With drought conditions occurring throughout much of South Dakota, it is important to begin monitoring field margins for grasshopper defoliation. Grasshopper populations are usually observed in grassy areas like ditches and field margins. They do not typically move into crops until later in the summer and early fall. However, grasses can prematurely dry down during droughts, causing grasshoppers to seek out other green plants. These green plants are often neighboring crops, which can undergo severe defoliation due to large concentrated populations of grasshoppers. In addition to the 2017 drought conditions, grasshopper populations were high in 2016 and have likely increased due to the long warm fall that we experienced.
Grasshoppers can be scouted using visual searches or by using sweep nets. However, scouting can prove difficult as grasshoppers move rapidly within crops. One of the easier methods for soybean is to scout based on defoliation. For soybean, the pre-bloom defoliation threshold is 40%. For corn and other crops, management is recommended once grasshoppers reach 21-40 per square yard or 3 sweeps. The three most commonly observed grasshoppers in South Dakota fields are the differential grasshopper, redlegged grasshopper and the two-striped grasshopper. There is the possibility that only a single species may be present, or that there may be a combination of two or three species present.
Managing grasshopper populations typically does not require insecticide applications. However, if excessive defoliation is occurring or populations exceed threshold levels, insecticide use should be considered. Please refer to the most current edition of the South Dakota Pest Management Guides for Corn and Soybean for insecticides labeled for grasshopper management.
Source: Adam J. Varenhorst, iGrow