What is less lovely than a summer blossom? The sight of a Japanese beetle on that blossom, shiny as an oil slick.
“We’re hearing quite a few reports about the beetles this year,” according to Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. The iridescent bugs feed on about 300 different species of ornamental plants, but especially favor roses, grapevines, and crabapple, cherry and linden trees.
The adult beetles eat the tissue between the leaf veins, leaving just the skeleton of a leaf. In the larval stage of their life cycle, they are white grubs that live underground and eat the roots of turf grass. “If you have a lot of grubs, they can eat so many grass roots that you can peel patches of grass up like a carpet,” she said.
The eggs and grubs thrive in moist soil, so rainy springs and overwatered lawns lead to both grub damage and beetle damage.
Adult Japanese beetles have oval, metallic green bodies about 1/4- to 1/2-inch long, with coppery brown wing covers. Originally from Asia, they entered the United States more than a century ago and are now considered an invasive pest.
Here are some tips from the Plant Clinic for coping with Japanese beetles. Learn more at mortonarb.org/japanese-beetles.
Don’t use traps. Japanese beetle traps use sex pheromones to lure beetles, which are trying to find mates. “The problem is that the pheromone in the trap will attract beetles from all over the neighborhood, far more than the trap can hold,” she said.
Handpick or shake beetles from plants. Fill a wide container, such as a quart-sized yogurt tub or small bucket, with soapy water. Either pluck the beetles and drop them into the water or hold the container under an infested leaf or flower and shake the plant so the beetle falls in. The soap ensures that the beetles die.
Try to get them early. When a beetle finds a good food source, it emits pheromones to attract other beetles. “If you can handpick the beetles before they send up their chemical flag, you’ll have fewer beetles,” Yiesla said. Don’t expect to completely eliminate them; more beetles can always fly in.
Go after the grubs. The adult beetles are only active for a few weeks in early summer until they mate. Then the female digs down to lay eggs in the soil during July and August. The eggs soon hatch into grubs, which feed on plants’ roots all through the summer, lie dormant in winter and resume feeding in spring.
Avoid overwatering. Moist soil in lawns also invites beetles to lay eggs, since the moist soil is easy to dig. Both the eggs and grubs thrive in moist soil. Lawns with automatic watering systems are prime grub habitats. “To discourage grubs, turn off the timer and only water when the soil of the lawn is actually dry,” Yiesla said. “Try to avoid watering the lawn at all in July and August, when beetles are laying eggs.”
Choose grub controls carefully. If you have a heavy infestation of grubs, you may be considering a treatment for the lawn. Only some insecticides are effective, and only when the grubs are young, in August and September. Be sure you are using a product labeled for grub control and that you are using it in the right way at the right time. Be aware that applying insecticides to your lawn can also kill beneficial organisms in the soil. Contact the Plant Clinic (mortonarb.org/plant-clinic) for more guidance.
Don’t treat linden trees. It is illegal to use the pesticide imidacloprid to treat linden trees against Japanese beetles. If you are considering applying any other insecticide to protect an especially valuable plant, be aware that insecticides can’t guarantee control. Contact the Plant Clinic before using any insecticide to be sure you are using the right product in the right way.
Consider biological control. You can enlist the help of beneficial nematodes — microscopic wormlike creatures that infest and kill grubs. These nematodes are not always sold in garden stores but are available through mail order and on the internet. Other products contain a species of soil bacteria, Bacillus thuringiensis galleriae, abbreviated Bt, that can be applied to the soil to kill beetle grubs, including Japanese beetles. Both nematodes and bacterial control must be applied in a particular way at a certain time and may take several years to build up enough to be effective. Another bacterial treatment, often called milky spore, is not recommended for the Chicago area because the bacteria don’t reliably survive the winter.
Plant more trees and shrubs. Japanese beetles avoid laying their eggs in the shade, so adding more beautiful, shady plants to your landscape will discourage them.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
