Agrotis, Amathes, Peridroma, Prodenia spp.
These caterpillars (the larvae of Noctuid moths) spend their days hiding underground and their nights feeding on the surface. They feed by wrapping themselves around the stem of a plant and eating it until the plant falls over. It can be very frustrating after you have spent weeks nurturing your transplants, to have them destroyed within days of planting out. If you suspect cutworm damage (it’s pretty obvious - the fallen plants will be laid on the ground with the stem still sticking out of the ground) dig down into the soil around the plant to find the culprit. During the day they hide in the soil near the fallen plant, so, it’s almost always there if you search around. If Cutworms are very bad you can deter them by putting little collars of cardboard or aluminum foil around the stem of each seedling. Night patrols can catch cutworms in the act.
Image: Eugene E. Nelson, Bugwood.org