Japanese beetles defoliating a basswood leaf. / Photo Credit: Wisconsin DNR

By Linda Williams, DNR Forest Health Specialist
Linda.Williams@wisconsin.gov or 920-360-0665

The Japanese beetle (Popillia japonica) is an invasive insect that feeds on many plant species. The adult beetles “skeletonize” leaves by eating the material between the veins.

Adult Japanese beetles feed on the flowers and leaves of more than 300 plant species, including trees, shrubs and herbaceous plants. Forest trees can be defoliated, with open-grown or edge trees suffering the highest defoliation rates.

Japanese beetles favor birch, mountain ash and basswood (linden) trees, but they have also been observed feeding on the leaves of young oaks.

An oak leaf has been completely defoliated by Japanese beetles, leaving its lacy network of veins. / Photo Credit: Wisconsin DNR

Adults emerge in southern Wisconsin in early July and can be found in northern regions shortly thereafter. In much of southern Wisconsin, the beetles have been present for quite a few years, and the population has stabilized at a level that is expected to continue each year.

In the north, Japanese beetle populations are still building up – for the first time, in some areas – and are expected to continue to build and spread.

Some people mistake multicolored Asian lady beetles for Japanese beetles, but they are distinctly different species.

While Japanese beetles typically don’t kill trees, their defoliation may make trees more vulnerable to other stressors. When birch trees are stressed by defoliation, they may be attacked by a native beetle called bronze birch borer, which can kill the top of the tree or even the entire tree.

Biological control can help to control beetles in the forest. The Winsome fly, which has been expanding its range into Wisconsin, lays its eggs on adult Japanese beetles. Their eggs look like small white dots near the heads of the beetles. When the eggs hatch, the fly larvae bore into the Japanese beetle to feed within the beetle and may kill the beetle. If you see fly eggs attached to Japanese beetles and can report them in the iNaturalist app, please take a moment to do so. You can see some great pictures of Winsome fly in iNaturalist.

Winsome fly eggs can be identified as white ovals near the heads of beetles. There may be one or several. / Photo Credit: Wisconsin DNR

Visit the University of Wisconsin-Madison Extension’s Japanese beetle webpage for information on control measures for adults and grubs. If you have questions or concerns about Japanese beetles on your ornamental trees and yard trees, visit Extension’s Ask A Gardener page to submit your question using the provided form.