Egg - Each milky white, bean-shaped larva is white and about 0.25 mm long. The second instar, also wingless, may be green or yellow with red eyes.
Prepupa and Pupa - These two stages resemble larvae in shape and color but begin to show developing wings. The prepupa has short wing pads and the pupa has longer ones. The pupa remains motionless unless disturbed.
Host Plants - Besides onion, garlic, and related plants, onion thrips injuriously attack cabbage, cauliflower, parsley, cucumber, melons, pumpkin, squash, kale, turnip, tomato, lettuce, bean, beet, pea, celery, blackberry, strawberry, and practically all other vegetable and truck crops. However, though potato, sweet potato, and mustard are hosts, they seldom are injured severely. Greenhouse-grown flowers, cucumbers, and tomatoes are damaged frequently. Many grasses and weeds also harbor this species of thrips.
Damage - Thrips rasp the tender parts of center leaves and/or terminal buds with their sharp mouthparts and feed on escaping juices. Leaves develop silvery blotches or scratch-like markings. Some leaves become distorted and curl upward. This symptom should not be confused with aphid injury which causes leaves to curl downward.
Light thrips infestations tend to delay plant growth and retard maturity. Heavy infestations may kill terminal buds or even entire plants. On onions, leaves become curled, crinkled, and twisted; growth stops; and plants die. When terminal buds on other types of crops are damaged, abnormal branching patterns result. Injury to the plant is always more severe under hot, dry conditions.
Life History - Onion thrips spend winter in sheltered areas of plants, such as remnants of onion plants left in the field or crowns of alfalfa and clover. In spring, development resumes and winged adults search for suitable host plants. A female thrips produces 10 to 100 eggs which she inserts singly into tender plant tissue. Eggs hatch 4 to 10 days later. The larvae feed for approximately 5 days before pupating in the soil. About 4 days later, new adults emerge from the soil to feed and lay eggs. Although winged adults are weak fliers, they are capable of flying from plant to plant and may be carried long distances by wind. Probably 5 to 8 generations occur each year in North Carolina.
Young onions being grown for bulbs should be sprayed as needed once the thrips population reaches 10 thrips/plant. Onions grown for seed may need spraying after plants begin to bloom. In this case, the seedhead should be checked for thrips. If three or more thrips fall out when the head is tapped, 20 to 30 more are probably still inside. An insecticide safe to pollinating bees should be applied late in the evening. For up-to-date recommendations, consult the current North Carolina Agricultural Chemicals Manual.
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