The 2019 growing season was challenging for farmers in many parts of the state, especially because cold and wet soil conditions in April significantly delayed planting. Thistle caterpillar was the most abundant insect statewide, though multiple species of caterpillars, Japanese beetle, soybean aphid, and soybean gall midge were observed in soybean.
Each year, the ISU Soybean Research Laboratory conducts insecticide efficacy evaluations for soybean aphid, but two new pests were included in 2019: Japanese beetle, an emerging pest in the Midwest, and soybean gall midge, a new pest not known to occur elsewhere.
We established one research plot for soybean aphid (Photo 1) at the Iowa State University (ISU) Northwest Research Farm (NWRF) in Sutherland, IA. Colonization by soybean aphid was slightly delayed in 2019, with initial colonization occurring in July. Exponential growth occurred in August, and we saw a 9-fold increase in aphid populations in just one week.
Peak aphid populations occurred in early September at 1,783 aphids/plant, which is nearly triple the economic injury level for soybean aphid. Since economic infestations occurred late in the season (at full seed set), yield differences among treatments were not dramatic.
Japanese beetle (Photo 2) evaluations occurred at the ISU Northeast Research Farm in Nashua, IA and the ISU Johnson Research Farm in Ames, IA. Japanese beetle abundance is difficult to estimate because this pest is highly mobile; however, peak beetle populations occurred in early August at 9.75 and 11 beetles/10 sweeps in Ames and Nashua, respectively.
Higher beetle numbers were observed in the untreated control and Transform (active ingredient: sulfoxaflor) plots; however, the level of defoliation was low and did not translate to measurable yield loss.
Soybean gall midge
Soybean gall midge (Photo 3) evaluations occurred at the ISU Northwest Research Farm in Sutherland and a commercial soybean field near Griswold, IA. Emergence of soybean gall midge adults began in June, and plants with larvae inside could be found shortly after.
Emergence was continuous throughout the summer with overlapping generations evident, though three distinct generations of adults were observed in emergence cages. Compared to Sutherland, midge pressure was greater at Griswold, where a commercial soybean field typically achieving 70 bu/A experienced yield ranging from 4.3 to 43.9 bu/A.
At this farm, yield was not protected by any insecticide or application timing evaluated, and pressure was not high enough to determine how midges affected yield at Sutherland.