Inspect the plants each day for cabbage loopers, if you've already planted the garden. Also, look for bright green caterpillars with two white stripes down their backs and a white stripe on each side. Inspect the plants for traces of the brown fecal matter the caterpillars leave behind on the plants.
Mix Bacillus thuringiensis (Bt), a type of bacterium often used as a pesticide, with water by following the directions given on the package, if you find caterpillars. Spray the mix directly onto the foliage of the affected plants. For smaller gardens or light infestations, remove the caterpillars by hand from the plants in addition to using Bt.
Install row covers to prevent further infestation after you eradicate the caterpillars. Establish dill with the plants as a secondary measure to repel cabbage loopers and other moths.