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Japanese beetles...Home, Yard, & Garden Newsletter article from 6-27-16

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Adult Japanese beetles have been reported in southern, central, and northern Illinois. Early control reduces damage through the six weeks that they are actively feeding as beetles are attracted to previous feeding damage.

Japanese beetles 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 feed on more than 100 plants, with favorites including smartweed, willow, linden, rose, buckeye, birch, crabapple, apple, cherry, hazelnut, currant, grape, and raspberry.

Japanese beetle adults on pawpaw.

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 (Sevin), bifenthrin (Talstar), cyfluthrin (Tempo), lambda-cyhalothrin (Scimitar), or permethrin (Astro) foliar sprays provides protection for about two weeks. A single soil application of imidacloprid (Merit, others) is also effective. Do not apply imidacloprid to linden and other hosts that bloom during the Japanese beetle flight season to avoid impacts to pollinating insects.

Hand-picking the beetles every couple of days is effective, but time-consuming. When disturbed, particularly in late afternoon, 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.

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. (Phil Nixon)

Phil Nixon