Adult Japanese beetles have emerged in central and southern Illinois and should be appearing soon in northern Illinois. They typically are present until mid-August, feeding on the leaves of rose, linden, crabapple, willow, birch, and many other trees and shrubs, as well as the flowers of rose and other plants. Smartweed is very attractive to them, so they commonly appear there first. They are attracted to the sun, being most numerous on the top of the host plant. They feed through the upper epidermis of the leaf, leaving the lower epidermis intact, which then turns brown, and through the leaf lamina, leaving only the major veins.
Evidence exists to suggest that Japanese beetle adults are attracted to previously damaged foliage. Thus, reducing early feeding damage now can result in much less damage later. Homeowners can prevent heavy damage by hand picking the beetles daily on their prized plants. Soapy water or rubbing alcohol in a jar or can suffices as a killing solution. Place the jar or can below the beetle and disturb it, causing it to fold its legs and drop into the container.
Various insecticides are labeled for the control of adult Japanese beetles. Synthetic pyrethroids are more effective than others. Carbaryl (Sevin) is particularly effective. Recent evidence suggests that Sevin may be mildly systemic, penetrating the leaf tissue and remaining there for a number of days. This frequently results in control lasting 2 weeks or more, much longer than one would expect this chemical to last in the hot summer sun. Synthetic pyrethroids usually provide control for about 2 weeks, as well. Most other labeled insecticidesí effectiveness lasts only a couple of days or so. Azadirachtin (Neem) is marketed in garden centers as a Japanese beetle repellent. Informal testing has not shown it to be effective.
Damage from this insect is primarily aesthetic. Its feeding occurs late enough in the season to cause relatively minor harm to the health of the plant. Leaves produce most of the food for the plant in the first few weeks of the growing season, with production dropping off as the season progresses. For this reason, one can reasonably choose to use insecticide on more obvious plants in the landscape and not treat very large trees or smaller plants in the backyard or other less-noticed landscape areas. Because control involves several insecticide applications, it reduces the amount of insecticide released into the environment and reduces the cost to the client, while still providing an attractive landscape.
Adult Japanese beetles are strong fliers. They commonly fly 1 to 2-3/4 miles in a single flight and are capable of 5-mile flights. These flights allow beetles to travel 10 to 15 miles from where they lived as larvae by the end of the season. Typically, one-third of the adult Japanese beetles fly to a new host each day. For this reason, controlling adult beetles to reduce the larval white grub population feeding later on turf is not effective. Neither is it effective to control the larval grubs to prevent adult damage.
Traps are available that use pheromone and floral lures along with yellow vanes to attract both sexes of adult Japanese beetle. It has been shown repeatedly that the use of these traps where a large beetle population exists results in increased landscape-plant damage compared to not using the traps. Apparently the traps attract beetles from a block or more away but do not lure many of the beetles all the way into them. Instead, the beetles feed on nearby plants and cause additional damage. Regardless, many homeowners use the traps, probably for the satisfaction of knowing that they trapped and killed at least some of the beetles.