Don't Confuse Japanese Beetles with Emerald Ash Borer

Japanese beetles join ash borer, long horn as suburban tree scourges

By Marni Pyke | Daily Herald Staff

There's a collective crunching noise in gardens across the region this month. And if you follow that sound to its source, you'll find a copper-winged insect with a green body.

What's this new pestilence to trouble the area?

More emerald ash borers? A resurgence of the Asian long-horned beetle? Gypsy moths?

Relax. It's just the Japanese beetle.

If you were to list all the insect villains that have broken the hearts of tree lovers in the last decade, the Japanese beetle is not among the top evildoers - at least in the suburbs. It's more like an annoying relative who stays forever and empties your refrigerator.

In downstate Illinois, however, the beetle is packing more of a punch, as it devours corn and soybean crops.

For answers on the latest insect plague, we turned to entomologists Philip Nixon and Kevin Steffey with the University of Illinois Extension and Fredric Miller of the Morton Arboretum.

What are these bugs?

Japanese beetles arrived in North America in the early 1900s. They made their first appearance in the Chicago metropolitan region in the 1960s. The sun-loving insects (they lie dormant when it's raining) start at the top of plants and eat their way down, turning the leaves brown. Adults are about the size of a dime, with copper-colored wings, green bodies and white spots.

What local species do they target?

Adult beetles have a fondness for roses as well as marigolds and hollyhocks. Favorites among trees include lindens, crabapple, birches and cherry.

Will they kill plants in my garden?

It's unlikely because Japanese beetles come out later in the summer, after plants have taken much of the nourishment they need from leaves through photosynthesis. It's rare the bugs will kill an established plant, although the munching can take its toll on young ones.

How can I protect my vegetation?

Pick the beetles off by hand in the evening, when they're less likely to fly away or apply insecticides such as Sevin or Bayer Advanced Multi-Insect Killer. But experts advise using bug spray sparingly, targeting heavily infested plants or ones in more public areas. Natural predators, such as skunks, will eat the beetles when they are in the grub stage.

Are there more beetles now than before?

In some areas of the state, experts said the pest is definitely more problematic than it's been before. Reasons could be extensive egg laying last year coupled with heavy snowfalls that insulated the soil and protected the grubs.

What's the life cycle?

One year. Adult Japanese beetles emerge in late June and early July. They live for four to six weeks. Before dying, females lay eggs in the soil, which hatch in July and August as white grubs about an inch long. The grubs feast on roots and are partial to turf, which creates another headache for homeowners. The grubs feed into the fall and spend the winter underground.

How can I learn more?

Visit www.urbanext.uiuc.edu/hortanswers.