The Spatial Trends of Japanese Beetles in Illinois Soybean Fields and Their Effects on Sampling and Production
A 2-year research study was conducted in Illinois soybean, Glycine max (L.) Merr., fields to determine the spatial trends of Japanese beetles, Popillia japonica Newman, and to measure their impact on soybean production. Commercial soybean fields were sampled intensively or extensively. Densities and distributions of Japanese beetles within fields were measured with two different sampling methods (sweep-net samples and visual counts) in both intensively and extensively sampled fields, and the influence of field border type (i.e., corn, grass, soybean, road) was analyzed. Additionally, the relationship between densities of Japanese beetles and corresponding percentage defoliation over time was analyzed in intensively sampled fields. Japanese beetle densities were considerably larger in 2009 than in 2010, although corresponding percentage defoliation in both years was well below the widely accepted economic threshold of 15 to 20% defoliation between bloom and pod fill. Densities of Japanese beetles were larger in field edges than in field interiors in both 2009 and 2010, but the difference in densities between field edges and field interiors was statistically significant only in 2009. In both 2009 and 2010, significantly greater densities of Japanese beetles were found in field edges that bordered cornfields than in field edges that bordered grass, soybean, or roads. Growers who wish to manage Japanese beetle populations in soybean must recognize that field edges, where Japanese beetle densities can be highly concentrated, may not be indicative of the overall densities in their fields. Furthermore, although larger densities of Japanese beetles were observed in field edges than in field interiors, percentage defoliation and yield were not significantly different between field edges and field interiors. Under the modest Japanese beetle population densities measured during this study, the defoliation in soybean field edges never reached a level where the yield differential would have justified the cost of managing Japanese beetles even in field edges. Sweep-net samples and visual counts to estimate Japanese beetle population densities were strongly correlated. Most soybean growers in the Midwest do not use sweep nets to sample insects, but they might be inclined to count Japanese beetles by visual observation to determine the need for insecticide application. Future economic thresholds based on numbers of Japanese beetles per meter of row likely would be more useful than the current, more subjective percentage defoliation thresholds.
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign