Answer: Japanese beetles are a pest both in the larval (grub) form and adult form. It sounds like you are having trouble with the adult beetle eating plant foliage. Here's a variety of options: Protect plants with floating row covers in late spring/early summer as adults emerge. (Be sure to check under covers anyway.) Beetles are sluggish on cool overcast mornings or evenings. Shake them off plants onto sheets of paper to squash them or into containers of soapy water. You can also handpick them. There are Japanese beetle traps that have a sex attractant to lure males and a floral lure to attract females and males. These are not a cure for a big infestation however. Attract birds and insect predators which love to feast on the beetles. Plant pollen and nectar plants to attract parasitic wasps and tachinid flies. (Note that if you spray chemicals you are also killing the beneficial insects that help keep others in check.) There are some sprays based on Neem that may have success.
You didn't mention whether you have a grub problem in your lawn, but it seems likely if you have that many adults. You may want to control that as well. Japanese beetle grubs are best controlled by spraying beneficial nematodes on the lawn and garden area. These microscopic worm-like creatures attack only the grubs in the soil and not plants, animals or humans. Spray them in spring when the soil temerpatures are above 55F and you should see a difference in summer. It's important to maintain soil moisture to keep the nematodes viable and so they can move easily through the soil. I suggest that you moisten soil well the day before you apply them, or wait until after it rains. You can buy the beneficial nematodes from Gardens Alive,
email@example.com; ph# 812/537-8650.
Another product is milky spore powder, a naturally-occuring bacteria that kills grubs; you simply apply the powder in a grid pattern on your lawn. The powder is harmless to earthworms and other creatures. The bacteria will continue to multiply, and will keep working for up to 20 years. (The results won't be immediate, however; it will take some time to notice a decline in the grub population.)
Finally, make sure you plants are healthy with appropriate water, soil and nutrients. That's the best way to keep pests at bay. Hope this info helps!
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