Cutworms decimate tender baby tomato plants early in the season, often cutting seedlings off right at or below the soil line. Mature tomato plants aren't as susceptible to cutworm damage as young plants are. You'll need to remain diligent through harvest, however, because tomatoes in all growth phases attract these pests. The younger the cutworms are, the easier they are to eradicate. You'll need to be proactive early on because pretreating plants and soil chemically isn't effective.
Inspect the stems of your tomato plants twice daily. Plant damage is easiest to see when it's fresh early in the morning. Cutworms are at their busiest late in the afternoon or early in the evening. Rake your fingers through a 12-inch diameter area of the topsoil around each plant. You'll turn up larvae hiding in the dirt.
Hand pick and squash any cutworms that you find.
Make a collar from a piece of 3- or 4-inch-wide cardboard. The diameter should be large enough to clear the stem of the tomato plant by about ½ inch. Set it around the plant and push it about 1 inch into the soil, leaving 2 or 3 inches above ground. These barriers are highly effective for preventing cutworm access to plants.
Apply Bacillus thuringiensis, packaged as SOK-BT, Bactur, Thuricide or Dipel. Follow the manufacturer's instructions carefully. Cutworms will sicken quickly and die soon thereafter.