Planning for Japanese Beetles
Japanese beetles are a serious pest of turf, trees and ornamental plants. Grubs (the larval form) feed on the roots of grass while adult beetles can feed on over 300 types of plants. Adults emerge from the soil in early July and quickly feed, mate and lay eggs. Feeding is most intense over a 6-week period (July), after which the beetles gradually die off. Over the 6-week period each female can lay a total of 60 eggs or more.
Japanese beetles adults prefer to feed in full sun so they generally start in the upper crown of trees, moving downward as the leaves are consumed. Adult beetles release an attraction pheromone that causes other beetles to aggregate on trees. At dusk, this pheromone is no longer produced and the females fly to turf to lay eggs.
Japanese beetles feed on a wide variety of trees and shrubs in Minnesota. Here are a few of their favorites:
Primary tree favorites = linden, elm, serviceberry
Secondary tree favorites = crabapple, paper birch, river birch, swamp white oak
Primary shrub favorites = roses, chokeberry, grape vine
Skeletonized maple leaf in July from heavy Japanese beetle feeding.
Adults fly long distances to feed on trees so beetle infestations do not necessarily indicate a turf infestation. Foliar spray applications are a combination of two types of insecticides to help control the beetles. A contact insecticide will kill any beetle that is actively feeding a particular tree that has been sprayed. However, since most Japanese beetles are not present at the time of spraying, a systemic insecticide (see pesticide warning below) must also be used. Any Japanese beetle feeding on the leaves following a spray application will ingest the systemic insecticide. Even after trees sprayed, adult beetles can still be seen flying around the property for the entire 6-week period. This is to be expected. Many of these beetles are newly hatched or were feeding on neighboring properties. The actual spray application will only kill 1-5% of the active beetle population on a site. The remaining beetles will be controlled with the systemic insecticide. They must feed on the leaves with the systemic insecticide to be controlled. The overall objective of the spray application will be to protect the trees, not necessarily kill all of the beetles on contact (since they are not there).
Japanese beetle activity in mid-July in Minnesota.
Pesticide Warning
In recent years, many pesticides have been banned from usage against Japanese beetles, and linden trees specifically. Some chemicals can harm bee and pollinators because the systemic chemicals are contained inside linden flowers. Pesticide labels must be read carefully and directions must be followed to avoid any concerns with pollinating insects.
Japanese beetles in the upper crown of little leaf linden trees in mid-summer in Minnesota.
