How to get the better of beetles

Be on the lookout: A living jewel could be feeding on your plants soon.

With their shiny emerald green coloring, Japanese beetles are beauties -- as beetles go. But they also are voracious. And, unlike emerald ash borers, Japanese beetles are already well established in several areas of Minnesota.

Most of us don't see Japanese beetles when they emerge from lawns in late June or early July. In fact, we often don't notice them until we see the adult beetles feeding on grapes, roses or any of the more than 300 plants that these insects love. Some gardeners don't notice the beetles at all. But it's hard to miss the damage they do: Japanese beetles can leave large holes in leaves or eat entire leaves or leaf clusters. And, if they defoliate a plant for a few years, they can kill it.

Know your beetle

This beetle is about 1/2 inch long with shiny emerald green on its upper back, lustrous brown wing covers on its central back and distinctive white markings. (There are other beetles similar in size, but they lack the markings on their sides and back.)

Vicious cycle

The adult beetles feed on garden plants in July and August, then lay eggs in the lawn, which grow into large white grubs. The grubs feed deep on the roots of grass in the fall, then again the following spring. In late June or early July, the grubs pupate into adult beetles, which fly out of grass and start feeding on garden plants once again.

It's important to note that the adults may or may not lay their eggs near the plants they've been feeding on. Their grubs require healthy grass to survive. If the grass is thin or weak, its unlikely to harbor grubs.

Battling beetles

Your best bet is to hand-pick adult beetles and drop them in soapy water to kill them. The best time to pick is early morning, when the beetles are slower-moving and likely to be found piled up (probably mating) on their favorite plants. The adults typically are found through July and August. After they lay their eggs, they die.

Pesticides aren't very effective against Japanese beetles because the beetles can just fly away and feed on other plants. If you can't stand to hand-pick, you could try neem, a low-impact biopesticide that alters the beetle's behavior and causes it to stop feeding.

You also can go after the grubs or larvae if you know they are living in your lawn. (There are insecticides designed to kill grubs in the grass.) However, most gardens have adults, not the larva. Pheromone traps are good at attracting the beetles but have not shown to reduce the numbers from a specific property.

A good defense is an early offense. If you see lots of Japanese beetles this summer, watch your lawn and look for beetles emerging next summer. Hand-pick the beetles as soon as you see them. With a bit of diligence, you can prevent Japanese beetles from damaging your favorite plants.

Mary Hockenberry Meyer is a professor and Extension Horticulturist with the University of Minnesota.