I have extensive flower beds and one of the larger ones tends to over-run with Garden Phlox. I pull and dig the unwanted ones, but last year and this something appears to be biting off the tops. Last year I thought it was deer, but now I realize I would see tracks or crushed foliage. The damage is only to the top, new leaves are growing, and last year they bloomed anyway. The stem looks neatly "cut". Any ideas what may be causing this?