Japanese beetles is an invasive insect capable of feeding on corn and soybean plants. This pest has been in Iowa since 1994 but its distribution in fields is sporadic around the state. Japanese beetles need about 1,030 growing degree days to complete it's adult development and will emerge in the plants around 2,150 growing degree days. Based on this Japanese beetles can show up from mid June- August.
Japanese beetles have a one generation per year in Iowa and will emerge from the grass in late June and begin feeding on low lying plants such a shrubs and work their way up to soybeans and corn.
Description
Adult Japanese beetles are 3/4 inch long and have an iridescent copper and green color. There will be white tufts of hair on their abdomen sides and tip. Larvae grubs will be 1/2 to 3/4 inch long and will be found in the soil.
Scouting
When scouting for Japanese beetles be sure to look all over the plant but they tend to be more apparent on the silks of corn or the leaves of soybeans. Scout the whole field to determine the extent of the infestation with problem areas more likely to occur on lighter textured soils.
Damage
Japanese beetles have a wide host range from fruits and vegetables, ornamentals, and field crops. On soybeans, the adult beetle will feed between the leaf veins and can ultimately consume most of the bean plant. On corn, adult beetles may clip the silks of the corn plant during pollination. Leaf injury is uncommon but could consist of parallel holes between leaf veins.
Threshold
The treatment threshold for Japanese beetles in soybeans is 30% defoliation before bloom and 20% after bloom. Take a look at the image below to compare how much defoliation has occurred on your soybean plant. In corn, you should consider foliar insecticide during tasseling and silking if there are 3 or more beetles per ear, the silks have been clipped to less than 1/2 inch, AND pollination is less than 50 % completed.
Source: Iowa State University Integrated Crop & Google Image.
