Onion thrips occur in all areas. Adults are small (1/25 inch long), slender, and light yellow to brown in color. They overwinter on plants or debris in fields, fence rows and weedy areas. Thrips puncture the outer layer of the leaves with their rasp-like mouthparts and feed on sap and bits of leaf tissue. They produce several generations each summer. Hot, dry weather is favorable for increased insect activity and plant injury. Small whitish blotches on the leaves are characteristic symptoms of thrips injury. Thrips are hard to control since they feed between the leaves.
Maintain plant vigor. Limited control can be achieved by hosing down the plants (early in the day) on a regular basis when injury is first noticed. Some control will also result from using insecticidal soap (more effective on larvae than adults) The most effective control measure is use of an insecticide labeled for thrips control in vegetables being careful to observe the days to harvest interval. Insecticide resistance has been a problem with onion thrips. Since the insects feed between leaves near the base of the plant, they are hard to reach with insecticides. Insecticides should be applied with sufficient water to ensure thorough coverage. Consult the most recent Commercial Vegetable Production Guide for specific control measures. Follow directions on the labels according to plant type when applying insecticides.
Red onions tend to be more susceptible to thrips than white onions, with yellow onions intermediate. Resistance to thrips infestation occurs in some varieties of Sweet Spanish onions. All varieties can tolerate populations of 25 thrips per plant. In well managed, irrigated onion crops, plants can tolerate high populations of thrips without reducing yields. Bulb size can be reduced if populations greater than 50 thrips per plant are allowed to develop and persist. In onions, waiting until you see crop damage is not recommended. Sprays need to be based on high populations but before feeding damage is readily apparent. Early crops can sometimes be harvested before damaging populations develop.
Onion maggot problems vary from year to year. Maggots are more of a problem during and after a series of wet springs. They rarely attack any crop except onion (other, related species attack other crops). As maggots infest young onions, the plants wilt and often die. Larger onions may survive an attack but the injured bulbs will often rot in the field or in storage.
The adult is a long-legged fly a little smaller than a house fly. The maggots are whitish in color and 1/3 inch long when fully grown. Onion maggots overwinter in a resting stage known as a pupae. Adult flies emerge in early spring and begin to lay their eggs in the soil near onions. Eggs hatch in three to four days and the maggots immediately bore into the plants. They feed and grow for about three weeks before changing to pupae. Adult flies emerge about two weeks later. There are three to four generations each year depending on the weather. The first brood is always more injurious to plants.
Do not plant onion bulbs in the same location as the previous year. Remove and destroy infested plants. Plants can be protected from the first generation of adults by using a floating row cover held at least 6" from the plant stems. Consult the most recent Commercial Vegetable Production Guide or specific control measures. Follow directions on the labels according to plant type when applying insecticides.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Shelby Fleischer