The best way to protect plants from cabbage loopers and cabbageworms—the larvae of two different moths—is to exclude the moths by using fabric row covers. You can also treat the loopers with Bt (Bacillus thuringiensis). For best results, treat the plants while the caterpillars are still young. The older and larger they get, the more resilient they become. Use Bt sparingly, however, because it is toxic to all caterpillars, including the larvae of your favorite butterflies.