By: Ron Gumz, Certified Arborist MN-0324A
The Japanese beetle is an exotic insect that is about a half-inch long. It has a green head and thorax along with shiny, copper-colored wings. These insects are just becoming established in southeast Wisconsin and will increase in range and population as time goes on. Having no natural enemies in the U.S. makes it an even more formidable threat.
Both the grub and the adult forms of the Japanese beetle can cause damage to trees. Grubs feed mostly on the roots of grasses, but also on the fine roots of trees. Control of grubs has proven difficult and not very effective. The grub stage has a hard time surviving in dry ground conditions, preferring instead, the irrigated turf of golf courses and well-watered lawns. As a result, their populations tend to be higher near those types of areas.
Adults cause the most visible damage. When beetle populations are high and trees are not protected, their feeding can cause the complete defoliation of a tree. Rose gardeners may have been the first to recognize the effects of Japanese beetles since roses are one of the preferred plants of the adult. Other favorites of the beetle are lindens, birches, crabapples, Japanese and Norway maples, and mountain ash. Japanese beetles also favor stressed trees of any type. The adults feed on the upper surface of the leaf between the leaf veins, leaving behind a skeleton of a leaf. This takes away the food production capability of the leaves and creates further stress for a tree.
Adult Japanese beetles are most active in warm, sunny weather, during the mid-day hours, and can readily be seen flying during the day. This summer has been cooler than normal, and still some areas had severe problems because of Japanese beetle feeding. High humidity retards their flight, but that only encourages them to feed more heavily on the trees they are already on. Feeding adults release a chemical that attracts other beetles to the target plant, causing further destruction to individual trees.
There is hope for your trees in the campaign against Japanese beetles. A fall application of a soil-injected systemic insecticide can provide a defense strategy for your trees. This application will help protect your trees from feeding damage the following year. You may still see minor feeding damage on the newest growth, but the trees are generally protected well enough to stay healthy. If beetle populations are very high, a summertime spray can be used for further control of the adult population.
It can be very discouraging and frustrating to see your yard that you have worked so hard to create, be devastated by an insect such as the Japanese beetle. The time to treat is here! Please call your Wachtel Tree Science Certified Arborist to treat for these little green insects this fall.
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