Rangeland Grasshopper Guide XVIII – 1

A Guide to Grasshopper Control on Rangeland

(Adapted from NebFact NF97-329)

John B. Campbell, P.E. Reece & Gary L. Hein

Grasshopper.

Grasshoppers can cause damage anywhere, but do so most frequently in areas with less than 30 inches annual rainfall. In most years, Colorado, Wyoming and the western two-thirds of Nebraska falls into that rainfall category and can be subject to severe grasshopper outbreaks.

There are about 50 species of grasshoppers found on rangelands in the High Plains; however, only nine species ever reach economic importance, doing about 95 percent of the damage. These are the whitewhiskered, striped, largeheaded, bigheaded, whitecrossed, migratory, spottedwinged, Kiowa, and fourspotted grasshoppers. Grasshoppers cause defoliation of grasses by direct feeding on leaf and stem tissue and by cutting off leaves or stems while feeding. High populations of grasshoppers on rangeland may damage the plant crowns so severely that many of the grass plants will not recover. With the exception of the migratory grasshopper, rangeland grasshopper species rarely feed on crops, except during years of very high populations.

Life Cycle

There are three stages in the grasshopper life cycle—the egg, nymph, and adult. The number of egg pods deposited by a single female may range from seven to 30, with eight to 30 eggs per pod, depending on the species. The eggs are well protected by the insulation of the pods and can survive extremely cold temperatures. Some grasshoppers prefer to lay eggs in soil surrounded by roots of grasses; other species select open areas with accumulations of surface debris.

Most grasshoppers winter in the egg stage, but a few species hibernate as nymphs. Most of these species are "band-wings," large grasshoppers that make a crackling noise when in flight and are seen early in the spring. They are normally few in numbers and usually do not seriously damage rangeland. Hatching time is influenced by weather, and predictions can be made by correlating the four developmental stages of eggs (clear, coagulated, eye spot, and segmented) with soil temperatures. Most nymphs start feeding within one day after egg hatch, and usually feed on the same plant species as the adult. Young nymphs are the most vulnerable to weather conditions, diseases, predators, parasites, and insecticides. Grasshopper nymphs generally develop to the adult stage in six to eight weeks. Hoppers begin egg laying one to three weeks after becoming adults. They may live up to two months or longer, depending on late summer and early fall weather.

Temperature Relationships

It is difficult to predict grasshopper development from air temperatures, because grasshoppers can regulate their body temperature by moving from plant shadows to bare soil or open areas. However, grasshopper activity and feeding begins as temperatures in their micro-habitat rise above 65-70°F. Growth begins earlier and progresses more rapidly on heavily defoliated compared to properly managed rangeland.

Cool-season grasses such as needlegrasses, wheatgrasses, and bluegrasses begin growth before grasshoppers become active, during late-March or early-April, and grow most rapidly when air temperatures are 65-75°F. By the time infestation of grasshoppers begin to heavily defoliate plants, cool-season grasses have completed their growth and recharged energy reserves needed for spring growth next year. Warm-season grasses such as bluestems, grama grasses, and buffalograss do not begin growth until May and grow most rapidly when temperatures are 85-95°F. Consequently, grasshoppers are most likely to damage warm-season grasses because they don't complete their growth until late summer.

Grass Response to Defoliation

Understanding how grasses respond to defoliation is critical for grasshopper management on rangelands. Each year rangeland vegetation is defoliated by livestock, wildlife, insects, hail and/or fire. Infestations of mature grasshoppers rapidly remove foliage from plants. Root growth stops and nutrient uptake is reduced for several days when more than half of the green herbage is removed from grasses. Lengths of "shut-down" and "slow-down" periods in roots increase as severity and frequency of defoliation increase. Removing more than 65 percent of the green herbage one time during the growing season can reduce total root length by 30 percent or more. When grasses are severely defoliated over several years by any combination of defoliation processes, plants become weak and die. Plants in excessively defoliated pastures are drought stressed even when precipitation is near average, because reduced root length and nutrient uptake activity limit their use of available soil moisture. Plants on shortgrass prairie are less likely to experience defoliation-induced drought because low infiltration rates limit the depth of soil moisture on these sites.

