Extension Educator, Horticulture
They're back and chewing on a rose near you. It's time to scout for Japanese beetles. Adult beetles are attracted to other beetles and damaged leaves. Therefore reducing feeding damage now can result in less damage later.
Japanese beetle adults are one quarter to one half inch long with copper colored wing covers, shiny metallic green head and prominent white tufts of hair along their sides. Kind of attractive in a bugly sort of way.
They also have the munchies for your favorite rose, linden, grape, raspberry and some 350 different plants. They generally do not feed on dogwood, forsythia, holly, lilac, evergreens and hosta.
Japanese beetle adults feed on flowers and fruits and skeletonize leaves by eating all the leaf tissue and leaving the veins. Adults are most active from 9 a.m. - 3 p.m. on warm, clear summer days. Feeding is normally in the upper portions of plants in the sun.
Adults are present in high numbers until mid August. After mating, females lay eggs in turf. Female beetles prefer to lay eggs in moist, actively growing turf. Therefore stopping or reducing irrigation during July results in reduced egg-laying, with fewer grubs. Grubs feed on plant roots until cold weather drives them deeper into the soil. Adults emerge in summer of the following year.
The bacterial control, milky spore, sold as Doom or Grub Attack is commonly recommended to control Japanese beetle grubs. However it only controls Japanese beetle grubs and not our predominate lawn grub, annual white grub. Common lawn grub controls such as halofenozide (GrubEx), imidacloprid (Merit) and beneficial nematodes will control several species of beetle grubs.
However controlling Japanese beetle grubs does not significantly reduce the number of adult beetles in your yard the following year, according to Phil Nixon U of I Extension entomologist. The beetles are good fliers and easily fly a couple miles in a single flight. They may travel 10 to 15 miles from where they lived as larvae.
Generally pesticide sprays of cabaryl (Sevin) and synthetic pyrethroids such as cyfluthrin will control the adults for up to two weeks. However, Sevin is toxic to bees and other beneficials. Imidacloprid is also sold as a systemic to kill adult beetles in trees. For best effectiveness it should be applied in April or May when the trees are actively growing. Also with imidacloprid the beetles must feed to ingest the insecticide so some leaf damage will still occur.
Picking them off by hand in early morning is another alternative. When disturbed, beetles fold their legs and drop to the ground. Hold a bucket containing rubbing alcohol or soapy water below the infested leaves. Move the plant and the beetles will drop to their death.
Japanese beetle traps are not recommended where a large beetle population exists. It has been shown repeatedly that the use of these traps results in increased plant damage compared to not using the traps.
A number of birds such as grackles, cardinals and meadowlarks feed on adult beetles. Two native predator insects and a couple of introduced parasites may help to keep Japanese beetle populations in check. Protect natural enemies by keeping the use of conventional pesticides to a minimum.
Although damage looks devastating, Japanese beetle feeding rarely kills woody plants. Therefore, confine control of beetles to plants in important landscape locations or plants of particular value.