The cutworms are out in full force right now. They are the caterpillars of a fairly nondescript moth, and I explained about protecting your transplants with a cutworm collar here: (scroll down to "adding a cutworm collar").
One of the things I do with seedlings which are impractical to protect with collars, is to leave a few weeds in the row so there's something to eat besides my seedlings. About the third week of June the cutworms will have pupated, and/or the stems of large plants will have gotten large enough and tough enough so you can remove the collars and weed close to the plants. As we've had a warm spring it may even be possible this change will take place by mid-June.
I find a few when digging transplant holes or weeding with a hoe, and dispatch them then. They usually curl up when you find them, but they may be out straight as well. They eventually grow to about an inch, (which this one was) but you may find much smaller ones now, just beginning to feed and grow.