Jeffrey Hahn, Asst. Extension Entomologist
The moment many people have been dreading has arrived - Japanese beetle (JB) are starting emerge. It is not necessary to panic as they are not out in force yet. There have been a few individuals that have been found early (normally JB is not out until the first week of July). However, you know the rest are not too far behind. In fact with the recent rains, we could be seeing large numbers will probably emerge within a week or less.
JB is a pest because the adults feed on the leaves and flowers of many plants while the grubs feed on the roots of turf grass. If you have seen JB grub damage in the past, July is a good time to treat your yard. Use a preventative insecticide, like imidacloprid, after you see adults flying, about late June or early July this year. By the time eggs are laid and grubs hatch, about two to three weeks, the insecticide will be taken up by the grass and the young grubs will be exposed to it.
As the grubs get older they are less affected by preventative insecticides. It is still possible to control them with a curative insecticide, such as trichlorfon (e.g. Dylox). You can effectively treat JB with a curative insecticide until about mid-August. Remember to only treat the grubs if you are experiencing problems in your lawn. It is not effective to treat grubs to reduce the number of adults that are seen in your garden. Adult beetles are good fliers and can easily fly into your yard from the surround neighborhood.
Another option is the use of a systemic insecticide, like imidacloprid (various trade names) and dinotefuran (Safari). They are easy to apply and are long lasting. They do not kill JB quickly but they do cause them to stop feeding with death coming later. One important drawback of these products is they are very toxic to bees. Avoid treating plants, like linden and roses, that are very attractive to bees. It doesn't matter that the trees and shrubs are not flowering at the time of application as these insecticides will be active for a year. Another important consideration is that it takes some time, especially for imidacloprid, for the tree to translocate the insecticide (3- 4 weeks for large trees). If you have plants that have been plagued by JB in the past, now would be a good time to treat them with a systemic so the insecticide can protect them before much damage is inflicted.
For more information see Japanese beetle management in Minnesota.