Grasshopper Biology and Management (continued)

E-272 (Revised), February 1997

Estimating Grasshopper Infestations
Monitoring Hatch and Population Development
Planning Management Programs

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Estimating Grasshopper Infestations

Estimating the grasshopper population requires several samples before assessing the need to apply an insecticide.

Egg assessment

The first fields that should be inspected are fields that had a high adult grasshopper population the previous summer and will be seeded during the present growing season. These may include last season's weedy summer fallow fields (crop-fallow rotation) and late-season crop fields (continuous crop rotation). In addition, the field borders (fence rows, road ditches) may contain a high number of eggs, which may result in grasshopper migration from these areas into the field. Egg sampling in the fall may not provide an accurate assessment of the potential population for the next year but can be helpful in identifying sampling sites and possible areas of high infestation for the following spring. Fall sampling may be most useful in summer fallow fields but should not be conducted till after a killing frost.

Spring egg sampling should be conducted as early as possible. Remove the soil from a square yard to a depth of about 2 inches and sift it through a 6 mesh hardware screen. Several soil samples should be taken at each site. If egg numbers from the field average 17 eggs or more per square yard, the threat of economic damage to the crop exists. If the egg numbers outside of the field (field border or roadside) average 120 eggs or more per square yard, a threat of economic damage to the crop exists (Table 1). If there is a threat of economic damage because of egg numbers in the field, outside of the field, or both and the field is to be cropped, early seeding of a small grain crop should be considered.

Nymph and Adult Assessment

Grasshoppers can begin hatching in late April and as late as early July. In general, a warm spring following a warm fall will favor an early hatching date, whereas a cool fall and spring will delay the hatching date. In most years, egg hatch begins in mid May and lasts until late June. Once egg hatch has begun in or near small grain fields, infested areas should be inspected at least once a week. After two or three weeks, if populations are threatening, inspections should be conducted every other day. Since broadleaf crop seedlings (sugarbeets, sunflowers, canola, dry beans and soybeans) do not have the damage tolerance level of small grains, these crops should be inspected at least every other day after the onset of egg hatch.

Newly hatched grasshoppers are extremely small (about the size of a kernel of wheat) and are extremely difficult to count. It is often necessary to search for young grasshoppers on hands and knees during the warm period of the day. Newly emerged grasshoppers do not usually need to be controlled unless the population is extremely large (see Table 2). Continue checking for possible reinfestation created by subsequent hatch or migration into the field. Insecticides should not be applied if nymph populations are low and a large number of eggs have not hatched, unless significant economic damage is occurring. If most of the nymphs are about 1/3 of an inch long, the egg hatch should be complete or nearly completed. If economic damage is occurring, nymphs are 1/3 of an inch long and egg hatch is still underway, an insecticide treatment should be considered.

Monitoring Hatch and Population Development

Central to any pest management program is field monitoring to see if the pest is present. For grasshopper programs, it is useful to know when hatching begins in the field.

Lilac as an indicator of grasshopper hatch

Recent research with rangeland grasshoppers, which included the migratory grasshopper, used the beginning of bloom in common lilac as an indicator of when grasshopper hatch is under way. Using plants as indicators can help the farm or range manager observe the influence of current weather on biological activity without using temperature-driven predictive models. The main variable influencing bloom time of common lilac, a landscape plant that often receives standard care, is temperature. On average, 10 days after common lilac were flowering 75% of the grasshoppers were first stage nymphs and 25% were second stage nymphs. Local variation will occur due to elevation differences. Earlier hatching and faster development is also expected on southern facing slopes.

Forecasting with degree days

The primary use of degree days (DD) in integrated pest management (IPM) is to time scouting for pest species. Degree days can be used to predict when a life cycle event will occur. If the number of DD accumulated to date is known, future degree days can be estimated by using average maximum and minimum temperatures, such as 5- or 30-year averages. These averages give only an approximation of when an event occurs, but they can be extremely useful in planning sampling and management activities. This eliminates unnecessary scouting, avoids missing injurious pest populations, and helps make better management decisions. These techniques help prevent economic losses and excess use of pesticides.

The first requirement for calculating DD is to recognize that growth only occurs within a range of temperatures. The minimum temperature below which no growth occurs is called the minimum developmental threshold or the minimum threshold. Growth will increase with higher temperatures up to a maximum temperature called the maximum developmental threshold. These thresholds are determined experimentally and are different for each species. Although a minimum threshold is required for insect development, maximum thresholds often are not included since insects rarely experience temperatures near the maximum threshold during springtime development.

Degree days can be easily and accurately calculated by using the average temperature minus the base temperature. Using the average temperature method, calculate DD for each day by determining the number of degrees above the threshold. Degree days for a single day are calculated as follows:

Degree days are totaled over a period of days to determine when an insect has reached a certain stage. The accumulations necessary to go from one stage to another are determined through research. Common starting times to begin accumulation is when temperatures exceed the minimum threshold.

DD are useful in predicting development of grasshoppers, but these predictions are only an estimate. The accuracy of a DD estimate is also extremely dependent on the temperatures used in calculating degree days. Consequently, degree days should be calculated with temperatures that represent the environment where the grasshoppers are developing. Temperatures at one site will give only a rough estimate of insect development at another site miles away.

Planning Management Programs

The use of lilacs to indicate grasshopper emergence and degree days to monitor population development can be beneficial when planning and implementing management programs.

To be effective, trap strips must be prepared prior to significant grasshopper hatching. If all green vegetation is destroyed prior to hatch and not allowed to regrow, the first instars will die. If hatch is begun, trap strips should be established to attract and hold the nymphs. Strips can be made the same width as the spray boom for later management with insecticides.

Using the DD information from Table 3, hatching of new grasshoppers should be nearly complete at 160 DD. Begin assessing the size of grasshopper populations prior to this time. Delay the use of insecticides beyond this point when possible. Treatments prior to this may lose their effectiveness before emergence is complete. Treatments made to hatching sites and trap strips at this time limit the number of migrating second instar grasshoppers.

After 340 DD, grasshoppers have reached third, fourth, and fifth instar and are becoming quite mobile. Food consumption by grasshoppers increases rapidly at the third instar. If infestations are at threatening levels, treatments should be made to limit plant injury.

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[ Life Cycle ] [ Damage ] [ Natural Enemies ]
[Cultural Control Methods] [Chemical Control]
[Descriptions of Economically Important Grasshoppers]
[Necessary Precautions in Controlling Grasshoppers]
[Insecticidal Control Recommendations]

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E-272 (Revised), February 1997