Q: Would spreading nematodes be helpful to combat the coming onslaught of Japanese Beetles? Would nematodes be a better option than Milky Spore? Also, I would like to put a systemic pesticide under my crepe myrtles so they also can fight off the beetles. Is there an ingredient I should look for in a systemic? Would a systemic work for my poor roses?
A: Both Milky Spore and nematodes take as long as a couple of years to get control in a landscape. And while the grubs in the lawn may be killed, any beetle within 100 yards can easily fly to your susceptible plants. Therefore, you may not see a decrease in adult Japanese beetle numbers for a couple of years, and if no one else in the neighborhood is treating for them, you may never see a decrease in their numbers.
The Heterorhabditis nematodes seem to be the most effective, but must be applied when the grubs are small, in August or Sept. Be sure to irrigate both before and after application.
For more information about nematode use, see an article called Using Nematodes to Control Japanese Beetle.