Japanese beetle is becoming a more common field crop pest in Iowa. A review of the literature regarding this insect shows that the adults need about 1,030 growing degree days (base 50 degrees F) to complete development, says Erin Hodgson, Iowa State University Extension entomologist. Japanese beetles will continue emergence until around 2,150 degree days. Based on accumulating degree day temperatures in 2014, Japanese beetle adults began to be active in some areas of southeastern and southwestern Iowa last week (see map, Fig. 1). You can expect adults to emerge in central and northern Iowa about this week and next as the days of warm temperatures have continued.

BECOMING MORE COMMON: Japanese beetles have shown up more and more in some parts of Iowa in recent years. This bug has a wide range of host plants it will feed on, including many species of fruit, vegetables, ornamentals and field crops such as corn and soybeans.

Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (January 1 - June 12, 2014). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

To more accurately predict adult emergence in your area this summer, Hodgson suggests you 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.

Crop injury and management of Japanese beetles
Japanese beetles have a wide host range that includes many species of fruit, vegetables, ornamentals and field crops, she says. Adults are metallic bronze and green with white tufts along the side of the abdomen (See Photo 1). There are some look-alike beetle species that may be confused with Japanese beetle.

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Photo 1. Japanese beetle. Photo by David Cappaert, www.ipmimages.org.

Adults prefer to feed between soybean leaf veins, but can ultimately consume most of the leaf (See Photo 2). The treatment threshold for Japanese beetles in soybeans is 30% defoliation before bloom and 20% defoliation after bloom. Hodgson says most people tend to overestimate plant defoliation, but this website 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 1/2 inch AND pollination is less than 50% complete.

Photo 2. Japanese beetles skeletonize soybean leaves. Photo by Mark Licht, Iowa State University.

Photo 3. Japanese beetles are strongly attracted to silking corn. Photo by Erin Hodgson, Iowa State University.