon., 1987b).
Acephate has demonstrated good residual control of grasshoppers (about 7 days) as well as ease of mixing and aerial application (Onsager and Mazuranich, 1979). As with other insecticides for grasshopper control, price and availability at time of need determine the volume and frequency of this material's use in large-scale range treatment programs. The acephate label describes restrictions on product use where lactating dairy cattle are present.
As a consequence of large-scale grasshopper treatment programs and numerous field trials, the pros and cons of insecticidal control for these range pests have been determined. Quick and efficient surveys for potential grasshopper problems appear essential because net returns from control programs are greatest if insecticides are used while insects are relatively young nymphs (Onsager, 1978). In addition to saving more forage by controlling insects at an earlier stage, land managers also realize the benefits of treating grasshoppers at a life stage when they are more susceptible to insecticides. Indeed, by delaying treatment of grasshopper populations until the insects are larger, more insecticide-tolerant, post-reproductive adults, land managers actually lose money on control programs.
Theoretically, another possible advantage of range treatment programs is that grasshopper populations may be suppressed for several years; hence, a comparatively high initial cost could be amortized over time. However, field observations suggest that not all treatment programs provide long-term freedom from damaging grasshopper populations. Blickenstaff et al. (1974) listed several reasons why long-term suppression may not be accomplished: 1) grasshoppers from untreated borders may re-enter treated areas; 2) surviving grasshoppers (e.g. where infested lands are skipped by the spray planes or rigs) may undergo explosive population growth; or 3) existence of diapause eggs (i.e. underground egg pods wherein growth and development are temporarily suspended pending an environmental cue such as a minimum time spent at or below a critical temperature). A contributing factor for (2) above may be the consequent loss in natural grasshopper enemies along with the grasshoppers. If their food source is dramatically depleted, grasshopper predators and parasites must either move or die, leaving the few surviving grasshoppers or invading grasshoppers with few natural curbs to a tremendous reproductive potential. Pfadt (1977) noticed the same problems in his follow-up studies of control programs in Wyoming; long-term (5 or 6 years) control could be observed after some programs but not after others.
Biological Control of Grasshoppers
Fortunately, a number of factors can cause grasshopper populations to collapse from one season to the next or from one year to the next. Summer drought can leave the insects without adequate food so that they die or leave the area; late-season moisture may revive forage and promote its growth over that of the relatively few surviving grasshoppers. Spring rains may give forage a head start over the grasshoppers so that there is (at least temporarily) adequate food for both grasshoppers and grazing animals. Natural enemies may also significantly reduce grasshopper numbers in an area; however, if forage is available, the area may be repopulated quickly by local movements of crawling nymphs or by flights of adult grasshoppers. Thus, both current and historical survey and weather data are useful in evaluating the destructive potential of grasshopper in various parts of the state.
At least 200 species of insects, mites, and nematodes attack grasshoppers (Lavigne and Pfadt, 1966; Rees, 1973). Various species of flies and wasps parasitize grasshopper nymphs and eggs while other flies, beetles (including blister beetle larvae in the genus Epicauta), birds, and rodents are significant predators. Natural enemies probably do cause enough mortality in grasshopper populations to make pest outbreaks infrequent. In Wyoming, Lavigne and Pfadt (1966) estimated that 11-15% of the mortality in their study populations was caused by robber flies. Biological controls may slow rapid population increases under most circumstances; however, when favorable weather and other conditions occur, grasshopper populations seem to overwhelm most of their natural enemies and quickly reach damaging epidemic levels.
Of the diseases that occur naturally in North American grasshoppers, the most common is a fungus, Entomophthora grylli. Prior to death, affected grasshoppers characteristically climb to the tops of stems or twigs, stiffen, bloat, and die. The dead grasshoppers remain clasping the stalks (MacLeod et al., 1980). Although this disease may occasionally cause certain grasshopper species to decline dramatically, extended periods of high humidity are usually required for the disease to occur (Pickford and Reigert, 1964)
The only biological control agent for grasshoppers researched for commercial production and manipulation in the environment is Nosema locustae Canning, a naturally occurring protozoan disease that affects several species of damaging range grasshoppers.
In a natural situation, the infection level for Nosema disease is usually less than 10% and is often less than 1% (Capinera and Sechrist, 1982b). Henry (1972) found that approximately 5% of grasshoppers collected in natural conditions where Nosema *occurred were infected; further, different species varied in susceptibility. Usually grasshoppers must ingest *Nosema spores, although spores are occasionally transmitted in grasshopper eggs to approximately 5% of the offspring of infected females (Anon., 1987b). Nosema is most effectively transmitted when grasshopper densities are 15-22/square yard, well above the threshold at which treatment programs are typically considered. Once grasshopper density declines to about 2/square yard, Nosema is no longer effective (Anon., 1987b).
