Most landscapes are affected by Japanese beetles to some extent. They can be voracious garden pests, chewing their way through the leaves and flowers of hundreds of varieties of common landscape plants. The tell-tale damage is small holes that eventually result in “skeletonization” of the leaf, leaving only the leaf veins (see picture). Additionally, the juvenile form (grubs) of Japanese beetles cause lawn damage.
Severity of Japanese beetle populations varies from year to year, from barely noticeable to quite severe. There are a few methods that you can use to treat Japanese beetles, and with careful management you can even begin to reduce their populations in your landscape over time.
If you observe just a few Japanese beetles on your plants, a chemical-free option is to simply knock them into a container of soapy water, then dispose of them.
To reduce feeding damage to your garden plants from adult Japanese beetles, spray affected plants with Bonide Eight (permethrin) or Captain Jack’s Dead Bug Brew (spinosad, a naturally occurring insect toxin). You can spray only the plants with visible feeding damage since despite being voracious, Japanese beetles are picky and generally stick to the plants they prefer. Because Japanese beetles eat the entire leaf, spraying the undersides of the leaves is not as important as it is when treating other pests. Repeat applications can be made in about a week if needed (always read and follow directions on all pesticides).
To control grubs, and over time reduce adult populations of Japanese beetles, apply milky spore to your lawn. The warm days of late summer (August-September) when the grubs are feeding is the ideal time to apply milky spore, though it can be effectively applied almost any time of year (read and follow the label directions for application).
As the milky spore kills the grubs, more of the spores are released into the soil by the dying grubs. After a few years, your lawn will be fully inoculated and the effects can last for up to 20 years, with no reapplication needed. Even better, milky spore affects only Japanese beetle grubs and is completely harmless to people, pets, beneficial insects and other wildlife.
What about traps? While some people swear by them, others find it may attract even more beetles to their garden. Generally, placement is the key to whether or not traps do more harm than good: keep traps as far away from your desirable plants as possible and assess if they are bringing in more beetles to your garden than you started with. Find more on the trap/don’t trap debate here.