Answer: Treating for the larvae is good, but you point out a common problem with them coming from other yards. Japanese beetles are tough to control. Charlie Nardozzi writing in "Vegetable Gardening for Dummies" recommends trying to get neighbors to treat for them in a neighborhood-wide offensive. (Not always easy to do, I know!)
Handpicking in morning and evening when beetles are sleepy and less likely to fly away is an option if you are diligent. Stomp on them to alleviate your frustrations! Insecticidal soap can be used against adult beetles. Pyrethrin and rotonone, which are considered botanical insecticides because they are made from plants, can be used. However, they are extremely toxic. They will kill a broad range of insects, including beneficials such as bees. Use pyrethrin in the evening when bees are not active. Use rotonone as a last resort. Don't use rotonone around water, as it will kill fish. Follow label instructions exactly. I hope this info helps.
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