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Gardening Columns

Japanese Beetles

First published on on August 13, 2009, by Suzanne DeJohn

Japanese beetles are difficult to control. Unlike many garden pests that come in, do their damage, and then leave, Japanese beetles emerge in June and hang around for months. And while many pests are host-specific, attacking just one type of plant, Japanese beetles feed on a variety of ornamentals and edibles. Finally, the beetles are more mobile than many pests, and can fly in from surrounding areas, making neighborhood control efforts necessary. The good news is that they rarely do permanent damage to plants, except perhaps when they entirely defoliate young trees. 

For small infestations, hand-picking may provide adequate control. Visit your garden in the cool of the morning, and knock the sluggish beetles into a can of soapy water. Larger infestations call for more drastic controls.

Do Traps Work?
Japanese beetle traps hanging from trees are a common sight in backyards, but do they really work? Yes and no. Most traps are baited with pheromones and/or food odors that the beetles find highly attractive. So yes, the traps do catch some beetles. However, the traps also attract beetles from surrounding areas, so they act as lures to draw even more beetles into your landscape. Since at least some of the new visitors won%t end up in the traps, the net effect may be more beetles, not fewer. 

Contact Insecticides
Some spray-on insecticides will kill Japanese beetles, but most are broad-spectrum, meaning they%ll kill other insects, including beneficial ones. And because the beetles are large compared to most insects, you need relatively heavy and frequent applications. Organic pesticides containing neem, pyrethrum, clove and peppermint oils, and insecticidal soaps will kill some beetles and repel others, so they can provide short-term control. Spinosad, a new type of organic insecticide made from fermenting a naturally occurring soil bacterium, has shown promise in controlling Japanese beetles with little harm to beneficial organisms. 

Sometimes the simplest solution is the best. Row covers draped over plants and secured at ground level will exclude the beetles, and this may be a good option for protecting young plants or keeping the beetles off ripening fruit. Kaolin clay-based sprays, such as Surround, coat plants with a thin white film that discourages feeding, but it leaves plants unsightly.

Grub Control
Japanese beetles lay their eggs in grassy areas, where the eggs hatch into the familiar white, C-shaped grubs. By targeting control efforts on the grub stage you can reduce the number of beetles, at least early in the growing season. Eventually, beetles from surrounding areas will fly in. Two organic products used to control grubs are milky spore and beneficial nematodes.

Milky spore is a naturally occurring bacterium sold as a powder that you apply to your lawn. The bacteria kill the grubs and the spores multiply inside them, so a single treatment is good for several years. However, milky spore takes a few years to build up in the soil so it's a long-term effort. Beneficial nematodes (microscopic worms) act more quickly to kill grubs, but must be applied yearly. A combination of the two may be a good bet. And an even better bet would be to convince your neighbors to treat their lawns, too.



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