Whitney Cranshaw, Colorado State University, Bugwood.org
They are an important pest of cabbage, cauliflower, and onions.
Immature onion thrips are small and milky white at first and after molting turn to green or lemon-yellow with red eyes. The adults can range from a pale yellow to dark brown and are usually smaller than 1/16 of an inch. Young thrips lack wings, but adult thrips have four wings that are very narrow and fringed with hairs. The females deposit eggs in tender plant tissues.
Adults and nymphs pass the winter in small grains, clover and alfalfa fields concealed in grass or other plant remnants. Activity may continue throughout the year in warmer parts of the state. In a few days wingless nymphs emerge, feed, and molt four times before reaching the adult stage. Growth from egg to adult can be completed in two weeks when temperatures are warm. In the midwest, development from egg to adult is estimated to take 20 to 40 days, with 5 to 8 generations per year. Development of an infestation is influenced by seasonal conditions. The pest breeds most rapidly when a mild, dry winter is followed by a hot, dry spring. This may be due to better survivorship during the winter, and then movement off of overwintering hosts that are stressed.
Presence of thrips in cabbage should be noted when scouting for worms, and onions should be checked regularly for thrips. Give special attention given to fields near small grains or alfalfa, at times of harvest of small grains and alfalfa, or during times of hot, dry weather. Yellow or white sticky cards may help detect thrips immigration into fields.
Thrips feed by piercing surface tissues and sucking up the exuded plant juices. On onions with continuing leaf growth, these feeding points elongate to give the typical onion thrips symptoms: whitish spots and streaks on the leaves. Intensive feeding results in a silvery-white stippled appearance sometimes referred to as white blast or silver top. Seedlings may be killed if large numbers of thrips are present. Feeding occurs on the interior of the onion neck, on developing leaves. The early bulbing stage is the most susceptible. If feeding on cabbage, the cabbage is blistered, scarred, and bronzed by thrips feeding on leaves; and heads are unmarketable with thrips present and evident. The damage on cabbage has also been described as forming whitish scratches or brownish blisters on foliage, and red-to-brown patches on heads. Where many thrips have fed, the discolored areas coalesce to form large brownish, blister-like areas. Severe infestations can lead to underweight and misshapen heads. Damaged tissue used for sauerkraut turns into dark blotches during processing.
Cultural controls are important. If possible, avoid planting cabbage, cauliflower or onions close to and immediately downwind of small grains or alfalfa. Choice of variety can also help. In cabbage, onion thrips tend to prefer varieties with very tight head, but host plant resistance tests have not always been consistent. Eastman et al. (1995, see reference below) list varieties with at least moderate resistance and varieties with the least tolerance. Although several natural enemies feed on egg and immature thrips, augmentative release of natural enemies has not been show to be effective.
In general, red onions tend to be more susceptible to thrips than white onions, with yellows intermediate. Resistance to thrips infestation occurs in some varieties of sweet Spanish onions. Davis et al. (1995, see reference below) lists tolerant, moderately tolerant, and susceptible varieties. In the Midwest, Davis et al. (1995) reports that all varieties can tolerate populations of 25 thrips per plant. In well-managed, irrigated onion crops, plants can tolerate high populations of thrips without yield reduction. 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 applied based on high populations but before feeding damage is readily apparent. Early crops can sometimes be harvested before damaging populations develop.
Insecticide resistance has been a problem with the onion thrips. Also, in onions, it is possible to have a different species of thrips - the Western flower thrips - and some insecticides that work on onion thrips may not work on Western flower thrips.
Since the insects feed between leaves near the base of the plant, they are hard to reach with insecticides. Insecticides should be applied in sufficient water to ensure thorough coverage. High water volumes (30 gal/A) is recommended in the Midwest Systemics activity helps. When thrips are increasing in a cabbage field, it is important to apply insecticides before cupping or curd formation. The Commercial Vegetable Guide list current labeled options.
Eastman, C., S. Mahr, J. Wyman, C. Hoy, & H. Oloumi-Sadeghi. 1995. Cabbage, broccoli, and cauliflower. Pp. 99-112 In Foster, R. and B. Flood. Vegetable insect management with emphasis on the Midwest Meister Publ. Co. 1995. Willoughby, Ohio.
Davis, M., E. Grafius, W. Crenshaw and T. Royer. 1995. Onions. Pp. 136-146. In Foster, R. and B. Flood. Vegetable insect management with emphasis on the midwest. Meister Publ. Co. 1995. Willoughby, Ohio.