Japanese beetles directly damage landscape plants as adults, while the larvae (grubs) damage turf grass. Adult beetle activity in this area commonly peaks during mid-July through mid-August. Adults feed during the day, consuming flowers and foliage, favoring hot weather, and growing plants in full sun. Adult females feed and lay eggs throughout the summer, laying 40-60 eggs in the soil. Eggs are only 1-3″ in the soil, a relatively shallow depth. The key to egg survival is adequate soil moisture. So, in a wet summer or in lawns irrigated regularly, the population has a greater chance of survival and increases. Conversely, grub survival decreases under conditions of extended drought.
Grubs hatch in 10-12 days and feed on turf grass roots until fall. By late October to November, when soil temperatures drop, grubs cease feeding and move down 6-12″ in the soil. Come spring, grubs move upward and continue to feed on grass roots. Grubs mature, molt, and pupate from late May through June, completing the life cycle. Check out the video below from Penn State University regarding the life cycle of the invasive Japanese beetles.
Control of grubs should be done with a granular insecticide or milky spore treatment. Adult control should be done with a contact or systemic spray. Make sure you follow the directions on the product you buy from your supply store, and it is indeed to be used for Japanese beetle control. Japanese beetle traps containing floral and sex-attractant lures that attract adult beetles are used as monitoring tools. Traps have been misused by the public, who mistakenly believe they control beetles, while in fact, beetles have been shown to land often and feed on plants close to the traps. (Potter, D. 1998).