Henry and Oma (1981) found that fat body, neural, and other tissues are damaged by Nosema. Development rates, activity levels, and fecundity are usually reduced, while cannibalism often increases. If large numbers of spores are ingested relatively early in grasshopper development, the deleterious effects of the disease may be more pronounced, although death may occur weeks later. In one optimal scenario, application of 1.12-1.68 kg of wheat bran per hectare containing 1.6 x 109 to 2.3 x 109 spores, distributed while Melanoplus sanguinipes is predominantly in instar 3, may produce 50-60% mortality in 4-6 weeks; survivors could have infection levels of 35-40% (Henry and Oma, 1981). Thus, applied by itself, Nosema does not provide short-term control of grasshoppers. Promoters of the product claim that long-term suppression could be realized (Wright, 1985; Onsager, 1987); however, performance of the product in a variety of large-scale field tests since 1975 has not been consistent, well defined, nor easily quantified (Onsager, 1987).
Henry and Onsager (1982) concluded that Nosema could be potentially useful in an integrated pest management program for grasshoppers on rangeland. Wright (1985) states that because of its relatively high cost per acre, Nosema would be most useful where environmental concerns were of primary importance. Nosema has been commercially marketed as a rangeland pest control product since the mid-1980s. It was first incorporated into an APHIS grasshopper management program in 1986 after undergoing extensive research trials for over a decade. In 1986, approximately 25,000 acres in the western U.S. were treated with Nosema for grasshoppers.
Research continues on additional species of Nosema, including N. acridophagus Henry and N. *cuneatum *Henry, that appear to be more pathogenic than N. locustae (Capinera and Sechrist, 1982b). Development of these products as biological insecticides has been slowed by problems with host insect culture; these other Nosema species apparently kill their hosts so rapidly that few spores can be harvested. The discovery that corn earworm, Helicoverpa zea (Boddie), could produce relatively large numbers of spores without killing the caterpillars suggests a possible new route for research and potential commercial development (Henry et al., 1979).
Cooperative Grasshopper Control Programs
While various agencies in the western states had attempted to organize grasshopper control campaigns since the turn of the century, the federal government became acutely aware of the magnitude of rangeland grasshopper problems in the 1930s. Grasshopper suppression over very large areas would require synchronized and cooperative efforts from land managers; the individual rancher treating pest problems alone would have little or no impact on pest populations even if the control program were exceptionally thorough.
In recent years, the USDA-APHIS and state departments of agriculture have agreed to conduct cooperative surveys and, if warranted, to implement grasshopper population control programs on both private and government-owned land.
Since 1986, APHIS' preferred alternative, an integrated pest management program (IPM) for grasshoppers on rangeland, has been developed to incorporate elements of an interagency IPM pilot project for long-range programmatic consideration. Elements of this rangeland IPM program are outlined in detail in the Environmental Impact Statement (EIS) (Anon., 1987b). Under the IPM alternative, malathion, carbaryl, acephate sprays, carbaryl bait, and Nosema locustae bait would be available for particular control projects; research would continue on other chemical and biological methods and on cultural/mechanical methods. Databases developed from survey results could be used to enhance outbreak prediction capabilities. As new methods become operational, APHIS would conduct environmental analyses tied to the current EIS for consideration in its program.
The EIS examines potential impacts on soils, vegetation, wildlife, water quality and aquatic systems, human health and worker safety, socioeconomics, historic and cultural resources, visual resources, and noise levels. Any potential adverse impacts would theoretically be avoided through adherence to operational procedures and mitigation measures provided in the document. To facilitate paperwork processing and to decrease response time in the event of a cooperative control program in a given county, a biological assessment is made of any endangered or threatened species of wildlife found within the county. Protective measures such as leaving an untreated buffer zone around the species habitat may be required to protect any species of concern. These documents may be reviewed and updated annually or as conditions require.
Widespread outbreaks of grasshoppers are usually preceded by several years of gradual population buildup associated with mild spring weather and a low incidence of disease, parasites, and predators. These conditions, plus late summer rains that provide adequate food for egg-laying females, usually result in an outbreak. Populations normally collapse in about a year when weather conditions are poor for egg-laying and grasshopper development, and also when parasites, predators, or disease levels have reached their maximum levels.
The adult survey conducted in late summer may not predict a grasshopper outbreak the following season, but can indicate local population trends. The nymph survey conducted in the spring is aimed at determining the need for a control program in a particular area. If an economic infestation exists in the spring, the surveyors use maps and other data to determine if the area meets other criteria required for initiating a cooperative control program.
For a full-fledged cooperative program involving federal and state regulatory agencies and area landowners, these criteria include: 1) blocks of rangeland must encompass at least 10,000 contiguous acres or an area must totally encompass an infestation; 2) no more than 20% total crop land (including fallow land) can occur within the 10,000 acre block; 3) at least 8 adult grasshoppers or an equivalent number of immature grasshoppers per square yard must infest the block; and 4) treatments must be applied before the majority of grasshoppers have become adults and started laying eggs. For crop lands to be treated, the crop involved must be listed on the insecticide label used in the grasshopper control program. The farmer pays 100% of the cost for treated affected crops.
