Key to Lettuce Pests
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.
A. Caterpillars that sever or leave holes in foliage
- 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 (Figure 1); damages bud and young leaves
- 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 (Figure 2); young larva on underside of leaf; mature larva deep within head; consumes tender leaf tissue, leaving most veins intact
- 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 (Figure 3); young larva on underside of leaf; mature larva deep within head; consumes tender leaf tissue, leaving most veins intact
- 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 (Figure 4); 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
- 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.
- Bean aphid – Dark green to black body with white appendages; adult up to 2.6 mm long; cornicles about same length as cauda (Figure 5A); nymph green, last instar with 5 to 7 pairs of white spots on abdomen (Figure 5B)
- 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 (Figure 6A); yellow-green nymphs with 3 dark lines on abdomen (Figure 6B)
- 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 (Figure 7)
- Turnip aphid – Adult and nymph both dull green; adult up to 2.2 mm long; swollen cornicles slightly longer than cauda (Figure 8)
- 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 (Figure 9)
- Leafhopper – Spindle-shaped pest up to 4 mm long; green body with yellowish to dark green to black spots (Figure 10); usually jumps instead of flies; extracts saps from underside of leaf causing leaf to crinkle and curl upward; can also cause yellowing of leaves.
Green Peach Aphid
Green peach aphid, Myzus persicae (Sulzer), Aphididae, HEMIPTERA
Adult – This soft-bodied, pear shaped insect is usually wingless and ranges from 1.6 to 2.4 mm long. The wingless female is pale yellow to green. The winged midgrant form has a yellowish-green abdomen with a dark dorsal blotch. Both forms have a pair of tailpipe-like appendages known as cornicles.
Egg – No egg stage occurs in North Carolina.
Nymph – Slightly smaller than the adult but similar in shape, the nymph is pale yellow-green with three dark lines on the abdomen.
Distribution – The green peach aphid is a cosmopolitan species.
Host Plants – The green peach aphid infests a wide range of plants. Some important hosts include cabbage and related cole crops, dandelion, endive, mustard greens, parsley, turnip, tomato, tobacco, potato, spinach, pepper, beet, celery, lettuce, and chard.
Damage – Green peach aphids extract sap from plants and excrete a sweet sticky substance known as honeydew. Black sooty mold grows on honeydew and, though not directly harming the plants, may block out sufficient light to reduce yield. Weakened plants become susceptible to secondary disease and may be inoculated with viruses carried by aphids. Among the virus diseases transmitted by green peach aphids are potato leaf roll, potato virus Y, beet mosaic, beet yellows, and lettuce mosaic.
Life History – Adults pass the winter on greens and wild hosts such as cabbage, collards, turnip, wild mustard, and dock. Winged forms migrate to other hosts in late spring. During these migratory flights, aphids may spread virus diseases from infected volunteer plants and weeds to healthy crop plants. Movement between host plants continues through summer and fall.
In southern states, the aphids are nearly all females. Successive generations of females, mainly wingless, are produced throughout the year. Winged migrants develop whenever overcrowding occurs or food becomes scarce. This type of development (all females, no males or eggs) occur as far north as Tennessee and Maryland. Many generations are produced each year.
Lady beetles and their larvae, lacewing larvae, syrphid fly larvae, and stilt bugs all feed on aphids. Fungus diseases, high temperatures, damp weather, and hard rains also reduce aphid populations.
Cultural practices are helpful in avoiding aphid infestations. Winter host plants (collards, mustard, dock, turnip) in the vicinity of seed beds should be destroyed before plants begin to come up. The purchase of certified seed from areas free of virus is also a good preventative measure.
A number of insecticides are available to control aphids on a wide variety of crops. However, repeated applications of certain carbamate insecticides within intervals of a week or less are frequently conducive to aphid buildups. For specific chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.
Potato aphid, Macrosiphum euphorbiae (Thomas), Aphididae, HEMIPTERA
Adult – This soft-bodied, pear-shaped insect may be solid pink, green and pink mottled, or light green with a dark stripe. Usually wingless, it is about 2.5 to 3.5 mm long and has a pair of long, slender tailpipe-like appendages known as cornicles.
Egg – The egg stage does not occur in North Carolina.
Nymph – Although slightly smaller than the adult, the nymph is similar in color and shape.
Distribution – Potato aphids occur throughout North America.
Host Plants – Potato aphids infest a wide range of host plants. Some important cultivated hosts include potato, tomato, eggplant, sunflower, pepper, pea, bean, apple, turnip, corn, sweet potato, asparagus, clover, and rose. Weeds such as ragweed, lambsquarters, jimsonweed, pigweed, shepherdspurse, and wild lettuce are also common food plants.
Damage – Sporadic in occurrence, potato aphid infestations are rarely severe enough to kill plants. Aphids pierce veins, stems, growing tips, and blossoms with their needle-like mouthparts. As a result, blossoms are shed and yield is reduced. New growth becomes stunted and curled. Heavily infested plants turn brown and die from the top down. Aphids tend to spread rapidly from field to field transmitting a number of viral diseases. These include various mosaics, leaf roll, spindle tuber, and unmottled curly dwarf.
Life History – In North Carolina, female potato aphids feed and reproduce year round. No eggs or males are produced. Without mating, wingless females give birth to about 50 live nymphs. During warm weather, each of these nymphs matures in 2 or 3 weeks. The life cycle continues in this manner until overcrowding occurs or food becomes scarce. At these times nymphs develop into winged adults and migrate to new host plants. Once settled down, these aphids begin reproducing and the life cycle continues as before. During winter, however, feeding and reproduction occur at a much slower rate. Many generations are produced each year.
Lady beetles and their larvae, lacewing larvae, syrphid larvae, and stilt bugs all feed on aphids. Fungus diseases, high temperatures, damp weather, and hard rains also limit aphid populations.
Cultural practices are helpful in avoiding aphid populations. Crops should be planted in well-prepared, fertile seedbeds to promote vigorous growth. When possible, avoid planting sites near infested fields or from which an aphid-infested crop has been removed.
A number of insecticides are available to control aphids on a wide variety of crops. However, repeated applications of certain carbamate insecticides within intervals of a week or less are frequently conducive to aphid buildups. For specific chemical recommendations, consult the current North Carolina Agricultural Chemical Manual.
Publication date: Feb. 10, 2003
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