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2018 Eastern South Dakota Grasshopper Forecast

By Adam J. Varenhorst
In 2017, researchers at South Dakota State University completed a grasshopper survey of Eastern South Dakota. The last grasshopper survey to be conducted in Eastern South Dakota was published in 1925. As we know, 2017 was an interesting growing season with areas of South Dakota experiencing severe drought conditions. In some of these areas, we found that grasshopper populations were causing defoliation injury to already drought-stressed crops. Factors that can lead to increased grasshopper populations include long, warm falls, decreased ground cover, and limited spring rainfall.
2017 Populations
Leading up to 2017, we experienced several warm falls where the first frost date occurred much later than normal. These conditions allowed grasshoppers to successfully lay eggs for a much longer period of time and were likely part of the reason why we saw increased grasshopper populations in the eastern half of the state.
Areas of South Dakota with grasshopper populations that reached or exceeded the threshold of eight or more grasshoppers per square yard included: Hyde, Hand, Beadle, Kingsbury, Spink, Buffalo, Jerauld, Brule, Aurora, Douglas and Hutchinson counties. In addition, there were several other counties that had multiple samples near threshold levels (Figure 1).
Figure 1. Map of 2017 grasshopper abundance for Eastern South Dakota. Green triangles indicate grasshopper populations that were below threshold, orange squares represent grasshopper populations that were approaching threshold, and red circles indicate grasshopper populations that exceeded the threshold. 
2018 Growing Season
One factor that may lead to reduced grasshopper populations in 2018 was the very cool spring we had in 2017. As a result, many of the grasshoppers that we observed were several weeks behind in growth. For most of South Dakota, the first 28°F frost occurred between the first and third weeks of October (Figure 2). This would have given the grasshoppers plenty of time to finish development and lay eggs. Like the previous years, most of South Dakota was warmer than average during the fall (Figure 3). However, trying to predict insect populations is difficult as many additional factors can contribute to their winter survival and overall spring populations. Our recommendation is to scout for spring grasshopper populations, as they will be the first indicator of potential problem areas. Depending on the 2018 growing season, areas with limited rainfall or drought conditions may experience increased grasshopper populations.
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