Once such a program seems imminent, the provisions of the New Mexico state law "Grasshopper and Other Range Pest Control Act," 76-5-2 through 76-5-10 NMSA 1978 provide structure to the organization of the cooperative project, particularly where it involves interagency cooperation; organization and operation of local rancher committees; assessment, collection, and use of fees; and alternative actions when disaster areas are declared Federal and state officials as well as state and county Cooperative Extension Service personnel will attend rancher committee meetings to explain the results of grasshopper surveys and control options available for the area. In New Mexico, pending the availability of funds and approval of all agencies involved, the federal government will pay for all control costs associated with strictly federal land in a control district that includes Indian reservations and land managed by other federal agencies, such as the Bureau of Land Management, U.S. Forest Service, National Park Service, etc.; federal, state, and private landowners will each pay 1/3 of the control costs for private rangelands infested with grasshoppers. For state trust lands included in the control district, a different cost-share formula may apply depending upon land use. Where ranchers are involved in the cost-share formula, cost assessments are made for each landowner proposing to be in the program; collected monies are placed in an account to pay program costs. Delinquent accounts are referred to the appropriate local district attorney for collection and enforcement. If the grasshopper population declines naturally before treatments begin, APHIS and state officials will call off the treatment program; otherwise, the program proceeds.
A delimiting survey is completed promptly for the infested area, just before treatment is to begin. At this point, the surveyors determine the exact boundaries of the economically infested area that could be treated.
According to the EIS, rivers or other bodies of water in the proposed treatment area will not be treated with synthetic organic pesticides. Small aircraft may be used near these sensitive locations, but there still will be a 500-foot buffer zone for aerial sprays of any synthetic insecticide, and a 200-foot buffer zone for carbaryl bait. No buffer is required for Nosema bait.
When properly applied, insecticides used in the cooperative control program are not harmful to livestock; however, there are restrictions on the use of acephate where lactating dairy cattle are present. With the exception of carbaryl bait and Nosema bait all insecticides used in the grasshopper control program are highly toxic to honey bees and other insects.
APHIS recommends that all bee hives be removed from the spray area. Letters announcing the possible treatment areas and dates would be sent to registered beekeepers as early in the planning stages of the program as possible. Local news media are used to make timely announcements on pending treatments and program progress. When ranchers sign an agreement for treatment, they agree to make sure that all bee colonies are moved or otherwise protected before treatment begins (Anon., 1987a).
USDA-APHIS has also adopted a "crop land protection program" that may provide relief for at least some farmers in New Mexico and other western states. The criteria for this program require: 1) economic grasshopper infestations or the presence of threatening migrating populations in the immediate area; 2) susceptible crops growing adjacent to the federal rangeland; 3) removal or protection of honey bees and pollinating bees by owners so that timely treatments may be applied. Rangeland treatment for the purpose of crop land protection will generally consist of 1/4-1/2 mile strip spraying of adjacent rangeland. These requirements are intended to reduce forage damage and grasshopper migration, maximize efforts to protect crop land adjacent to federal lands, reduce long-term populations, keep costs reasonable, and ensure the use of methods and chemicals with the least environmental impact. USDA-APHIS has not conducted grasshopper control programs on crop land, per se, since 1950 when a state federal task force studying the situation reasoned that available chemicals and improved application equipment made it possible for growers to control grasshoppers on high-value cropland on their own or with only periodic assistance from government agencies. Another important reason for not treating crops is that environmentally acceptable and economical insecticides used in a large-scale control program are not as effective in killing grasshoppers on crops at the application rates recommended for range. Also, label restrictions do not allow application of the insecticides used in grasshopper control to all crops.
If farmers or ranchers have land that does not qualify for any sponsored grasshopper control programs, they can contact their county extension agent for information about appropriate pesticides and application techniques for particular crops or situations. In these cases, costs are borne strictly by the farmers or ranchers.
Beginning in the 1980s, environmental concerns became increasingly significant not only to regulatory agencies but also to land managers and the public. Laws were promulgated effecting tighter controls on pesticide labeling and use. Federal agencies were also grappling with the concepts of threatened and endangered species; whether and how to protect these species and their habitats from pest-control programs and other human activities is still hotly debated. Public and scientific concern had already mounted about long-known problems involving pesticide drift, contamination of nontarget habitats (especially water), fates of pesticide residues, and collateral problems of grasshopper control as it affected rangeland plant and animal communities. As a result of many of these legal changes plus economic cost-benefit analyses and a general decrease in available state and federal funds, large-area grasshopper control programs face a very uncertain future in many parts of the U.S., including New Mexico.