Differentiating Common Caterpillar Pests of Late-Season Vegetables

Beet armyworm (BAW) caterpillars can range in color from pale green to nearly black with stripes along each side (Fig. 1). They have a characteristic black spot on each side above the second pair of legs (counting back from the head). BAW attack a wide range of vegetables including beans, beets, cole crops, eggplant, onion, pepper, potato, cucurbits, spinach, sweet potato, tomato, ginger, and others. Additional weedy hosts include pigweed and common labsquarter. BAW are resistant to pyrethroids and some populations in the region may be resistant to diamides.

The corn earworm (CEW), also called the tomato fruitworm, soybean poworm, and cotton bollworm has a very broad host range and can potentially be found in nearly every type of field and vegetable crop grown in the Delmarva region. Late-season infestations are common in tomatoes, cole crops, sweet corn, snap and lima beans, and pumpkins. CEW larvae vary in color, ranging from green to pink to dark brown (Fig. 2). They typically have several thin, dark stripes running along bodies as well as black spotting. Head capsules are typically light green or tan colored. CEW is resistant to pyrethroids as well as all Bt sweet corn varieties except Attribute II/Attribute Plus varieties.

Yellow striped armyworms (YSAW) can be found attacking late season tomatoes and peppers, as well as fall cole crops. Larvae are pale gray or greenish to jet black in color with a yellow stripe along each side and a pair of small, triangular spots on top of most segments. Potentially confused with the beet armyworm due to the black spot located near the head, YSAW can be differentiated by the presence of a second, larger black spot located behind the last pair of legs towards the middle of the body. Numerous insecticides are effective in controlling small larvae, however, larger caterpillars become more difficult to control with many products.

Diamondback moth (BDM) caterpillars are small, pale green caterpillars (Fig. 4) that can be recognized by their habit of wriggling violently when disturbed. DBM will consume only the tissue on the underside of the leaf, causing a transparent “window-paning” effect. They are notorious for developing insecticide resistance and effective management relies on adhering to good rotation of different modes of action.

Both cabbage and soybean loopers can be found damaging fall vegetables in the Delmarva region. The two species are virtually impossible to differentiate in the field. Both appear as a pale-green caterpillar with thin white stripes along their back and sides (Fig. 6). When disturbed, loopers “loop” along leaves, travelling in a motion similar to an inchworm. Cabbage loopers are a common pest of brassica crops, and both cabbage and soybean loopers can also damage tomatoes. Soybean loopers are resistant to pyrethroids, and diamides offer only partial control.

Imported cabbageworms (ICW) are large, velvety green caterpillars (Fig. 5) that feed primarily on the upper leaf surface near the midrib of various cole crops. They are the larval stage of the cabbage white butterfly often observed flying in brassica fields. ICW is effectively controlled with all recommended worm products.

Cross-striped cabbage worms (CSCW) are a bluish-grey color on top and green underneath, with numerous black bands across their backs and a yellow stripe down each side (Fig. 5). Numerous larvae are typically found on a single plant and damage will often be severe on one plant while neighboring plants remain undamaged. Treatment thresholds are much lower for CSCW, but most labeled worm products should work well.

Beet webworms are pests of spinach, beets, Swiss chard, and amaranth and are also commonly found on pigweed and lambsquarters. Larvae are translucent, pale greenish-yellow with a dark stripe running down their backs (Fig. 7). Beet webworms are resistant to pyrethroids.