I’m having a problem with Japanese beetles. I have beetle traps hanging around my garden and they catch a lot of them, but beetles are still chewing on my wisteria and rose bushes. I don’t want to spray, so every day I hand-pick them off. What else can I do?
Those Japanese beetle traps work by using a chemical that mimics a pheromone produced by the female Japanese beetle to lure the male beetles. You now see how well this works, as you are drawing more and more Japanese beetles into your yard where, on the way to the trap, they stop to dine on your plants.
Extension agents say that if you use these traps, it’s best to place them a good distance from any plants you want to save, so they will be lured away from the plants.
Generally, most experts advise against using them at all.
Good for you for choosing not to spray chemical killers – the honeybees and other pollinators are safe in your landscape. Handpicking the beetles or knocking them off into soapy water is an effective short-term method for reducing the population in your garden. The best time of day may be early morning, when they apparently are more sluggish.
Biological control of Japanese beetles begins with understanding their life cycle. Young grubs hatch in the soil in late summer and feed on roots, then spend winter in the soil. They become active in spring and continue to feed. Adult beetles begin to emerge in June.
You can treat the lawn with milky spore, a bacterium that kills the grubs in the soil. Milky spore builds up in the soil over several years and suppresses the grub population over time — no grubs, no Japanese beetles to hatch and harass your plants.
The University of Tennessee Extension has a publication about controlling Japanese beetles. You can read it here.