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GardenSMART :: Japanese Beetle Facts

Japanese Beetle Facts

By Bayer Advanced
Photo courtesy of Bayer Advanced

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In early summer, little beetles with big appetites make their annual appearance. Whether your landscape passion is a perfect lawn or pretty ornamentals, Japanese beetles are probably not one of your favorite insects. Discover some interesting — and helpful — tidbits about this lawn and garden nemesis.

They Like It Hot

Adult beetles are sunbathers, feeding and mating in clear, warm weather. On sunny days, individual beetles start appearing on plants midmorning. Peak numbers occur mid- to late afternoon. If you’re knocking insects into soapy water, do it twice daily: a little before midday and again by late afternoon — before females fly off to lay eggs.

There’s Never Just One

Feeding beetles release a congregation pheromone that attracts other adults. Wounded leaves and flower petals also release a chemical adult beetles can detect. The result is the insect equivalent of a flash mob. A single beetle doesn’t eat much; group feeding is what really decimates plants.

They Have a Short Life Span

Adult Japanese beetles have a short life span: 30-45 days on average. Females feed, mate and lay eggs — repeating the process every 24 to 48 hours. At each egg laying, female beetles deposit one to five eggs 2 to 4 inches deep in soil.

Water Keeps ‘Em Going

Soil moisture is vital to keep eggs viable and prevent newly hatched grubs from drying out. Lawns with automatic irrigation, including golf courses, typically have higher grub populations than non-irrigated lawns, especially during droughty summers. Mature grubs boast drought tolerance.

Tops in Turf

Japanese beetles are the nation’s No. 1 turf pest. Grubs devour grass roots, making it impossible for grass to absorb water and ultimately killing grass. Grub feeding produces irregular brown spots in a lawn. High numbers attract predators like skunks, moles and armadillos, which dig up turf to unearth grubs. Beetle grubs also feed on other plant roots.

Learn how to deal with lawn grubs, including fall control tips.

Smuggled in Soil

The coppery-green beetles entered the United States in 1916, purportedly hidden in a shipment of irises from Japan. Prior to the insect’s export, it was known to occur only in Japan, where it’s not a major pest problem.

Still Invading

Japanese beetles are still invading the United States. Established populations occur most heavily east of the Rockies and in the Midwest. Small pockets of beetles have been identified on the West Coast. Adult beetles conquer geographical barriers by hitching rides on freight, vehicles or even air cargo. Eggs and grubs travel in soil of nursery stock.

A Strong Flight Risk

Strong flying abilities enable adult beetles to infest areas several miles away from where they emerge from soil. Most typically, however, they fly short distances between feeding and egg-laying areas.

Refined Palates

Japanese beetles feast on more than 400 plant species, but find certain plants tastier than others. Favorites include roses, hibiscus, grapes, raspberries, linden, sassafras, Japanese maple and Norway maple. Plants seldom damaged by beetle feeding include boxwood, red maple, flowering dogwoods, hollies, magnolias and lilacs.

Beetle Impersonators

Several beetles are often confused with Japanese beetles. The list of doppelgangers varies by region, but includes bumble flower beetle, masked chafer beetle, June beetle, green fruit beetle, hoplia beetle and sandhill chafer (sometimes called false Japanese beetle). While all of these adult beetles have distinguishing characteristics, the grubs are nearly identical, requiring entomological expertise to tell them apart.


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