Q: In preparation for the inevitable onslaught of Japanese beetles, I was wondering whether you had any thoughts regarding spreading beneficial nematodes? I realize nothing is 100% successful but I hate to harm beneficial creatures.

A: Nematodes are microscopic worms, some of which attack white grubs in the soil. My main concern with using nematodes is that it takes a while to get control in a landscape, perhaps a couple of years. And while all of the grubs in your lawn may eventually be killed, any beetle within 100 yards can easily fly to your susceptible plants. In other words, you may not see any decrease in adult Japanese beetle numbers for a couple of years. And if no one else in the neighborhood is treating for them I’m not sure you’ll EVER see a decrease.

One source for insect-eating nematodes is gardens alive!. The Heterorhabditis nematodes are reputed to be most effective, but must be applied when the grubs are small: in August or September. Be sure to irrigate both before and after application.

See also:
Entomopathogenic Nematode News

Japanese Beetle Flight Activity (PDF)