Description: Common throughout the United States, the imported cabbageworm (Pieris rapae) chews large, irregular holes in the leaves of cabbage and cole crops (broccoli, cauliflower, kale, turnip). As it feeds it may bore into the center of heads and contaminate them with its fecal pellets. Although this garden pest moves sluggishly, it is extremely destructive, especially later in the growing season when populations have built up significantly.
The imported cabbageworm (1-1/4 inch long) is velvety green in color and has many short fine hairs and faint yellow strips down its side and back. Adults are white or pale yellow butterflies (1-2 inch wingspan) with three or four black spots on their wings. They are frequently noticed fluttering about the garden from early spring to late fall.
Life Cycle: Adult females emerge from green pupal cases in early spring and lay up to 200 tiny yellow eggs on host plants, usually on the undersides of leaves. These hatch in 7 or more days (depending on temperature) into young caterpillars (larvae). After feeding for about 15 days they pupate on lower leaf surfaces or nearby garden objects. During warm weather, the pupal stage lasts 10 days before a new generation of butterflies emerges. There are 2 to 4 overlapping generations each year.
Cabbageworm Control: While this insect has many natural enemies, including predatory beetles, spiders, yellow jackets, lacewing and parasitic wasps, it is often an annual garden pest that requires attention. Protect plants with floating row covers to prevent adults from laying eggs. Use pheromone traps to determine the main flight period for moths, then release trichogramma wasps to destroy eggs. As soon as damage is noticed (large irregular holes in leaves, fecal pellets on plants and ground), begin handpicking caterpillars and spray Dipel Dust (Bt-kurstaki) or Monterey Garden Insect Spray (Spinosad). Use botanical insecticides only as a last resort. After harvest, till under garden debris to destroy overwintering pupae before adults emerge in early spring.