Act fast to eradicate destructive Japanese beetle

They are wretched, awful beasts. They lurk beneath the leaves, chomp casually upon the most perfect flowers. They turn the lush, green leaves of bodacious ornamentals into thin, pathetic lacy networks, remnants of former beauty.

A simple, succinct morsel of advice: If you see them, squish them. Don’t delay.

Japanese beetles have found their way into the gardens of central and coastal Maine over the past few years. For gardeners, this is very bad news, to say the least. Of all horticultural pests, the Japanese beetle is among the most destructive.

The Japanese beetle is perhaps most destructive in its larval stage, as a grub. Skulking beneath the soil, beyond our view, the larvae attack the roots of turf grass and herbaceous plants. In fact, the beetle spends most of its life cycle within 2 inches of the soil surface. Larvae pupate into adults underground, emerging from the soil primarily in late May and June.

The mature adult beetles are easily spotted with their shiny, metallic green head and body and coppery brown wing covers. Tufts of short, white hairs protrude from under the wings. Adult beetles are most active in warm, sunny weather, gathering to feed on the foliage, flowers and fruit of woody ornamentals including the American chestnut, apple, black walnut, Japanese maple, linden, cherry, crab apple and, most disappointingly, the rose.

Adult Japanese beetles can measure up to one-half inch in length. They chew out the plant tissue between veins of flowers and leaves, resulting in lacy skeletal remains. Damage from the destructive pest can be a sore sight. Most established, vigorous plants, however, can withstand the damage of adult beetles, including rather extensive defoliation.

Japanese beetles tend to cluster together while feeding, congregating on individual fruits, flower buds, petals or leaves. For protection they spontaneously fly off or rapidly drop to the ground if threatened. Female beetles produce a pheromone – a sex hormone – that attracts males to them to mate.

According to a publication produced by the University of California, milky spore disease, Bacillus popilliae, is a commercial control used by horticultural growers and landscapers to combat the insect in its larval phase. The microbial insecticide is applied to turf near trees and shrubs in an effort to infect and kill immature beetles before the adults emerge to feed on landscape plants. This method of control is preferred over more harsh, broad-spectrum insecticides because it doesn’t harm earthworms in the soil or harm beneficial insects such as parasitic wasps that attack the Japanese beetle larvae, and thus help naturally control the pest.

If a localized population of Japanese beetles is found in the yard, say, on one particular rose shrub, a simple, low-tech control may be to place a sheet beneath the shrub and shake the stems of the plant until the beetles fall to the ground. Gather the beetles up in the sheet and dump them in a bucket of soapy water. This technique is best undertaken on a cool morning, when the beetles have an inhibited capacity to fly away.

Many home gardeners employ pheromone traps used to trap the adult beetles, but their efficacy is debatable. Pheromone traps appeal to the beetles’ reproductive urges. Adult beetles seek out the source of the pheromone and become trapped. With this in mind, traps should be placed away from susceptible ornamental species, since one could easily lure misguided adults to a food source rather than the trap. As one gardener once said, “It might be better to convince your neighbor to set out a trap. That way, all the beetles from your yard will be lured over to theirs!”

While that particular technique may be questionable – albeit amusing -whatever the mode of attack on the Japanese beetle, it’s likely worthwhile.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, RR1, Box 2120, Montville 04941, or e-mail them to dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number.