PESTS OF LETTUCE

Caterpillars attacking the fall crop are usually the most troublesome field pests of lettuce. In the greenhouse, aphid and cabbage looper infestations often give problems. Cutworms, whiteflies, leafminers, and slugs are slightly less important greenhouse pests. Regardless of growing site, aphids cause additional problems by transmitting several virus diseases, including lettuce mosaic.

KEY TO LETTUCE PESTS

A. Caterpillars that sever or leave holes in foliage

  1. Beet armyworm - Green or black larva, up to 30 mm long; three pairs of legs near head; five pairs of fleshy prolegs; three lightly colored stripes running length of body; black spot on each side of the second segment behind the head (Fig. 133); damages bud and young leaves

  2. Cabbage looper - Green caterpillar with longitudinal white stripes; body up to 30 mm long, tapers toward the head; three pairs of legs near head; three pairs of fleshy prolegs (Fig. 134); young larva on underside of leaf; mature larva deep within head; consumes tender leaf tissue, leaving most veins intact

  3. Cutworms - Fat, basically gray, brown, or black caterpillars 40 to 50 mm long when fully grown; three pairs of legs near head; five pairs of fleshy prolegs (Fig. 135); young larva on underside of leaf; mature larva deep within head; consumes tender leaf tissue, leaving most veins intact

  4. Imported cabbageworm - Velvety green caterpillar up to 32 mm long; yellow stripe down back; row of yellow spots down each side; three pairs of legs near head; five pairs of prolegs (Fig. 136); feeds deeper in plant and more likely to eat small veins than the cabbage looper; leaves wet, greenish-brown excrement deep among leaves

B. Small (less than 4 mm long) piercing-sucking insects that extract sap and create discolored areas on foliage

  1. Aphids - Soft-bodied, pear-shaped insects with a pair of dark cornicles and a cauda protruding from the abdomen; may be winged or wingless - wingless forms most common; feed in colonies; cause discoloration or mottling of foliage; often transmit virus diseases; excrete honeydew on which sooty mold grows.

    1. Bean aphid - Dark green to black body with white appendages; adult up to 2.6 mm long; cornicles about same length as cauda (Fig. 137A); nymph green, last instar with 5 to 7 pairs of white spots on abdomen (Fig. 137B)

    2. Green peach aphid - Pale yellow to green wingless adult up to 2.4 mm long; winged adult with dark dorsal blotch on yellowish-green abdomen; cornicles over twice as long as cauda and slightly swollen toward tip (Fig. 138A); yellow-green nymphs with 3 dark lines on abdomen (Fig. 138B)

    3. Potato aphid - Adult and nymph both solid pink, green and pink mottled, or light green with dark stripe; adult up to 3.5 mm long; long slender cornicles about twice as long as cauda (Fig. 139)

    4. Turnip aphid - Adult and nymph both dull green; adult up to 2.2 mm long; swollen cornicles slightly longer than cauda (Fig. 140)

  2. Greenhouse whitefly - White moth-like insect about 1.5 mm long; found in conjunction with tiny yellow crawlers and/or green, oval, flattened, immobile nymphs and pupae (Fig. 141)

  3. Leafhopper - Spindle-shaped pest up to 4 mm long; green body with yellowish to dark green to black spots (Fig. 142); usually jumps instead of flies; extracts saps from underside of leaf causing leaf to crinkle and curl upward; can also cause yellowing of leaves.

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