Japanese beetles have a wide range hosts. The range of crops this insect can feast on includes many species of fruit and vegetable crops, ornamentals and also field crops such as corn and soybeans. The adult Japanese beetles are metallic bronze and green with white tufts along the side of the abdomen (See Photo 1). "This pest is becoming a more common corn and soybean pest in Iowa," says Erin Hodgson, Iowa State University Extension entomologist. "Adults started to emerge in late May last year, but this year the cooler temperatures that were experienced during April, May and into June have slowed down its development in 2013."
Photo 1. Japanese beetle adults are metallic bronze and green, and have white tufts of hair along the side of the abdomen. Photo by David Cappaert, www.ipmimages.org.
Hodgson and graduate student Cody Kuntz at ISU offer the following information and observations to help you cope with this pest.
Research shows the adult stage of this insect needs about 1,030 growing degree days (base 50 degrees F) to complete development. Japanese beetles will continue emergence until around 2,150 degree days. Based on accumulating degree day temperatures in 2013, Japanese beetle adults should have been active in some areas of southern Iowa this past week (See Figure 1).~~~PAGE_BREAK_HERE~~~
However, a few adults were already collected in pheromone traps in Story County (central Iowa) on June 20, so you should expect adults to show up in northern Iowa in about seven days (or around June 27 this year) if warm temperatures continue.
You can use this website to predict emergence of Japanese beetles in your area of Iowa this summer
To more accurately predict adult emergence in your area this summer, use this website to generate up-to-date information. Click on the "View Degree Day Map" button in the left corner of the page, and then set the parameters for degree days to create a new map. Make sure to set the start date to January 1 of the current year and the end date to today; set the base temperature to 50 degrees F and the ceiling temperature to 86 degrees F.
Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (Jan. 1 - June 19, 2013). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
How to assess damage, and then take these management steps
Adults prefer to feed between soybean leaf veins, but can ultimately consume most of the leaf (Photo 2). The treatment threshold for Japanese beetles in soybean is 30% defoliation before bloom and 20% defoliation after the soybean plants start blooming. Most people tend to overestimate plant defoliation, but this reference can help with more accurate estimations.
In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping silks during pollination (Photo 3). Consider applying a foliar insecticide during tasseling and silking if there are three or more beetles per ear, silks have been clipped to less than ½ inch, and pollination is less than 50% complete.
Photo 2. Japanese beetles skeletonize soybean leaves. Photo by Mark Licht, ISU.
Photo 3. Japanese beetles are strongly attracted to silking corn. Photo by Erin Hodgson, ISU.