Key to Onion Pests
The onion thrips and the seedcorn maggot are practically the only onion pests in North Carolina. The onion maggot does occur here but poses a severe problem only in the northern states. Onion and seedcorn maggots cause the same type of damage and are virtually indistinguishable in the field.
- Onion thrips – Foliage-rasping pest; pale yellow to dark brown body 2 mm or less in length; adult with 2 pairs of narrow, fringed wings (Figure 1); causes silvery blotches or scratch-like markings on leaves; some infested leaves distorted, curling upward
- Seedcorn maggot – Bulb-feeding pest; white to yellow-white maggot up to 7 mm long; legless body with pointed head (Figure 2); feeds underground in bulbs, causing plants to yellow and die.
Onion thrips, Thrips tabaci Lindeman, Thripidae, THYSANOPTERA
Adult – Pale yellow to dark brown in color, the onion thrips adult is slightly less than 2 mm long. Its two pairs of narrow wings are fringed with long hairs.
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.
Distribution – Onion thrips occur throughout the vegetable-producing areas of the United States and Canada as well as in many other regions of the world.
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, sweetpotato, 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.
Good cultural practices can limit onion thrips populations. Destruction of volunteer plants and crop residue after harvest eliminates many favorable overwintering sites. Since onion thrips populations build up rapidly on cucurbits, crucifers, strawberries, roses, and carnations, these crops should not be planted near or rotated with onions.
Young onions being grown for bulbs should be sprayed as needed once the thrips population reaches 10 thrips per 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.
Publication date: Feb. 10, 2003
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