How to combat Japanese beetle infestations

It's prime garden time — and that means it's also time when certain pests wreak havoc on our garden beds and lawns.

Japanese beetles have become an unwelcome bunch in late June and early July, according to Shane Bugeja of the University of Minnesota Extension Education for Blue Earth and Le Sueur Counties. The beetles do a number on lawns during their grub stage, when they feed off the roots of grass. When the insects become adults, they feed off flowers, fruits, vegetables, trees and shrubs.

"A lot of times Japanese beetle damage is more annoying than fatal by itself, so that's important to note," Bugeja said. "But then if you have this extra stress with droughts and then you have Japanese beetles feeding on it, it could be pretty bad for a plant."

Japanese beetles were first discovered on the East Coast of the United States in 1916, according to the Minnesota Department of Natural Resources. It's only fairly recently that they've become more prolific in Minnesota.

Adult beetles, active now, can fly several miles and spread from area to area. The Minnesota Department of Agriculture has identified the Twin Cities, Mankato, Albert Lea and Rochester as the main population centers for the pests in the state.

"A lot of those big population centers are where those insects have been established," Bugeja said. "But you'll start to see more confirmed sightings in the outskirts, the rural areas."

Adult beetles lay eggs on grass and other turf covers. The grubs hatch and burrow into the grass. In winter, they dig deeper into the soil before reemerging as adult beetles.

The adults, active now, are easily visible: small beetles with metallic green and copper-colored wing covers.

The simplest way to go after them is to try to submerge them in soapy water. Bugeja recommends going out early.

"Let's just say they're a little sluggish in the morning so they're easier to get in the water," Bugeja said. "You can lightly tap the plant and get them in the soapy water."

Research has shown that traps sold at stores actually attract beetles to your yard. And some insecticides are toxic to pollinators, so Bugeja favors physical removal.

Another way to deter the beetles is to be strategic about what you plant — or, rather, don't plant — in your yard.

As far as preventing lawn damage from Japanese beetle grubs, using an insecticide in June or early July could help. However, the university recommends a "wait and see" approach before treating your lawn.

Evidence of grubs — yellowing or browning of grass or dying patches that pull easily — usually appears in late July through September. Once the grubs have stopped feeding and started to move downward in late fall, insecticides are not effective against them, according to the university's website.

The best deterrent is to maintain a healthy lawn, which includes increasing mowing height by 1 inch in midsummer so grass can better tolerate stress from heat and winds that can dry out lawns.