An infestation of Japanese beetles can be very disheartening. To deal with these pests it is good to start by knowing a little about the life cycle of a Japanese beetle.
The Japanese beetle starts life as a white grub, living just below the thatch line in your lawn. Adult females lay eggs in summer, which mature into grubs by fall. The grubs go dormant over winter and awaken in spring to continue developing and emerge in the summer as a Japanese beetle.
If you have a large number of grubs under your turf in spring, chances are you are going to have a problem in your garden the following summer.
The grubs are a grayish-white with a brown head and two rows of spines and they usually lay curled in the shape of a 'C'. The adult has a blue-green body and head with copper wings.
One way to detect large grub populations is by monitoring mole, raccoon or blackbird activity. These animals eat white grubs. If you are having problems with moles, you just might have grubs. It is important to note that a low population of grubs is not necessarily a bad thing. Having a few grubs around ensures the continuation of Bacillus popilliae, a disease of Japanese beetles, from year to year. Small quantities of grubs also aid in aerating your lawn.
Japanese beetles cause damage in both the grub stage and as an adult insect. As a grub they eat through the roots of your grass causing large patches of wilted or dead grass. In cases of severe damage you will be able to lift the turf right off the ground where the grubs have eaten through the roots.
Adult Japanese beetles move on from your lawn to close by flower borders and vegetable gardens eating leaves, flowers and fruits. They tend to eat the soft tissue between leaf veins leaving a green skeleton behind, which will eventually fall off the plant.
Milky Spore Disease - Bacillus popilliae, Milky Spore Disease, is a naturally occurring disease of Japanese beetle grubs that is not harmful to humans or other creatures. There are several commercial dusts available. The grubs eat grass roots that have been dusted with the spores, become infected with the disease, die, decompose and then release more the spores into the soil. While this is not usually a quick fix, it can take up to two seasons to significantly reduce populations, once the cycle becomes established it can provide years of protection.
Beneficial Nematodes - Nematodes are microscopic parasitic worms. While this may sound unpleasant there are many types that benefit our environment. Heterorhabditis heliothidis and Steinernema carpocapsae are both parasitic to Japanese beetle grubs but harmless to other plants, insects and animals. Soil moisture is essential for these nematodes to take effect. A film of moisture is needed for nematodes to attack grubs. Water both before and after nematodes are applied to your lawn.
Hand Picking - You can hand pick or even vacuum adult Japanese beetles. This should be done in early morning when the dew on their wings and cooler air make them lethargic. If this is done early enough in the season, before the females have had a chance to lay eggs, you can further reduce the population of grubs in your lawn.
Plant Traps - Japanese beetles are attracted to certain plants. You can use the plants as traps to make hand picking them easier. Some of these plants have the added benefit of being poisonous to the Japanese beetle. Just be aware that what is poisonous to the Japanese beetle is harmful to people and animals as well. Good trap plants are four o'clocks (poisonous), larkspur (poisonous), castor bean (poisonous), borage, marigolds, light colored zinnias, and white roses.
Neem Oil - Neem is an extract from the seeds of neem trees. Since it is derived from a plant, it is biodegradable and breaks down in the soil without harming the environment. Depending on the infestation apply a diluted concentrate about every 3 to 7 days. Saturate the entire plant, and be sure to get the underside of the leaves and the canes. Neem is also effective control against whiteflies, cucumber beetles, aphids and many types of caterpillars.