Japanese beetles have started showing up in gardens; the first discoveries of them were made in late June. It does not appear the late spring slowed down their emergence too much as the first week of July is about when we expect to see these beetles. It will be interesting to see how abundant they are this year. Because they were well insulated under the snow, it is not expected that last winter's cold weather had much effect on their populations.
However, a factor that does impact their relative numbers is the soil moisture at the time eggs are laid. The eggs and young white grubs are particularly susceptible to dry soil conditions. Eggs are laid soon after adults start to emerge, generally early to mid-July. However, older grubs are much more tolerant of dry soil. So the number of Japanese beetles that are present this year is related to soil moisture last summer. Although we have experienced drought in many recent years, 2013 was a fairly wet year. We could expect Japanese beetle numbers to be rebounding in at least in some areas.
Jeff Hahn, Univ. of MN Extension
Photo 1: Handpicking Japanese beetles on daily basis and putting them in soapy water can effectively reduce feeding injury on plants.
The researchers found that physically removing Japanese beetles on a daily basis in general helped to significantly reduce damage on grapes compared to removing no beetles. This is because damaged leaves give off a chemical volatile which attracts Japanese beetles and increases colonization and damage to plants. Keeping numbers of Japanese beetles low helps to reduce the attractiveness of them to plants. The researchers additionally found that the best time to remove Japanese beetles is at 7:00 p.m. (compared to 8:00 a.m. and 2:00 p.m.). This is because Japanese beetle activity peaks in mid-afternoon and feeding continues overnight. Grape leaves damaged overnight were more effective in recruiting new Japanese beetles than freshly injured ones. Reducing Japanese beetles at this time was the most effective time to minimize feeding injury to plants through physical removal.
Jeff Hahn, Univ. of MN Extension
Photo 2: If you have had problems with Japanese beetle grubs, treat them preventatively now.
For those looking for a low impact approach to treating white grubs, consider parasitic nematodes, especially Heterorhabditis species. Apply nematodes late in the evening. Be sure that they are watered in and that the soil is kept moist for at least a week (two to three weeks is even better). Parasitic nematodes are available from garden catalogs or biological control companies. Milky spore disease is also a low impact insecticide; however in research trials it has not been very effective against Japanese beetle grubs.
There are several traditional preventative insecticide options that are very effective. Look for imidacloprid (various trade names) or clothianidin (Green Light Grub Control with Arena), both neonicotinoids, or chlorantraniliprole (Scott's GrubEx) a type of anthranilic diamide. If Japanese white grubs are not treated preventatively now, they can be treated curatively with Trichlorfon (Dylox) or chlorantraniliprole (Scott's GrubEx) until mid to late August. After that, the grubs are generally 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.
For more information see, Japanese beetle management in Minnesota.