This past week I have been asked several questions regarding Japanese beetles. Typically, Japanese beetles emerge around the last week in June. This year, earlier emergence can be attributed to our warmer than average temperatures that are causing growing degree days (GDD) to be accumulated at a quicker rate than most years. Plant and insect development are both correlated with growing degree days. At any rate, expect to see increasing numbers of Japanese beetles in the landscape over the next several weeks.
Japanese beetle adults are known to feed on over 400 species of plants. The larval or grub form of Japanese beetles are known for the damage they cause by feeding on the roots of turf grasses, but they also feed on the roots of ornamental trees and shrubs as well as garden crops. The damage done by adult beetle feeding is described by the term “skeletonizing”. This term is used because the beetles eat the leaf tissue between leaf veins but leave the veins alone. This gives the leaf a lace-like appearance. Leaves that have been fed upon to this degree will wither and die. Adult beetles will also feed on flower buds and fruit.
Often it may seem to the gardener that the home landscape is overrun with Japanese beetle adults and the only control options are to either live with the damage or use a chemical pesticide. There is, however, a non-chemical control option available to the diligent gardener. The first Japanese beetle arrivals to a garden landscape begin to feed and release a congregation pheromone (odor) that is attractive to adults that emerge later. These odors attract additional adults to gather in masses on the plants first selected in the landscape. Also, beetle adults are drawn to plants that have been partially skeletonized because skeletonized leaves release odors that are highly attractive to the adult beetles. Knowing this, if the gardener can eliminate these early arrivals, damage to plants in the landscape can be minimized. This calls for the gardener to perform what some may call a daily Japanese beetle patrol. This involves filling a container with soapy water and then doing a daily walk about the landscape inspecting plants and knocking any beetles that are found into the soapy water. This control option works well as long as the gardener is diligent about the daily beetle patrol.
For those who are unable to control beetles using the non-chemical method, and still want to protect plants that are being eaten by the Japanese beetle adult, a chemical pesticide can be used for control. Be aware that all chemical pesticides carry some degree of toxicity and that it is important to follow label directions.
Outside of the home gardens and flowerbeds, Japanese beetles are also a major pest in agronomic and horticultural crop production. In soybeans, it is important to scout flowering soybean fields for the presence of the Japanese beetle and the extent of defoliation. Estimate the number of beetles per foot of row and examine the plants to determine if any blooms are being destroyed. Estimate the percent defoliation in at least five different areas of the field. Insecticidal treatments should be considered when defoliation reaches 30% before bloom and 20% before bloom & pod fill.
In corn, estimate the number of beetles per ear and then examine the ears to determine the extent of silk clipping. Be sure to look at a representative portion of the field. Populations of Japanese beetles are usually clumped together as a result of the pheromones they emit that cause them to aggregate. There are usually clumps of Japanese beetles near field edges, probably due to beetles moving from areas of emergence to host plants within the area. If sampling only occurs near areas where these clumps of Japanese beetles occur, densities across the field may be overestimated. Even though densities may appear to be extremely high, the average density of beetles may be below levels of economic concern. An insecticidal treatment should be considered during the silking period if there are 3 or more beetles per ear/ silks have been clipped to less than ½ an inch and pollination is less than 50% complete.
In fruit and vegetable crops, Japanese beetle populations can get out of hand very rapidly. Japanese beetles have only one generation per year, but these beetles emerge over a long period from late June through August, and they live for over 30 days. They can feed on the foliage of various fruit and vegetable crops grown in Ohio, causing damage to the plant, and increasing the risk of fungal diseases. Their emergence during mid-summer can also result in their presence during harvest of some fruit/vegetable crops. They are highly mobile insects and can fly into fields from surrounding areas, making control even more challenging.
Frank Becker is an OSU Extension Agriculture and Natural Resources Program Assistant and IPM Program Coordinator. He may be reached at 330-264-8722.
CFAES provides research and related educational programs to clientele on a nondiscriminatory basis. For more information, visit cfaesdiversity.osu.edu.