Douglas, that was going to be my suggestion ... cutworms.
I found one yesterday.
Cutworms are moth larvae that hide under litter or soil during the day, coming out in the dark to feed on plants. A larva typically attacks the first part of the plant it encounters, namely the stem, often of a seedling, and consequently cuts it down; hence the name cutworm.
Cutworms are usually green, brown, grey, or yellow soft-bodied caterpillars, often with longitudinal stripes, up to 1 inch (2.5 centimeters) in length. There are many variations among the genera. There also are variations in their biology and control, so the following extension material must be applied only as appropriate to the region.
Invasive plants are Earth's way of insisting we notice her medicines.
Everyone learns what works by learning what doesn't work.