July 28, 2002

Over the past couple of weeks I have been answering a lot of questions from neighbors and customers regarding Japanese beetles. This summer the beetles are numerous and feeding on many of their favorite plants: roses, raspberries, cherry and plum trees. They also have a liking for basil. Telltale signs include large holes in leaves and flower buds. The rose flowers may not open properly if at all after the beetles have eaten away part of the buds.
The beetles are easy to find on roses. They usually can be found in the petals of the flowers, sometimes several of them together. On trees, however the damage is usually evident but the beetles may or may not be found. The leaves will have holes and very often they are completely skeletized with only the veins remaining.
Insect pest populations vary from year to year for several different reasons. Periods of drought or heavy rains at certain stages of some insect's life cycles can cause populations to increase rapidly or decline. Temperature is another factor. Some insects will be high in number after a mild winter or relatively low in number after a severely cold one. And there are insects such as the cicada that are cyclical, having large broods every so many years.
Plants that are under stress are prime targets for insect attack. The hot dry spring and summer we are experiencing this year has most of the trees and shrubs under stress. Early in the season aphids and caterpillar activity was high. Now beetles and spider mites are the common pests. Bees and wasps are also very active searching for nectar and water.
Japanese beetles are difficult to control in their adult stage. The recommendation is to use and insecticide containing carbaryl- the active ingredient in the product called "Sevin". They are much easier to eliminate in the grub stage. As soon as the beetles have eaten their full, which will be in the next couple of weeks, they will mate and begin laying eggs in turf. After the eggs hatch, tiny grubs begin to burrow into the soil. These grubs will feed on roots under the soil. As the weather turns colder the grubs migrate deeper into the soil to escape freezing temperatures. When the weather begins to warm in the spring the grubs begin their migration back to the surface. After feeding some more they then pupate, later emerge as flying adult beetles and start the cycle once again.
By applying grub control on the lawn in either spring or early fall, the beetle populations are more easily controlled. The grubs are small, soft-bodied larva that are easy to kill. There are various products on the market that will kill the grubs. Granular forms that can be applied with a spreader are preferred. They are available with or without fertilizer. Diazanon, a popular grub control for many years, is no longer being manufactured. It was taken off the market by federal regulation last year. However it is still available in most stores until 2003. Whatever product you choose to use, always follow the application directions carefully and keep children and pets off the treated areas for several days until the insecticide has been watered into the soil.
Another popular product for controlling these pests is the Japanese beetle trap. Traps such as these were originally used by researchers to monitor insect pest populations. The trap contains a lure, usually a pherenome(sex hormone) to attract the insect to the trap. The beetle trap is capable of attracting beetles from up to a half of a mile away- much more than would ever naturally come to your yard! I always advise against using the trap for that reason.
The Japanese beetles can also be controlled organically without the worry of exposure to harmful chemicals. Hand picking the beetles from the infested plants is considered to be as effective as spraying with insecticide. Early morning and at dusk the beetles are quite sluggish and easy to pick off plants. Carry a container that is about half full with soapy water or vegetable oil and drop the beetles into it. When you are finished collecting, cover the container, give it a good shake and toss it into the trash. Do this several times while the beetles are active. This method may not be for the squeamish, but often a curious child can be enlisted to do the job. You will notice a dramatic decrease in the number of beetles in your garden next year.
Beth Nardone, botanist, is a resident of Hillsborough and the nursery manager at the Belle Mead Co-Op. Questions and comments can be sent to bethtanical@rcn.com