Grazing Management Objectives

•Make the environment as unfavorable as possible for grasshopper growth and survival of pest species of grasshoppers.

•Maintain high levels of plant vigor and range conditions to minimize the effects and occurrence of outbreaks.

Grasshoppers are small, cold-blooded, and profoundly affected by temperature and relative humidity in their microhabitat. All developmental stages of grasshoppers can be advanced by high temperatures or retarded by low temperatures. Increasing the time required for grasshoppers to mature increases juvenile mortality, which reduces defoliation and the number of eggs produced for next year's grasshopper population.

Temperatures and relative humidity near the soil surface are directly related to the height and distribution of herbage. Increases in herbage that delay grasshopper growth also provide habitat for natural predators and pathogens, including birds, mammals, reptiles, predatory insect species, fungi and other pathogens. Management practices that minimize favorable habitat for pest species of grasshoppers also maintain high levels of plant vigor and range condition. The number of grasshopper species and the total number of grasshoppers may be greater on properly managed prairie compared to overgrazed prairie; however, reduced grasshopper growth rates, higher mortality of immature grasshoppers, and higher productivity of plants minimize the effects of these populations.

Herbage Allocation

Soil can be shaded or insulated from direct sunlight with standing herbage or litter. On healthy rangeland, standing herbage consists of a mixture of residual herbage from preceding years and current-year herbage. A minimum amount of standing herbage must be retained at all times for protection against erosion and to enhance infiltration of precipitation. About half of the herbage produced each year is needed to maintain levels of residual cover typical of healthy rangeland. Disappearance of the other half by the end of the summer grazing season is the result of nearly equal defoliation by cattle and natural processes. On properly stocked rangeland, cattle will use about 25 percent of the current-year herbage resource.

Published estimates of herbage reduction caused by grasshoppers are highly variable. While these insects are a natural part of range ecosystems, herbage losses caused by above average populations must be offset by reduced livestock use. Estimates of daily dry matter intake for grasshoppers range from 30 to 250 percent of body weight compared to 1.5 to 2.5 percent for beef cattle. A 1250 pound cow would consume 19 to 31 pounds of herbage each day. The same amount of herbage could be consumed by eight to 104 pounds of grasshoppers in a single day. Many ranchers develop a mental picture of what pastures should look like when it is time to remove livestock. Cattle should not be placed in grasshopper-infested pastures that appear to be near or below that amount of cover envisioned as adequate by the rancher.

Plant Communities

Opportunities to shade the soil surface with standing herbage increase as composition of mid- and tallgrass species increases. Full growing-season deferment from green-up to killing frost can dramatically increase mid- and tallgrass herbage production after one to five drought-free years of overgrazing. The probability of increasing shade by improving range condition or plant vigor declines as the history of abuse increases. Decades of overgrazing can reduce mid- and tallgrass prairies to shortgrass dominated pastures that do not respond well to best management practices. Deterioration of rangeland is accelerated when drought and heavy defoliation are combined.

Best Grazing Management Practices

Cattle should be excluded for one year from pastures that are severely defoliated to replenish residual herbage and to provide uninterrupted plant growth for maximum replenishment of roots and energy reserves. It may be necessary to rest a pasture for an additional year if precipitation is below average or if the vegetation is heavily defoliated by other processes during deferment.

Recovery of plant vigor is maximized with full growing-season deferment and is not affected by dormant-season grazing at proper stocking rates. Grazing practices that alternate the season of grazing or shift summer grazing dates by 30 days or more among pastures prevents repetitively favoring the same pest species of grasshopper in consecutive years. Periodically changing grazing between growing and dormant seasons will change grasshopper environments dramatically and maximize plant vigor. Pastures should be grazed once from June to August to avoid severe defoliation of warm-season grasses. Grazing is generally distributed more uniformly throughout pastures with rotation compared to continuous grazing. Consequently, rotation grazing at proper stocking rates will minimize the development of large open areas near water that are common to continuously grazed summer pastures.

