Japanese Beetles

The Japanese beetle is a major pest of lawns and ornamental plants. Adult beetles feed on more than 275 species of plants, but especially like plants in the rose family. Grubs (larvae) feed on the roots of turfgrasses such as Kentucky bluegrass, fine fescues, ryegrasses, or bentgrasses. Life Cycle and Habits The entire life cycle requires about one year. Japanese beetles are metallic green with coppery-brown hardened front wings, and are just a 1/2 inch long. Beetle emergence begins in mid-June and peaks in early July. Beetles will remain active for about 4-6 weeks. Most of the egg laying occurs in July and tapers off rapidly by the first week in August. Generally, the amount of daily beetle activity will give some indication of grub populations that can be expected later in the summer. Most beetles remain close to the emergence area, and adult females lay 40-60 eggs near the adult feeding site. Beetle eggs hatch in about 10 days and grubs begin feeding on grass roots. Grubs are "C- shaped", creamy colored, with brown heads, 3 pairs of legs and are about 1 inch long when mature. Root loss from grub feeding during August and early September may be so extensive that the turf can be rolled back from the soil like a rug. By mid October, the grubs move down into the soil 6-18 inches and remain inactive until spring. In late March or early April, they return to the surface and briefly feed on roots. During late May, they stop feeding, pupate and emerge as adults in June. Beetle Control Options From a management standpoint, it is important to recognize that both the adults and larvae can cause damage. Moreover, since Japanese beetle adults are capable of flying in from other areas, controlling one life stage will not preclude potential problems with the other. Although several species of birds feed on adult beetles, their efforts are insufficient to prevent injury to ornamental plants, and even the best insect controls will only provide temporary relief when adult populations are heavy. Unfortunately, most pest controls remain effective only for a few days and any new plant growth will not be protected. Due to the continuous migration of beetles, 2 or 3 Ornamental Tree and Shrub spray applications may be needed to prevent serious injury to plants like roses and sand cherry bushes. Control of the grub stage requires a properly timed grub application. Trapping Beetles The commercially available yellow Japanese beetle traps are baited with a feeding attractant and capture large numbers of adults. However, randomly spaced, single traps cannot provide effective control. Frequently, a single trap attracts beetles from the entire neighborhood and concentrates the damage in a small area close to the trap site. Research has demonstrated a 31-40% increase in adjacent plant damage when traps were used close to susceptible plant species.