When do I spray for Japanese beetles?

Anything with permethrin in it. Check with your local nursery.

https://www.almanac.com/pest/japanese-beetles

When they start appearing

The best time is when they are still grubs, which are fat, ugly wormlike things you find under the grass. If you have yellowish to brown spots in your lawn, it’s likely grubs that are causing it. Kill those & you will get rods of your closest source of Japanese beetles.

You don't have to spray - get one of these - they work great! https://www.amazon.com/Safer-Brand-Japanese-Beetle-Attractant/dp/B0000BYCTG

Look into Milky Spore as a non-toxic long lasting way to get rid of your and eventually your neighbors Japanese beetles.

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