Japanese beetles also were present during the last full week of June in central Illinois and at The Morton Arboretum in northern Illinois. We usually expect them to appear in early July, so they are somewhat early. These beetles tunnel into the soil to lay their eggs, which will hatch into white grubs. These white grubs will eat turf roots and cause turf dieback in the fall. Japanese beetles appear to need damper soil than masked chafers to survive.
Unlike masked chafers, Japanese beetles feed as adults, typically being present for about six weeks. They feed on the upper leaf surface, eating through the epidermis and mesophyll, leaving the lower leaf surface (epidermis) intact. This lower surface is initially light-colored, but soon dries and turns brown. Japanese beetles will feed on more than 100 plants, with favorites including smartweed, willow, linden, rose, buckeye, birch, crabapple, apple, cherry, hazelnut, currant, grape, and raspberry.
They tend to feed on the upper parts of plants, causing the upper third or more of favored trees to be heavily damaged and eventually defoliated as damaged leaves drop. This tendency to feed at the top of trees allows one to accept damage on tall trees without it being very obvious to the general public.
Traps are available that will attract male beetles to a pheromone and female beetles to a floral scent. Research has shown that these will attract beetles from a considerable distance outside the typical residential landscape, but many of these attracted beetles will not be caught in the trap. This results in more beetle damage in areas that have traps than in areas that do not.
Adult Japanese beetles are difficult to control. Carbaryl (sold as Sevin) sprayed on foliage will provide protection for about a week. Synthetic pyrethroids, such as bifenthrin (Talstar), cyfluthrin (Tempo), lambda-cyhalothrin (Scimitar), and permethrin (Astro), will provide control for ten days to two weeks. Azadirachtin is marketed to homeowners as a Japanese beetle repellent. However, casual tests by the author and others have not shown dramatic effects.
The author and others have found that hand-picking the beetles every couple of days is effective. When disturbed, the beetles fold their legs and drop to the ground. Hold a can or jar containing rubbing alcohol or water with dishwashing detergent below the foliage; the beetles will drop into the container and be killed. Although this method is too labor intensive for commercial situations, home gardeners may find it useful.
Because Japanese beetle adults feed on many plant species and require multiple treatments for effective control, we recommend that professional landscapers and others avoid spraying most of the landscape. Although Japanese beetles cause obvious aesthetic damage, they are unlikely to cause dieback or death to healthy ornamentals. We suggest that you select for multiple treatments those susceptible plants that are focal points of the landscape, such as roses and crabapples at building front entrances, or small lindens in front yards. Large trees and ornamentals along back property lines can usually be left untreated. Of course, the decision of whether or not to treat a plant requires consultation with the client, who may be disinclined to pay for multiple sprayings of large trees and ornamentals in less significant parts of the landscape.