As the cool, wet summer marches on, the Japanese beetles have emerged in our yard. I check the gardens morning and evening for these unwelcome guests. They are sluggish and clumsy at these times and can be easily picked off and dropped into a container of soapy water.
The Japanese beetle (Popillia japonica), as its name suggests, hails from Japan. It was accidentally introduced into New Jersey in 1916. As with many other non-natives, the lack of natural enemies here and the perfect habitats--lawns, golf courses, parks--has caused the beetle to spread throughout the eastern U.S. It is considered the worst garden pest.
The beetle is easily identified as an adult with its oval-shaped body about 1/2-inch in size, metallic green head and thorax, and coppery elytra or hardened wing covers. The elytra protect the hind wings, which are used for flight. They usually feed in groups. Late June through July is their peak feeding time. During this period females occasionally drop to the ground to lay a few eggs in moist soil; they eventually lay as many as 40 eggs.
These eggs hatch into white grubs, which in turn live in the soil for 10 months, feeding on the roots of grasses, crops, and other plants during the growing season. They stay buried and inactive during winter. When the spring temperature reaches 50F the grubs start munching on roots again. If you dig into your lawn or garden you are likely to find at least a few inch-long, creamy white grubs curled into a crescent-shape. They then pupate and emerge as adults in June.
Signs of heavy grub feeding appear as patches of dead grass. As they feed they starve grass of water and the lawn dies from drought-stress. The adults feed on leaves, chewing the plant tissue between the leaf veins, leaving behind a lace-like appearance to the leaf.
Japanese beetles feed on 300 or so different plants, although they have some favorites such as roses, grapes, basswoods, sassafras, Norway maple (itself an invasive plant), hollyhocks, fruit trees. In our yard they are attracted to the beaked hazelnut, comfrey, sugar snap peas, and hollyhocks. I stopped planting hollyhocks because they were so attractive to Japanese beetles.
Mid-summer rains are good for Japanese beetles. Well, this should be a fine year for them, because we have rain. They of course like expansive lawns and golf courses, especially ones that are watered during drought periods.
For most of us, picking off the adults when they are slow (morning and evening) is the best control. The Japanese beetle traps that look like a green bag with a yellow cap hanging in some yards are not effective. They attract hundreds of beetles to the yard and not all are captured so it makes the problem worse. A diverse plant community in your yard helps reduce beetle damage. The grubs are eaten by skunks, moles, and birds, which will appreciate the food source and also like a diverse yard. Reducing the amount of lawn and avoiding excessive watering will also help.