Many homeowners, tree care companies and landscapers are looking ahead and worrying about defoliation of trees by the Japanese beetle adults again this summer. The Japanese beetle is "the worst landscape insect pest" in much of the eastern USA where it is established. The adults feed on the foliage flowers and fruits of over 350 types of plants. Favored hosts include linden trees, grapes and roses. Foliage is consumed by eating the tissue between the veins, a type of feeding called skeletonizing. Flowers and fruits are devoured completely, often by a horde of a dozen or more beetles at a time.
Eliminating Japanese beetles is not possible though over time the infestation does moderate. Populations (and damage to plants) will not always be as high as it is at the peak of an infestation 5 to 8 years after the first discovery in an area. Japanese beetles are at different levels around the state and within a community. Some properties do not yet have Japanese beetles, others are experiencing the frustration of the peak explosion, and others have seen the worst of the damage diminish as populations moderated. In other words, time will help to some extent, though experience in the eastern USA, where Japanese beetles have been present for nearly 100 years indicates they never completely go away and there will continue to be spotty outbreaks of severe defoliation.
Management options are as follows:
1. If possible tolerate the defoliation of trees. Defoliation of linden, crabapple and other trees is a temporary leaf loss. Defoliation makes trees unsightly but it does not mean the tree is dead. Healthy, well-established trees tolerate defoliation.
2. Screening, handpicking and spraying infested foliage with contact insecticides works for small plants (roses, shrubs, etc.). Young, newly-planted trees may benefit from protective actions but larger trees will survive without treatment. Using insecticides to keep the green leaves on trees requires repeated, thorough applications (due to the short residual of registered products; 1 to 3 weeks protection of foliage) for the duration of the beetle emergence period (late June to September). Insecticide labels prohibit spraying plants that are in bloom.
Contact insecticides for adult Japanese beetle control include carbaryl and pyrethroids (bifenthrin, cyfluthrin, deltamethrin, esfenvalerate, permethrin and lambda cyhalothrin). Note that carbaryl and some pyrethroids are toxic to bees and extra caution is required. Botanical alternatives such as Neem and pyrethrin products may provide 3-4 days of feeding deterrence. Never spray an insecticide on blooming plants or when bees are foraging or under windy conditions.
3. Soil-applied systemic insecticides such as imidacloprid were used in the past to protect tree foliage when by application prior to the arrival of the beetles (it takes time for the insecticide to make its way up to the foliage). However, we do not recommend soil-applied systemic insecticides for this purpose since the insecticides that protect the foliage may also move into the nectar and pollen where they are toxic to honey bees and other pollinators.
For trees that bloom early in the season (crabapples, etc.), it would seem that the risk to pollinators would be low. However, this assumption is being challenged as we learn more about the persistence of these insecticides in the trees, and whether the systemic insecticide is present in flowers the following spring.
Trees that bloom in summer (e.g., lindens) are the most problematic. Applying systemic insecticide before Japanese beetles arrive (and before the blooms appear) puts honey bees and other pollinators at risk. Applying after bloom may be too late for effective defoliator control and the systemic insecticide may be present in flowers the following summer.