White butterflies are not a problem for your lawn as they innocently gather nectar from flowers. Their larvae, however, are major pests of cabbage family crops. Somewhere nearby, the larvae, known as imported cabbage worms, are probably feeding on cabbage, broccoli, cauliflower, Brussels sprouts, kale, collards, or kohlrabi. The larvae are velvety green and up to 11?2 inches long. Feeding larvae chew holes in the leaves and heads of these crops, leaving behind telltale green pellets of frass. You can keep your cabbage worm-free by covering plants with a floating row cover so the adult butterflies can't lay their eggs on the plants. If your cabbage already has some worms blissfully chomping away, use the bacterial insecticide Bt (Bacillus thuringiensis). Insecticidal soap or chemical insecticides may also be used to control them.