It is the time of year that adult Japanese beetles appear in Illinois, and they are very numerous around Collinsville in the southern part of the state. They are only present in small numbers in central and northern Illinois, but those numbers will increase as adult emergence continues. Adult Japanese beetles feed on many deciduous trees, shrubs, and vines such as linden, sassafras, Japanese maple, sycamore, Norway maple, birch, elm, Virginia creeper, and grape. Their favorite plants are those in the rose family, which includes rose and crab apple. Japanese beetle adults also feed on flowering herbaceous plants such as zinnia, marigold, and hollyhock. They generally do not feed on dogwood, forsythia, holly, snowberry, or lilac.
Adult Japanese beetles cause plant damage by skeletonizing leaves, removing all leaf tissue between the veins.The leaves then turn brown and fall off. Their feeding damage can reduce a plantís aesthetic quality and subject it to stress. Japanese beetle adults are generally found feeding on plants in groups. They are most active during the warmest parts of the day and prefer feeding on plants that are fully exposed to sunshine. For this reason, adults generally start feeding on the top of plants.
Japanese beetle adults are metallic green, 1/2 inch long, with copper-colored wing covers. Unlike other beetles, they have a series of white hair tufts extruding from the end of the abdomen. Japanese beetles overwinter as late instar larvae (grubs) in the soil, where they feed on plant and turfgrass roots. Adults pupate in late May or early June. They emerge and start feeding from mid- to late June with peak densities occurring around July 1. Adults generally stay around until early to mid-August. They are present for approximately 6 weeks, living from 30 to 45 days. After mating, females lay small groups of eggs below the soil surface. Female beetles can lay between 40 to 60 eggs, which hatch into grubs during early August. Grubs feed on plant and turfgrass roots until the arrival of cold weather, which forces them to migrate deeper down in the soil profile. Japanese beetles are annual white grubs and take one year to complete their life cycle.
Insecticides such as carbaryl (Sevin) and cyfluthrin (Tempo) can be used to manage Japanese beetle adult populations when they are actively feeding. These are both contact insecticides so they must be applied when beetles are numerous and exposed in order to obtain maximum control. Because these insecticides generally last only one week, repeat applications may be necessary. Avoid using Japanese beetle traps as they generally attract more beetles and can result in higher levels of plant damage, especially if susceptible plants are present. Monitoring plants that are highly susceptible to Japanese beetles helps minimize potential damage. In addition, planting resistant varieties of roses and crab apples may result in fewer adults feeding and lead to less aesthetic injury to plants.