It’s time to keep an eye out for Japanese beetles, one of the most easily recognized and destructive garden pests. In June and July, grubs in the soil of lawns turn into adult beetles that start feeding on foliage.
“Japanese beetles are double trouble,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. They do damage both in the grub stage, when they eat the roots of grass plants, and at the adult beetle stage, when they feed on many plants’ leaves.
Adult beetles feed on about 300 different species of ornamental plants, but especially favor roses, grapevines, and crabapple, cherry and linden trees. They eat the tissue between the leaf veins, leaving just the skeleton of a leaf.
Japanese beetles have oval, metallic green bodies about ¼ to ½ long, with coppery brown wing covers. “The first adults to emerge feed at the tops of trees, so it’s hard to spot them,” Yiesla said. “Often people don’t realize they’re there until they attack the roses.”
How can you battle the beetles? Here are some tips from the Plant Clinic. Learn more at mortonarb.org/japanese-beetles.
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Handpick beetles from plants. “It’s easiest to catch them by placing a soapy water filled container directly under the leaf that they are chewing on and then shaking the leaf so they drop into the water,” Yiesla said. The soap ensures that the beetles die.
Try to get them early. When a beetle finds a good food source, it emits a chemical pheromone that attracts other beetles. (The beetles like crowds because they are trying to find mates.) “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.
Don’t use traps. Japanese beetle traps use the pheromone to lure beetles. “The problem is that it will attract beetles from all over the neighborhood, far more than the trap can contain,” she said. Go after the grubs. The adult beetles are only active for a few weeks, until they mate and 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. “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.
Avoid overwatering. The eggs and grubs need moisture, so they will thrive in lawns that are well-watered. Lawns with automatic watering systems are prime grub habitats. Moist soil in lawns also invites beetles to lay eggs, since the moist soil is easy to dig. “To discourage grubs, don’t water the lawn in July and August, when beetles are laying eggs,” Yiesla said.
Don’t treat linden trees. It is now illegal to use the pesticide imidacloprid to treat linden trees against Japanese beetles, which was formerly a common practice. 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 (mortonarb.org/plant-clinic) before using any insecticide to be sure you are using the right product.
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.
Consider biological control. You can enlist the help of beneficial nematodes — microscopic wormlike creatures that infest and kill grubs. Beneficial nematodes are not always sold in garden stores, but are available through mail order and on the internet. They must be applied in a particular way when the grubs are newly hatched. A bacterial treatment often called milky spore is not recommended for the Chicago area. Contact the Plant Clinic for more guidance.
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.