During 2019 we have received varying reports regarding grasshopper populations. Many reports have indicated that grasshopper numbers are down. However, we have also received reports of very large grasshopper populations in some areas of South Dakota. So why such a difference?
The abundant rainfall and resulting vegetation during 2019 has provided grasshoppers with more food than normal. Therefore, it is possible that grasshoppers are still content feeding in non-crop areas. However, populations have recently started moving into scouted areas. This is probably responsible for the reports of large populations.
Figure 1. Adult differential grasshopper. Note the black chevron markings on the hind leg. Courtesy: Adam Varenhorst
It is important to scout emerging winter wheat for grasshoppers and feeding injury, especially in areas where large grasshopper populations are being observed. Due to the reduced amount of green vegetation present in the fall, newly emerging winter wheat is a very attractive host to grasshoppers.
Figure 2. Adult redlegged grasshopper. Note the red hind leg. Courtesy: Adam Varenhorst
Grasshoppers are capable of causing stand loss to emerging seedlings by clipping back the emerging plants. This type of injury will be more noticeable along the field margins, as grasshoppers reside in the nearby vegetation.
Figure 3. Adult twostriped grasshopper. Courtesy: Adam Varenhorst
To minimize the impact of grasshoppers on emerging winter wheat, it is important to monitor fields and determine grasshopper densities before and after planting.
The grasshoppers that are most abundant every year are the differential grasshopper (Figure 1), redlegged grasshopper (Figure 2) and the twostriped grasshopper (Figure 3).
Grasshopper Management Options Prior to Planting
Before planting, scout the vegetation around the field edges and count the number of grasshoppers that are present in approximately one square yard.
Management should be considered if grasshopper populations in the non-crop borders or within field are in the range of 11-20 grasshoppers per square yard. Here are a few management options to consider:
- 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 stands around the edge will slow the movement of the grasshoppers into the rest of 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. 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. Please refer to South Dakota Pest Management Guide: Wheat for insecticide seed treatment options and recommended rates.
- Foliar insecticide sprays may be applied to the non-crop field edges to prevent the grasshoppers from moving into the wheat fields. Before selecting an insecticide, check the label to ensure that it can be sprayed on non-crop areas. See Table 1 for a list of non-crop and pasture insecticide options.
Grasshopper Management Options After Planting
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. Here are some management options to consider for emerged wheat:
When 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. Please refer to the South Dakota Pest Management Guide: Wheat 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.