Jeffrey Hahn, Extension Entomologist
Japanese beetles have just recently started to emerge; watch for them in your gardens and landscapes. If you find just a handful of these insects, you can easily tolerate any damage they cause. If higher numbers are found, there are steps you can take to protect your plants from them although some damage is likely to occur regardless of what you do.
One non-chemical method to avoid is traps. They have a floral lure attractive to both sexes and a mating pheromone that draws in just male Japanese beetles. Once deployed, traps can catch what appears to be an impressive number of insects. However, in areas where Japanese beetles are common, this is a drop in the bucket compared to what is actually present. Research from the University of Kentucky has shown that these traps actually attract more Japanese beetles than they capture; often plants in the area actually suffer more damage than without the traps.
If you are interested in using an insecticide, consider a low impact product like Neem or pyrethrins (containing PBO). However, these products are generally not very effective against large numbers of Japanese beetles. If you would like to use a product with a longer residual, consider a pyrethroid, such as permethrin, lambda cyhalothrin, or cyfluthrin. Another option is carbaryl (Sevin). Depending on the Japanese beetle numbers, you may need to make more than one application. Be careful not to apply one of these insecticides when bees are active.
Photo 3: Don't use Japanese beetle traps. You will probably draw more beetles into your garden than what you actually catch.
Another option is the use of imidacloprid, a type of systemic insecticide (dinotefuran is a similar systemic insecticide but is less effective against Japanese beetles). It's easy to apply and is long lasting so only one application during the summer is necessary. It does not kill Japanese beetles quickly but it does cause them to stop feeding, then they die a little later. One important drawback of these products is they are very toxic to bees. Avoid treating plants, like linden and honeylocust, which are attractive to bees. It doesn't matter that the plants are not flowering at the time of application as these insecticides will be active up to a year. Another important consideration is that it takes some time for imidacloprid to be translocated in trees, up to three to four weeks for a large tree. You have to think ahead if you want to use this product.
Another option is to have your trees sprayed with chlorantraniliprole (Acelepryn). This insecticide is effective against Japanese beetles, is long lasting and has low impact on bees. It is only available to licensed applicators so you would need to contact a landscape company for this treatment.
While you're dealing with adult Japanese beetles munching on your garden and landscape plants, you might also have to worry about immature grubs in the soil feeding on the roots of turfgrass. If you experienced problems with Japanese beetle grubs last year, you can expect to have problems with them in your lawn again this year. Now is a good time to use a preventative insecticide, just as the adults are starting to become active. 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.
Parasitic nematodes, especially Heterorhabditis species, can be an effective, low impact treatment. Apply nematodes late in the evening. It is important that they are watered in and that the soil is kept moist for at least a week (two to three weeks is even better). Nematodes are typically mailed ordered from garden catalogs or biological control companies. Milky spore disease is a common and familiar treatment. However, it isn't very effective against Japanese beetle grubs. There are several traditional preventative insecticide options that are very effective. Look for imidacloprid (various trade names), chlorantraniliprole (Scott's GrubEx), or clothianidin (Green Light Grub Control with Arena).
Photo 4: The smaller Japanese beetle grubs are, the easier they are to kill. By late August they will be too large to kill very easily.
As the grubs get older they are less affected by preventative insecticides. It is still possible to control them with a curative insecticide. Trichlorfon (Dylox) and chlorantraniliprole (Scott's GrubEx) are effective curative insecticides. You can effectively treat Japanese beetles until about mid to late August. By then, the grubs are getting too large to manage very well with any insecticide. Remember to only treat the grubs if you are experiencing problems in your lawn. It is not effective to control grubs to reduce the number of adults that are seeing. Adult beetles are good fliers and can easily fly into your yard from the surrounding neighborhood.
The University of Minnesota Extension and the Minnesota Department of Agriculture are conducting a survey to track where Japanese beetles are found in Minnesota. If you find Japanese beetles, contact Jeff Hahn, firstname.lastname@example.org to report it. Please include a digital picture when you e-mail your report.