There are long, black caterpillars with thin yellow stripes (one on each side) munching on my pond's water lily leaves. They're doing a lot of damage to the leaves. I think they are the yellow-striped armyworm. I've never noticed them in my tiny pond in past years, but yesterday I saw 3-4. Is it OK if I remove and kill them, or are they benign or beneficial and thus should I just remove them from the pond and put them in the grass?
We'd have to see a photo to give you a positive identification. You can attach them to a reply.
If it is an armyworm, it is a pest, but this late in the season it will not have much impact on your water lilies.