Adventures in Gardening: Japanese Beetles in America

by David Soper

Gardeners in the eastern half of the U.S. are facing another onslaught of billions of new Japanese beetles attacking their plants and gardens. The voracious insect was accidentally introduced in New Jersey around 1916 and has steadily moved southward and westward since. The USDA attempted to quarantine the insects in the 1970s but that failed.

Adult beetles look for sweet, new grass in sun to lay their eggs. This year, El Nino has supplied just the right amount of rain for a major crop. Under these nearly ideal conditions, beetle eggs will quickly (with two weeks) hatch into grubs. These grubs have voracious appetites. They feed on grass roots (and have been known to completely devastate a yard in just a few days. They continue to feed until winter weather forces dormancy.

Of course, they don't limit themselves to grass. They'll also attack roots of more than 270 different species of trees and shrubs, ornamental plants and flowers. The effect is the stunting the plant's growth, if not death. In about a year, they emerge as hungry adults and the cycle repeats. The adults are gregarious in nature. Mature Japanese beetle adults are often found feeding in masses on flowers, foliage, or fruit of a few plants leaving others nearby uninfested. On most hosts, leaves are skeletonized and mature fruit or flowers are damaged.

In their homeland, native predators seem to keep the numbers in check. Not so here. The beetles resist pesticides and extreme weather conditions. Even if birds could penetrate the tough shells, and many can't, Japanese beetles are not as tasty to birds as are other insects so are avoided.

While traps are often tried as a means to reduce damage, some entomologists question their use. They suspect the baits just serve to attract even more beetles to the garden!

There is a defense that works against the grubs and I strongly advise its use, particularly this year. The USDA has developed Milky Spore Powder. This is the stuff they used on the grounds of the White House and the U.S. Capitol. This is not a chemical pesticide but is a naturally-occurring microscopic bacteria. It is safe to use around people and pets, ponds and water. The great thing about Milky Spores (aside from the fact it works) is that a single treatment can last from 10 to 20 years in a single application. Typically, the powder is applied in a grid pattern when the soil is warm. The grubs are only a few inches beneath the surface, so are easily affected by the Spores.

It is not too hard to find Milky Spores and is a relatively inexpensive product. Gardener's Supply Company carries it in their catalog or you could call 800 801-0061 for yet another source.

Sources: U.S.-Japanese Beetle Survey, St. Gabriel Laboratories and The Center for Integrated Pest Management, North Carolina State University

David Soper, The Garden Guy, writes and lectures on gardening topics. Check out his books at

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