Pest Management Fact Sheet #5006
James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician
Charles D. Armstrong, Cranberry Professional & Staff Entomologist
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Imported cabbageworm (Pieris rapae) larvae prefer to feed on cabbage, cauliflower, broccoli, Brussels sprouts, turnips, radishes, kale, lettuce and weeds of the mustard family. In fact, this pest is one of the most damaging and destructive enemies of these plants. The caterpillars chew irregular holes in the leaves and usually eat their way into cabbage heads from the bottom. In addition to the feeding damage, the insects’ frass will stain cauliflower.
Description & Biology
The first sign of imported cabbageworm is often the 1½” butterfly which has yellowish wing undersides but is otherwise white. This stage of the insect emerges early in the spring. The females fly during daylight hours looking for suitable plants on which to lay their eggs. The yellowish, elongated eggs, are laid singly on the underside of leaves.
In 5 to 7 days, the eggs hatch to become velvety green caterpillars which will grow to 1½” long. They have a thin yellowish-orange stripe down the middle of the back. It is common for high larval populations in July and August to damage plants so severely that they die or become unmarketable.
When fully grown, the larva forms a greenish-brown pupa (chrysalis) which can often be found hanging from the bottoms of leaves or other protected areas. Ten days later, the second generation of butterflies emerge, continuing the cycle. There may also be a third generation in Maine. Adults may be seen almost anytime during the summer.
Controlling weeds around the garden, especially plants of the mustard family, should help decrease the numbers of this pest. Destroying and removing the remains of plants in the fall, as well as fall tillage, reduces the number of over-wintering pupae. Hand-picking the larvae may be labor intensive but can significantly cut the numbers and keep damage down. Early Globe, Red Acre, and Round Dutch cabbage have shown some resistance to cabbageworm.
B.t. (Bacillus thuringiensis), an organic insecticide derived from a bacterium and sold as Dipel®, Bactur®, Sok-Bt® or Thuricide®, is a management method. B.t. is less effective under cooler conditions. Other options include spinosad, insecticidal soap, Sevin® (carbaryl) and malathion. The smaller the caterpillars are, the easier they are to kill. Insecticides should be applied in late afternoon or early evening to minimize bee exposure.
When Using Pesticides
ALWAYS FOLLOW LABEL DIRECTIONS!
Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473
1.800.287.0279 (in Maine)
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