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Greenhouse Project > Western Flower Thrips: Biology And ControlIntroduction Damage There is no cure for this disease, and one WFT adult can infect a plant after feeding on it for only 30 minutes. Because both the virus and the thrips have such a wide range of plant hosts (including weeds), it may be difficult to eradicate the virus once it is found in a greenhouse. Identification Biology
The adults can survive 30 to 45 days and lay 150 to 300 eggs, depending on temperature and host plant, and are found feeding in protected areas of the plant, such as flowers and terminals. The pest’s rapid development time (egg to adult in 7-1/2 to 13 days at fluctuating temperatures) and reproductive rate can allow an undetected infestation to quickly become a major problem. They fly readily, although they are not strong fliers, and can be carried on wind currents or on clothing to nearby greenhouses. They can fly from a sprayed to an unsprayed area and can move into or out of a greenhouse through doors or vents. Effective chemical control is further complicated by the problem of insecticide resistance. Resistance to certain organophosphate, carbamate and synthetic pyrethroid insecticides has been documented in certain populations of western flower thrips. Control Early detection of a thrips infestation is critical because the symptoms of their feeding are often not noticed until the damage has occurred and because an infestation is easier to control when it is small. Yellow sticky cards provide an easy way to detect the onset of an infestation. These should be placed just above the crop canopy, one per 2,000 square feet, and near doors, vents and over thrips-sensitive cultivars to monitor the movement of thrips. Recent research has shown the light-to medium-blue sticky cards catch more thrips than yellow ones. However, other kinds of insect pests are also attracted to yellow cards, therefore it may be more practical to use yellow cards for general pest monitoring. The number of thrips per card should be recorded and graphed weekly to monitor population levels and movement in or out of the greenhouse, and thus aid in control decisions. Flowers can also be checked for thrips by tapping a blossom over a sheet of paper, although it is more efficient to use sticky cards to detect and monitor numbers of thrips. Yellow or white flowers seem to be particularly attractive to the thrips. Although effective thrips management can be difficult, adequate control can be achieved by a combination of physical, cultural and chemical control measures. Prevention is the obvious first step in a management program. It may be easier to prevent an infestation than to manage an established one. A grower should try to reduce the number of thrips present in the greenhouse at the end of the spring/summer season. The thrips will overwinter in the greenhouse and may result in a large infestation the following spring. Growers should also avoid purchasing plant material infested with thrips. Research in California has shown that the use of 400 mesh fine screen or barriers over vents can help prevent the movement of thrips into a greenhouse. Research in North Carolina is examining the use of materials such as Visqueen to cover plants on greenhouse benches. Greenhouse workers should avoid wearing yellow or blue so that thrips are less likely to be spread on worker’s clothing. Adequate chemical control involves the proper selection of insecticides, the number and frequency of application, the appropriate application method and spray particle size, and pesticide rotation. Insecticides that are registered for thrips control and have shown fair to good activity are shown by chemical class in Table 1.
It may be possible to improve the short-term effectiveness of an insecticide by combining it with a pyrethroid insecticide. It is thought that the pyrethroid insecticide, being an irritant, may serve to flush thrips out of protected locations to become exposed to more insecticide. Examples of pyrethroid insecticides include Talstar, Mavrik, resmethrin and Tempo. Several insecticide applications should be made at 5 day intervals to reduce a thrips infestation. None of the recommended insecticides are effective with one application. Research has shown that 5-day application intervals are more effective than 7-day intervals. Ideally, insecticides should be applied with equipment that produces very small spray particles (<100 microns). Spray particles of this size are best because they penetrate deep into the protected areas of the plant where the thrips are found and can provide the most efficient use of insecticides if coverage is thorough. Smokes or aerosol fog formulations of effective insecticides may also improve control. Rotating the use of insecticides from different chemical classes may be an effective way to delay the problem of insecticide resistance. However, it is best to maintain the use of an effective insecticide for more than one generation of a pest before rotating to another insecticide. Given the duration of the life cycle of WFT, an effective insecticide should be used for 2 to 3 weeks before switching to an insecticide from another class of chemicals. Dealing With Thrips And Tomato Spotted Wilt Virus There are at least two strains of TSWV. The strain most commonly found in greenhouses is called (INSV) Impatiens Necrotic Spot Virus. This virus is not easy to identify visually on many greenhouse crops. The use of an indicator plant, a plant in which the symptoms of the virus will develop quickly and be easily identified, should be considered the second step in the detection of the virus. The petunia cultivars, Calypso and Summer Maddness, serve well as an indicator plant. Indicator plants should be routinely used in greenhouses, especially in houses where incoming stock is placed, so that the plants can be assessed for disease and insect contamination. Indicator plants and sticky cards should also be used in empty greenhouses to monitor for thrips carrying the virus. The key to controlling thrips and Impatiens Necrotic Spot Virus (TSWV) is maintaining a clean, weed-free greenhouse. Weeds and crop debris serve as reservoirs for the virus. Physically remove all weeds or, if the house is empty, apply Round-up to kill actively growing weeds. Then apply Surflan to prevent weed seed germination. Water the Surflan in with about 1/2 inch of water to activate the herbicide and prevent volatilization. Use these herbicides in empty greenhouses only, to avoid any problems. The addition of granulated sugar to a tank mix in a wet spray of insecticide improves the effectiveness of most insecticides in controlling thrips. The sugar serves to draw thrips out of protected areas. Granulated sugar is used at a rate of 1 pound to 100 gallons of water and should be dissolved in warm water before adding it to the tank mix. References W.R. Allen and J.A. Matteoni. Petunia as an Indicator Plant for Use by Growers to Monitor Thrips Carrying the TSWV in Greenhouses. Plant Disease. Vol.75, No.1. Professional Plant Growers Association Newsletter. Vol. 20, No. 11, November 1989. Adapted from an article by John Sanderson, Cornell University Links to Further Resources on the Web See also our factsheet Impatiens Necrotic Spot Virus and Tomato Spotted Wilt Virus and the Web Resources listed there. ATTRA's exhaustive "Pest Management Technical Note" on Sustainable Thrips Control contains tables of Species of Thrips in Greenhouses and the thrips life-cycle, information on crop scouting and trapping, and citations of new research on thrips, sticky cards and indicator plants. Includes lists of plants that repel thrips, predatory mites, pirate bugs and parasitoids, least-toxic or "soft" pesticides and insect growth regulators. Extensive References and Further Reading sections, plus lists of specialists in thrips control and of Biological Control Suppliers. North Carolina Cooperative Extension's "Insect Note 72" Western Flower Thrips contains great photos of adult and juvenile thrips and the damage they do when feeding on seedlings and mature petals and leaves. The University of Minnesota's Factsheet on WFT:
Western Flower Thrips in Commercial Greenhouses. Includes a
good current table of pesticides labeled for WFT in greenhouses. Impatiens Necrotic Spot Virus, Tomato Spotted Wilt Virus and the vector Western Flower Thrips. Factsheet by Gary Moorman of Pennsylvania State's Cooperative Extension. Selected IPM Articles |
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