Do Japanese beetles bug you?
Japanese beetles are 1⁄3-inch long, metallic green and reddish beetles with white tufts of hair around the edge of their reddish wing covers. They have been around about four years and are fast becoming a problem.
Both adult beetles and grubs cause damage.
From late June to early August, adults feed on leaves of hundreds of species of plants. Adults feed on the upper surface of foliage and leave a lace-like skeleton. Defoliation, especially once fruit begins to form, will hurt yield.
The grubs feed below the ground on roots of turfgrass and ornamental plants. This reduces the plant’s ability to take up nutrients and water. First evidence of injury appears as localized patches of pale, dry grass. As the grubs get larger and feeding increases, damaged areas enlarge and combine to a point where the turf can be rolled back like carpet. At this point, it is likely too late to save the grass.
Japanese beetles have one life cycle per year. As soon as the females emerge from the ground they start laying eggs. Eggs hatch in two weeks and begin feeding on roots. Grubs grow quickly and can withstand most conditions. They overwinter in the soil as deep as 20 inches.
Adults: Physical removal and trapping can help for small areas. The mere presence of beetles will attract more of them and traps attract more beetles resulting in more damage.
If you use traps, place them at least 50 feet away from plants you wish to protect. For small plantings, floating row covers will protect plants from the beetles; they should be put in place just after blossoms have fallen to allow pollination.
When using insecticides — always follow label instructions and apply in the afternoon when beetles are most active. However, these may just kill the adults and others will rapidly invade.
Grubs: Withholding irrigation is an option because eggs and young grubs don’t survive well in dry soil conditions. However, moisture can help damaged turf recover from their damage. Unfortunately, not all insecticides perform equally. There has been some promise shown when applying preventative control products prior to the laying of eggs in mid to late June.
This article was condensed from a UW Extension publication authored by R. Chris Williamson, a turf and ornamental specialist in the Department of Entomology at the University of Wisconsin–Madison and the University of Wisconsin–Extension, Cooperative Extension.