Grasshopper Threat To Soybeans And Winter Wheat

By Anitha Chirumamilla and Ada Szczepaniec

The 2013 Grasshopper Survey prepared by the USDA predicted a low to moderate risk of high grasshopper populations in 2014 for western South Dakota. Although this prediction still holds true, isolated spots in central and eastern SD experiencing drought stress appear to be in a moderate to high risk for high grasshopper numbers.

While in most cases the grasshoppers are feeding on grasses and weeds in the ditches, reports of grasshoppers moving into soybeans are coming in. Soybeans are still in the pod and seed setting stages and grasshopper feeding on leaves and pods might cause yield losses. Treatment decisions are usually based on estimating percent defoliation or by counting the nymph and adult grasshoppers per square yard. Spraying is recommended when percent defoliation exceeds 20% between bloom and pod fill (Table 1). The sweep net sampling method is used to estimate nymph and adult grasshopper counts. Sampling is done at three random locations in a field by swinging the net 40 times in each location while walking in a semi-circle pattern. The total number of grasshoppers at each location is then divided by ten to estimate the number of grasshoppers per square yard. A general threshold of 30-45 nymphs or 8-14 adults per square yard justifies a pesticide application (for list of pesticides see Table 1).

Table 1: Insecticides labeled for foliar application on soybeans and wheat in South Dakota

Winter wheat planting is around the corner and parts of SD where populations of grasshoppers are high are at a greater risk for grasshopper damage. A majority of the grasshopper species are grass feeders, which makes the emerging winter wheat seedlings extremely vulnerable to grasshopper attack. As grasshoppers move from field borders and ditches, doubling the seeding rate on outer rows is recommended to account for seedling loss to grasshopper feeding. Also, insecticide applications targeting the border rows might help prevent the inward movement of grasshoppers into the field (for list of pesticides see Table 1).

Dry and warm climate favors development and reproduction in grasshoppers, but there are natural enemies of these pests that can attack grasshoppers and contribute to lowering their populations. For example, most of the grasshoppers spotted in central SD were infested with red parasitic mites, Eutrombidium locustarum (Figure 1). These mites attach themselves to the underside of grasshopper wings where they can suck the blood of the host without being dislodged (Husband and Wohltmann, 2011). Although it does not result in death of grasshoppers, high numbers of the parasitic mite weakens the host thereby increasing its susceptibility to predators and diseases.

Figure 1. Grasshopper infested with parasitic red mite. Photo: Richard Orr, www.marylandinsects.com

Source : SDSU