Since you're asking, I assume you've not yet seen the culprit. Look in your garden at night with a flashlight and you may find some slugs. If you don't, it's probably something else.
Do you have a lot of Japanese beetles around?
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OK, I looked at them last night after dark and I didn't see any slugs. I did see some little white looking moths or fly type bugs (whiteflys maybe?) Also have a lot of grasshoppers.