Several reports from ISU Field Agronomists have indicated Japanese beetles are emerging in southern Iowa. The emergence is about 7-10 days behind the last few years, due to slowly accumulating degree days in 2019.
Literature shows Japanese beetle adults need about 1,030 growing degree days (base 50°F) to complete development and will continue emergence until around 2,150 degree days. Based on accumulating degree-day temperatures in 2019, Japanese beetle adults should be active in some areas of southern Iowa this week (Figure 1).
Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (as of June 16, 2019). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy. Click Image to Enlarge
The false Japanese beetle (Photo 1) often emerges just before Japanese beetle (Photo 2). It is important to distinguish the two species. The former is not considered a field crop pest, but the later can be a pest on a number of crops, ornamentals, and garden plants. The insects resemble each other in the general size and shape, and are in the same subfamilies of beetles, called shiny leaf chafers (Rutelinae).
True Japanese beetles are more iridescent with a metallic green head and thorax with copper-colored forewings. The false Japanese beetle is not quite as shiny (sorry, that is up for your interpretation!) and the white tufts of “hair” along the sides and tip of the abdomen are not as obvious.
Photo 1. False Japanese beetle. Photo by Erin Hodgson. Click Image to Enlarge
Photo 2. Adults Japanese beetles are metallic bronze and green with white tufts along the side of the abdomen. Photo by Teresa Cira. Click Image to Enlarge
Plant Injury and Management
Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf. The treatment threshold for Japanese beetle in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom. It is important to note most people overestimate plant defoliation.
