Japanese beetles are small beetles with a shiny bronze-colored shell.
The invasive species was first detected in Wisconsin in the 1960s but didn’t make major population gains until the 1990s.
They’re now fairly well established in the southern part of the state.
Linda Williams is a Wisconsin DNR forest health specialist based in Woodruff.
She says the way a new invasive species like the Japanese Beetle works is the population numbers build (that’s millions and millions of them in this case) before they drop down to level that they will likely maintain in perpetuity.
Right now, the Northwoods is still in the build up phase.
“When Japanese beetle first shows up in an area, the numbers do get very, very high and very concerning for people,” said Williams.
Unlike invasives like Emerald Ash Borer, Japanese beetles aren’t focused on just one species. Adults feed on the flowers and leaves of more than 300 plant species.
In the northern forests, their favorite leaves to munch on appear to be birch, mountain ash and basswood.
Williams says they also will go for young oaks.
“They eat most of the material out from between the veins, and then those leaves curl up and die, and they can drop off the tree,” said Williams.
Healthy, well-established trees can weather the beetles just fine, even for several years in a row.
Younger or recently planted trees may not fare as well.
“In forests, there's not really much that can be done or generally needs to be done about Japanese beetles. Forests typically will have species that Japanese beetle does not like to feed on, mixed in with species that they do like to feed on,” said Williams. “The forest stand is generally going to remain healthy enough that those trees will continue to come back year after year.”
The winsome fly, another invasive species, is a biological control for Japanese beetles that will follow them north and eventually help with population control.
Williams say there are ways for people to remove beetles in their yard:
1. Soapy Water:
If you see a Japanese beetle crawling on trees or plants you can pluck it off and drop it in a bucket of soapy water.
2. Pesticides:
There are some pesticides that can repel them from plants or used on the ground to target the grubs. Willaims urges people to carefully read labels, some will work better than others. She also reminds people keep in mind that many of the flowers that the beetles feed on also support pollinators.
3. Beetle Traps:
You can find beetle traps in stores that use pheromones to attract and trap Japanese beetles. They are not recommended by UW Extension because while they do capture a large number, they may also attract many more adults to the area that are not trapped and can cause additional damage. Williams says if you are going to use them, place that far away from the plants or trees you’re trying to protect and check the traps daily.
“You might actually see more damage in the area right around the trap that you weren't anticipating,” said Williams.
UW Extension has more information and resources about Japanese beetles.
