Answer: To be honest I have never known slugs to be a problem on these plants when they are so large late in the season. Slugs leave that tell-tale slug-slime and eat the leaves in from the edge, not leaving exactly a Swiss cheese pattern. Is it possible Japanese beetles or grasshoppers are eating them? To eliminate slugs as the culprit, check the plants a couple of hours after dark and you will find them either on the plants, the stems or the ground close by (you will ONLY see them at night); beetles and grasshoppers work during the day. Beetles can usually be found from mid morning onward (and knocked into a container of soapy water); grasshoppers are harder to "catch in the act" but are only a faint possibility unless your garden is currently infested. The only other culprit I can think of would be some type of larva (caterpillar wormy thing) -- check the undersides of the leaves very carefully?
The last thing I can think of is mechanical damage such as older hail damage -- as the leaves grow though the season the holes become more apparent. This is the type of thing we usually notice first on hostas and then notice even later on other plants.
Good luck with your sleuthing!
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