Black caterpillars with yellow stripes munching on your broccoli could be cross-striped cabbageworms. They are voracious feeders that eat all summer long and can cause extensive damage to home gardens. Practicing good cultivation techniques and employing bacterial sprays may help you control these garden pests.
The adult cross-striped cabbageworm is a small yellowish brown moth with a 1.5 to 2 cm (1-inch) wingspan. Eggs are oval, light yellow and about 1/20 inch. Circular masses of 20 to 30 eggs are laid on the undersides of leaves. The egg masses are flattened and overlapping. Newly hatched larvae are small grey caterpillars that mature into bluish grey with black and yellow stripes running along their sides.
Caterpillars overwinter in the pupal stage just under the surface of the soil and emerge in late spring. Females deposit eggs on the undersides of leaves. In six to eight days, eggs will hatch and larvae will begin to feed. Depending on the temperature, larvae will mature in 30 to 45 days. They will then descend into the ground where they will pupate and emerge in 10 days. The cross-striped cabbageworm usually hatches two to three generations per year.
Cabbage, broccoli, cauliflower, turnips and collard greens are favourite foods of the cross-striped caterpillar, but related crops can also be damaged. Larvae leave large oval holes in central leaves and terminal buds. They also burrow into cabbage heads. Larvae are voracious eaters and can leave crops riddled with holes. In small home gardens they can decimate entire crops.
Handpicking and destroying larvae and egg masses is time-consuming but effective. Floating row covers can help prevent adults from laying eggs. At the end of the season, be sure to remove all crop debris and destroy it. Till the soil to expose overwintering pupae to predators. Planting nectar plants like alyssum near your broccoli can help attract beneficial insects that eat caterpillars. If you decide to use pesticides chose a spray containing the bacteria Bacillus thuringiensis. It only targets caterpillars and won't harm beneficial insects.
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