WESTERN FLOWER THRIPS

Thrips, especially the western flower thrips, or WFT (Frankliniella occidentalis) are the main insect pests in many greenhouses. WFT feeding injury on leaves and flowers of numerous host plants, as well as virus transmission (including impatiens necrotic spot virus and tomato spotted wilt virus) makes these insects very serious problems. They are extremely difficult to control once an infestation becomes well-established.

WFT development consists of the egg (deposited in plant tissue), two larval stages, two transformation stages (which usually occur off the plant), and the adult. Feeding injury is done by larvae and adults. Only adults transmit viruses, which are acquired when larvae feed on infected host plants. The life cycle is dependent on temperature, with development ocurring between about 50 to 90 degrees F. Thrips can survive cooler temperatures than 50F, but there is no development at these temperatures. At warmer temperatures, the egg to adult cycle is about 10 to 14 days.

No management method used alone will control these insects. The most important parts of a thrips management program are:

1.Detection. There are three good ways for thrips detection: sticky traps, plant inspection and indicator plants. The best procedure is to use all three methods. Yellow or blue sticky traps, used at about 5-10/acre (plus some near doors, vents, etc.) can be very useful. Traps should be placed just above the tops of the plants and inspected frequently (once or twice per week). If thrips are found on traps, there are thrips on crops in the greenhouse. Unfortunately, if no thrips are found on traps, there still may be thrips on crops in the greenhouse. Therefore, plants should be looked at as well, especially those crops known to be susceptible to thrips or viruses, and/or are flowering. Look for feeding scars on leaves or flowers, small dark spots of fecal matter, pollen "tracks" on crops such as African violets. Shake flowers gently over a piece of white paper. This will dislodge some thrips. Petunia plants (e.g. 'Calypso', 'Summer Madness') and fava bean ('Toto') are excellent indicators for thrips and virus presence. These plants show feeding injury and virus symptom are more visible than on many greenhouse ornamentals. Indicator plants can be used much like sticky traps and in about the same numbers.

2. Exclusion. The research being conducted using screens and/or positive air pressure for thrips exclusion shows that WFT are extremely difficult to exclude because they are so small. However, it is possible to reduce thrips invasions from outside the greenhouse by screening. Not all openings may need screening, because most of the thrips may be moving into the greenhouse from one direction. The first priority is to establish whether the thrips are moving into the greenhouse from outside. If this is the case, then contact an agricultural engineer or screening supplier for information on exclusion screening options. There is a software program available from North Carolina State University that will help with these calculations.

Research on the use of refelective aluminum shade cloth to repel thrips shows some promising results. In this case, no screening may be needed.

3. Pesticides. There is no single pesticide that will provide total WFT control. Pesticide resistance is a problem. One of the most effective products (Avid) does not mention thrips on its' label. However, Avid is registered on greenhouse crops for control of other pests. Mesurol 75WP is now back on the market and is registered for thrips control. Both Avid and Mesurol have controlled WFT quite well. DDVP (Vapona) fogs or smoke generators, plus Dibrom vapors may also be effective. Other products that will provide some control include Azatin, Decathlon, Orthene, Thiodan, Preclude aerosols (aerosol formulation of fenoxycarb). Fungus-based products (Mycoinsecticides) containing Beauveria bassiana (BotaniGard, Naturalis-O) have been reported to be effective by some growers using it - especially for thrips control in open flowers. Marathon is alsoregistered for thrips control, but will not control them in flowers. Our results have been quite good using a tank mix of Avid and Azatin, or Marathion granular treatment followed by Precision or Azatin sprays. The granular insecticide Pinpoint 15G has given excellent control on some flower crops - even in open flowers. Unfortunately, Pinpoint has a very limited label in terms of listed crops. It also can injure plants. Use this material with caution, but it is worth a try.

4. Application interval. The application interval is extremely important. and will need adjusting, depending upon the thrips numbers, crop and temperature. Applications may be required at 3, 5, or 7-day intervals. Change pesticide classes after every two or three applications. Use different application methods whenever possible. The low-volume sprayers, smoke generators and aerosol products are effective here.

5. Biological control. At this time on most commercial greenhouse ornamental plants there are no practical effective biological controls. Predatory mites and insects, as well as insect-attacking nematodes have been and continue to be evaluated.