Grasshoppers & Winter Wheat

Sep 16, 2016

By Adam J. Varenhorst

Assistant Professor & SDSU Extension Field Crop Entomologist

Reports of large grasshopper populations came in from much of South Dakota this year. In some areas, fields have been treated with insecticides in efforts to reduce these populations. With winter wheat planting underway, it is likely that some fields will be negatively impacted by grasshopper populations.

Due to the limited amount of green vegetation present in the fall, newly emerging winter wheat is a very attractive host to grasshoppers. Grasshoppers are capable of causing stand loss by clipping back the emerging plants due to the limited amount of foliage on seedlings. This type of injury will be more noticeable along the field margins as grasshoppers reside in the nearby vegetation. To minimize the impact of grasshoppers on emerging winter wheat, it is important to monitor fields and determine grasshopper densities before and after planting.

Common Grasshopper Species

Grasshopper reports this year have primarily been for the differential grasshopper (Figure 1). However, it is possible that other species, such as the redlegged grasshopper, may also be present this fall (Figure 2). Grasshopper populations will naturally decline during the fall, but there is still the possibility for significant populations to remain until the first hard frost. In areas where grasshopper populations have been consistently high, it is important to scout emerging winter wheat for grasshoppers and feeding injury.

Fig. 1. Adult differential grasshopper. Note the black chevron markings on the hind leg. Photo by Adam. J. Varenhorst.

Fig. 2. Adult redlegged grasshopper. Note the red hind leg. Photo by Adam. J. Varenhorst.

Pre-Planting Management Options

Before planting, scout the vegetation around the field edges and count the number of grasshoppers present in one square yard. Consider management if populations in the non-crop borders or within the field are 11-20 grasshoppers per square yard. Management options to consider include:

Delay planting in areas where grasshopper populations are high to reduce the time that grasshoppers will be feeding on the emerging wheat.
When there are increased grasshopper population densities in areas surrounding the field, double the planting population for a strip that is between 60-120 feet wide around the field edge. As grasshoppers move into the field they will feed on the emerging wheat. However, the increased plant stand around the edge will slow the movement of the grasshoppers into the field. The increased plant stand will compensate for the lost seedlings and allow for a reasonable stand to exist after grasshoppers have finished foraging for the year.
Insecticide seed treatments are labeled for grasshopper management in winter wheat, and can provide protection during emergence (Table 1). For grasshoppers, treated seed can be planted to the field edges in a strip that is 60-120 feet wide, or it can be planted to the entire field. For effective management of grasshoppers, the highest registered rate of the product should be applied to the seed.
Foliar insecticide sprays may be applied to the non-crop field edges to prevent the grasshoppers from moving into the wheat fields (Table 2). Before selecting an insecticide, check the label to ensure that it can be sprayed on non-crop areas.

After-Planting Management Options

If grasshopper populations are causing severe defoliation or clipping in newly emerged winter wheat, it is important to determine if the populations are above 11-20 grasshoppers per square yard. Additionally, determine how far into the field the grasshopper populations are. Management options to consider for emerged wheat include:

If injury and grasshoppers are only present around the field edge, a foliar application of insecticide to that area may be sufficient for reducing grasshopper feeding pressure. It is important to remember that there will be little residual activity of the foliar insecticides due to the limited leaf area of emerging wheat. See Table 2 for a list of foliar insecticides that may be applied to wheat.
If injury to the field edge is severe due to heavy grasshopper feeding pressure, it is possible to replant those areas after the first hard frost occurs. At this time, grasshopper populations will have declined significantly.

Table 1. Insecticide seed treatments labeled for managing grasshoppers in wheat.1

Insecticide	Rate (for grasshoppers)
Cruiser 5FS (thiamethoxam)	1.33 fluid oz. per 100 lbs. of seed
Gaucho 600 (imidacloprid)	2.4 fluid oz. per 100 lbs. of seed
1This list is not meant to be comprehensive. Always check and follow label instructions.

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