Onion Thrips
Onion thrips are tiny insects that range in color from yellow to black and are only 1/16” in length. They spend the winter as adults in crop remnants, alfalfa, wheat, greenhouses and weeds along the border of crop fields. Adults lay eggs singly in the epidermis, nymphs feed on leaves, and pupation occurs in the soil. There are at least two generations per year in the Northeast. Thrips have rasping mouth parts which they use to tear open plant cells to feed on plant juices. Populations are favored by hot, dry weather. Heavy rain or overhead irrigation can lower populations.
Scout plants along field margins where infestations build early, as well as checking across the field. Scout weekly to determine if populations are increasing. Look closely between the leaf blades to find the light yellow nymphs or darker adults. Though tiny, they are visible moving about on the leaf when the leaves are parted. Count number per plant and note number of leaves per plant to determine if thresholds are reached. The number that constitutes an economic threshold varies with the stage of plant growth, efficacy of insecticide to be used, water availability, health of the plants. A widely used threshold is three thrips per leaf or 30 per plant.
Cultural practices that have been shown to reduce the thrips include sanitation at the end of the season, avoid use of last year’s onions for sets, avoid imported transplants which may be infested with thrips from southern areas, eliminate volunteers, use straw mulch, and alternate onion rows with carrot rows. Avoid planting onions near alfalfa, wheat or clover, that can harbor large populations of thrips, may increase thrips problems because they migrate to onions when these crops are cut or harvested.
In onions feeding occurs in protected, succulent areas where new leaves are emerging, deep between the leaf blades. Damage may appear as silver lines, white patches, tip dieback and curling, slowed growth, reduced bulb size and yields, or if severe enough can result in plant death. Plants are most sensitive when still small and when bulbs are forming. Healthy vigorous plants can tolerate moderate populations. Scallions are particularly sensitive because the whole plant is marketed. Lacewing larvae, pirate bugs and predatory thrips are important natural enemies.
Thrips damage can increase occurrence of purple blotch (Alternaria porri), as fungus can penetrate the plant through wounds caused by feeding.
Insecticides for onions
Broad-spectrum products include numerous synthetic pyrethroids (including Warrior, Pounce, Decis, Ammo, Proaxis, Mustang) and carbamates (Lannate, Malathion 57E). See 2010-2011 New England Vegetable Management Guide for more details on rates. Selective or organic products include Beauveria bassiana (Mycotrol O, takes 7 to 10 days after application to see control. OMRI listed); kaolin (Surround WP, suppression/repellence only. OMRI listed); pyriproxyfen (Esteem 0.86EC, insect growth regulator, dry bulb onions only, suppression only); spinosad (Entrust, OMRI listed; Spintor 2SC, has both contact and ingestion toxicity); spinetoram (Radiant SC); pyrethrin (PyGanic EC5.0); pyrethrins + piperonyl butoxide (Pyrenone). Note that Spintor 2SC is no longer on the market, having been replaced by Radiant, a closely related material, which has activity against adults and larvae.
In Brassicas, thrips are primarily a problem on cabbage where they feed on inner leaves which are difficult to target by spraying. Thrips cause rough, golden or brown scars to form on leaves or produce a discolored layer within cabbage heads. Thrips damage can be confused with edema. Controls must be applied before head formation in order to be effective.
In Brassica crops such as broccoli, kale, collard or cabbage, thrips are more often a late-season problem. They may damage open leaves and cause scarring, rust or yellow-colored areas and generally reduced vigor in the plants. Do not plant cabbage or other Brassicas near Alliums (onion family), alfalfa, or clover, that can harbor large populations of thrips which may migrate into Brassicas when these crops are cut or harvested. Onions tend to dry down around the same time that late Brassicas are put out, so close plantings can be a source of high and damaging populations of thrips.
Insecticides for Brassicas: Broad-spectrum products include numerous synthetic pyrethroids (including Warrior, Pounce, Capture, Baythroid, Ammo, Proaxis, Mustang) and one neonicotinoid, imidacloprid (Admire Pro). Biorational or organic products include spinosad (Entrust, OMRI listed; has both contact and ingestion toxicity); spinetoram (Radiant SC); novaluron (Rimon 0.83EC, insect growth regulator, not for mustard greens); pyrethrin (PyGanic EC5.0, OMRI listed); Insect growth regulators affect immature stages only, causing death during molts.
If thrips are a perennial problem on cabbage on your farm, plant more tolerant varieties (Bobcat, Ducati, Fresco, Little Rock, Matsumo, Rio Verde, Ruby Perfection, Solid Blue 770 or 780, Blue Pack, Ruby Ball, Heads Up, Bravo, Brutus, Green Cup, Roundup, Superette, Vantage Point, and Zerlina). Avoid planting highly susceptible varieties, such as Atlantis, Columbia, Morris, Ramada, Supergreen, Market Prize, Princess, Charmant and Solid Blue 690.
If repeat applications are needed, use a 7 to 10 day spray interval. Rotate between insecticide groups after 2 applications to help prevent resistance. Use a shorter interval in hot weather. Wetting agents or spreader-sticker are recommended to improve coverage and control. Apply in early evening, using moderate to high pressure, 100 gal water/A, and appropriate nozzle spacing to achieve best possible control. Note that products labeled for thrips control are not exactly the same for onions and Brassicas.
Updated June 2010.
R. Hazzard, UMass Extension.
Identifying and Enhancing Natural Enemies in Vegetable Crops
Insects
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