Those uninvited summer guests that ate much more than expected are on their way out.
It’s not unusual to spot Japanese beetles in June and July, but the number of them was much higher this summer with outbreaks in Cincinnati, Dayton and Columbus as well as in northeast Ohio, said Joe Boggs, an entomologist with Ohio State University Extension. OSU Extension is the outreach arm of the College of Food, Agricultural, and Environmental Sciences (CFAES) at The Ohio State University.
Though Japanese beetles typically thrive on the leaves of linden trees, grape vines and roses, this summer, they branched out, devouring other plants in Ohio including scotch pine and jewelweed.
“Sometimes during outbreaks, they’ll feed on strange things,” Boggs said.
First introduced into the United States in 1916, Japanese beetles seem to have had a resurgence in Ohio during the last three summers, he said. But the populations still do not come close to what Ohio experienced in the 1960s and 1970s, Boggs said.
“Then there were so many beetles, it drove people out of parks.”
At that time, Japanese beetles had no natural enemies. Since then diseases as well as predators, including parasitoid wasps, have caught up with them and helped curb the number of the pests.
After flying to a plant, Japanese beetles send out pheromones, chemicals that once released into the environment, trigger other Japanese beetles to respond to the apparent message of: Free Food.
Then they’ll swarm a plant, munching until the leaves appear skeletonized.
“They’re almost like toddlers,” said Andy Michel, an entomology associate professor in CFAES. “On any given day, you don’t know what they’re going to like to eat.”
One woman who visited a Dayton suburb and later contacted Michel, observed how virtually all of the linden trees in the area were reduced to lacey leaves and referred to it as a “plague.”
With their iridescent copper bodies and green heads, Japanese beetles can be easy to spot, but hard to get rid of.
Flicking them away with your fingers doesn’t always work because they generally return. About the only way to keep them off plants without using pesticides, is to pull them and dunk them into a container of soapy water, Michel said.
In a field of crops, Japanese beetles most often gravitate to the edges. They don’t generally do enough damage to reduce the yield on soybeans and corn, he said.
On a soybean plant, even if they remove the leaves at the top, as long as sun can filter down to the lower leaves, the plant can still perform photosynthesis and remain healthy. For commercial soybean growers, a pesticide isn’t necessary unless about 30 percent of each leaf is gone on the majority of the plant before it has flowered or 20 percent after it has flowered, Michel said.
A pesticide may also be necessary on corn plants if the beetles eat the silk of corn, leaving less than a half inch, or if the beetles are numerous and feeding while fewer than half of the corn plants have been pollinated, he said.
Whatever damage they may have done, Japanese beetle populations are significantly decreasing now as they tend to do in late July and August.
“So if we have good growing conditions and the plant can put on new growth, you might save yourself a spray,” Michel said.