Monitoring Hopper Populations

Economic thresholds for grasshopper densities in pastures or rangeland may vary from eight to 40 grasshoppers per square yard. The thresholds are influenced by several factors including, the cost of insecticide treatment, projected forage yield, and value of the area being considered for treatment. These and other factors are considered in a computer program called "Hopper" that can be used to determine the threshold in individual situations. This program was developed by USDA-APHIS and can be obtained through your local Cooperative Extension office or state USDA-APHIS Plant Protection and Quarantine offices.

Several methods can be used to estimate grasshopper populations on rangelands. One is to count the number of grasshoppers seen leaving visually delineated square yard areas ahead of the surveyor as the person approaches each sample area. With this method, 30 or more samples should be taken. Sample areas should be at least 50 to 75 feet apart. It may be necessary to separately sample different vegetation types or aspects if major differences occur in plant communities and/or topography. Another method is to count grasshoppers in 18 different one square foot areas with 50 to 75 feet between samples. Divide the total number of grasshoppers counted in all 18 square foot areas by two to determine the number of grasshoppers per square yard. The most reliable method may be to use a standard sweep net equipped with a bush net. The surveyor would sweep a square yard at a time and after several square yards are swept, count the grasshoppers in the net. With this method, both numbers and species can be determined. Also, a screened frame, 36 by 36 inches or 12 by 12 inches , could be thrown out ahead of the surveyor, and the number and species of grasshoppers can be determined as the frame is raised. For methods other than the frame, it is advisable to lay out a square yard or square foot to familiarize yourself with the size of the unit.

Chemical Control

If control is needed, these practices are most effective when applied to grasshopper hatching areas while hoppers are in early nymphal stages. If populations are reduced to less than one grasshopper per square yard, control measures may not be needed for several years unless the area is reinfested through migration from other infested areas.

Grasshoppers may be controlled by applying insecticides as sprays or baits. The insecticides registered for use on rangeland are malathion, acephate (Orthene), methyl parathion (Penncap M) and carbaryl (Sevin). Rates for these products are listed on the labels. If larger grasshoppers are targeted, the higher labeled rates should be used. Other insecticides are labeled for control of grasshoppers in forages grasses, alfalfa, and other crops. See Guide to Grasshopper Control in Cropland (included in this guide) for control information in these situations. Read labels thoroughly before using any insecticide, and observe safety and grazing restrictions.

Recent research at the University of Wyoming has demonstrated the effectiveness of a new grasshopper control strategy in rangeland. This strategy has been termed Reduced Agent/Area Treatment (RAATs). The insecticide that shows the most effectiveness with this method is Sevin XLR (ultra-low volume, aerial applications) which is used at half the recommended rate (eight ounces per acre) instead of the full rate (16 ounces per acre). Also, with this method only 50 percent of the area is treated by leaving every other spray strip untreated. This method cuts control costs by 60 percent, and will significantly lower the economic threshold for grasshoppers in rangeland. Grasshopper control in the RAATs area lagged a little behind control in the full rate areas, but by six days after treatment both treatments showed the same level of control. The reasons for this dramatic control are thought to be due to grasshopper movement into the treated strips while the insecticide is still effective, and to the preservation of natural enemies in the untreated strips.

Ranchers may also need to consider protection-spraying "barriers" around valuable forage production areas, such as highly productive hay meadows or seeded crops like alfalfa or annual forages. Protection-spraying may require continual surveys during the summer. As the vegetation on upland range sites matures or dries, grasshoppers will move into areas with succulent vegetation. Thus, it may be necessary to spray at two- or three-week intervals to provide protection for these valuable forage resources.

Baits, once the most popular control method, have been replaced by sprays. However, baits are still used occasionally in some circumstances on rangeland with short, dry vegetation. Carbaryl (Sevin) 5% bait is available. Control of some grasshopper species will be severely limited because they will not feed on the bait. For successful control, this method requires uniform distribution of bait and re-application if the bait no longer is attractive to the grasshoppers. Attractiveness of the bait will be reduced substantially by rain or heavy dew.

Categories: Insects, Grasshoppers, Range

Date: 11/18/2001