What are these beetles?
Adult Japanese beetles are 3/8-inch long, metallic-green beetles with copper-brown wing covers and can feed on about 300 species of plants ranging from roses to poison ivy. Some favourite plants include Japanese maples, hollyhocks, black walnut, crabapple, sassafras, and grapes. The adults spread by flying and can travel up to 5 miles in one season.
Life cycle
Adult beetles emerge, mate, and lay eggs from late June until early August. The eggs hatch in about two weeks and the tiny grubs grow quickly. Japanese beetles spend approximately 10 months of the year in the soil as a grub. These grubs are C-shaped, off-white in color with a dark head. They prefer to feed on the roots of living plants, especially turf. In grassy areas, yellowing patches of sod usually appear in late August and September, when the grubs are vigorously feeding and the turf is otherwise water-stressed. In October or November, when soil temperatures begin to cool, the grubs stop feeding and move deeper into the soil, where they spend the winter. They return to the root zone and resume feeding early the following spring.
What to do
Hand-picking the adults is one method to control Japanese beetles but is often not feasible if you have a fairly large garden. In such cases, it's much easier to control the grubs than to try to attack the adults. Check your lawn and garden for grub numbers. Just a couple grubs per square foot are not a problem to an otherwise healthy garden. Ten or more per square foot are necessary to justify treatment.
Predatory nematodes are available for use in Canada and the US as a biological control for Japanese beetles. The use of these nematodes requires that the soil be kept moist and it is very important that the application instructions for this product be followed closely, as nematodes are living organisms.
Treat the entire garden. Do not attempt to control Japanese beetles by spot applications. Water the garden thoroughly after nematode application to help them penetrate deeply into the soil.
When to treat the garden?
The younger the grubs are, the easier they are to control. The best time to apply grub control measures is from mid-July to August when the grubs are small and near the soil surface. Although treatments can be made after this time, grubs will be more difficult to kill (because of their larger size). The second best time is March to April when the grubs are once again near the soil surface but a little larger.
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