Plants damaged by Japanese beetles attract even more beetles

“There are bright-colored beetles here and there in my garden that are feeding on leaves and rose flowers. I think they are Japanese beetles and I would like some suggestions on what to do to protect my plants.”

— Sal Malek, Evanston

The Japanese beetle (Popilia japonica) is approximately half an inch long and has a colorful, metallic green color with coppery wing covers. The beetles feed on the leaf tissue between the veins, which results in a skeletonized leaf. They are active now in Chicago gardens, but I have not seen any yet in my garden. Severely injured leaves turn brown and fall off the plant. Damage done by this insect is distinct and mostly cosmetic, though heavy damage to a plant’s foliage creates stress on the plant.

The adults feed from approximately 9 a.m. to 3 p.m. on warm, sunny days and are active in the garden from approximately mid-June until mid-August. You may also see some stray Japanese beetles in the garden in early September. Normally, they start feeding at the top of the plant and work downward, with a preference for plants in direct sunlight.

Japanese beetle adults feed on more than 300 plant varieties with a preference for plants like linden, elm, rose, raspberry, grape, viburnum, zinnia and Japanese maple. They are attracted to plants that are members of the rose family.

Plants that have been damaged by Japanese beetles attract more of them. A volatile odor is given off by a plant that has beetle damage, which attracts other Japanese beetles. To help reduce damage to your plants, begin monitoring your plants early and remove beetles as they appear. If you can reduce damage in the first couple of weeks, it is likely that you will have less damage overall. Hand-removal of beetles is generally effective on smaller plants.

In the early morning, late afternoon and evening, disturbed beetles fold their legs and drop to the ground. Hold a container of rubbing alcohol (isopropyl alcohol) or soapy water under the beetles and poke them to get them to drop into the container. This is faster than picking them off one by one.

If you are seeing large numbers of Japanese beetles and significant damage on your shrubs, you can use a pyrethrin-based insecticide to spray them. Be sure to follow all label directions and be careful to minimize killing nontarget insects.

Netting can provide more complete protection for small plants, but it is not practical for large numbers of plants. The product you choose must have a fine enough mesh to exclude the beetles while maximizing light transmission. A summer-weight garden fabric used for row covers should work well as a cover. I have never used this technique, and would consider using it only for a very valuable plant.

There are traps available that contain a pheromone attractive to male Japanese beetles and a floral lure attractive to female beetles. These pheromone traps are useful for detecting beetle emergence, but not recommended as a control method. Research shows that beetles are attracted from a considerable distance to areas near the traps but then switch to food plants, which can result in heavier plant damage near traps.

Japanese beetle populations vary from year to year. The beetle has a one-year life cycle but spends most of its life in the soil as a grub. Females lay eggs in the lawn and other grassy areas just under the soil surface in summer. They prefer to lay eggs in turf that is well irrigated. Eggs soon hatch and young larvae begin to feed on roots of nearby grass until cold weather forces them to move deeper into the soil for the winter. As the soil temperatures warm in the spring, the grubs return to the surface to complete their life cycle.

The bacterial milky spore disease, Bacillus popillae, kills Japanese beetle grubs in the soil. The dead grubs then produce bacterial spores that remain in the soil to infect future grubs. Milky spore disease is applied to the soil, but it takes two to three years before spore counts build up enough to become effective.

Treating your lawn may reduce Japanese beetle populations some in your garden in the long term but is not a permanent solution, as they fly in from other yards.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.