WESTERN FLOWER THRIPS By Dr. R. Lindquist OSU/OARDC Department of Entomology Wooster, Ohio 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 [INSV] and tomato spotted wilt virus [TSWV]) make these insects very serious problems. They can be extremely difficult to control once an infestation becomes well-established. WFT development (Photo 1) consists of the egg (deposited in plant tissue), two larval stages (0.5 to 1mm long), two transformation stages (which usually occur off the plant), and the 1 to 2mm long adult (Photo 2). Feeding injury (Photo 3) (Photo 4) is caused by both larvae and adults. Only adults are able to transmit viruses (Photo 5), which are acquired when larvae feed on infected host plants. Therefore, adult thrips that do not already carry virus cannot transmit virus from infected to non-infected plants. The length of the life cycle is dependent on temperature, with development occurring between about 50 to 90º F. Thrips can survive cooler temperatures than 50ºF, but there is no development at these temperatures. At warmer temperatures, the egg to adult cycle is about 10 to 14 days. No single pest management method 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 - especially when producing plants susceptible to INSV or TSWV virus. Yellow or blue sticky traps (Photo 6) (Photo 7), 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. WFT may fly less when plants are flowering and during winter months. Because thrips' flight habits vary, 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 flowers of crops such as African violets. Shake flowers gently over a piece of white paper. This will dislodge some thrips. Certain cultivars of petunia are suggested as indicator plants. The cultivars 'Blue Carpet', 'Burgundy Madness', 'Cascade Blue', 'Red Cloud', 'Summer Madness', and 'Super Magic Coral'. are excellent indicators for thrips and virus presence. These plants show visible feeding injury (Photo 8) before it appears on many other plants, and virus symptoms are visible more quickly than on most other greenhouse ornamentals - thus serving as kind of an early warning system. Indicator plants can be used much like sticky traps and in about the same numbers. Flowers and buds should be removed, and a non-sticky trap should be placed next to the petunia to attract more thrips. Petunias are not good host plants for WFT development (according to people who are supposed to know these things), so there is little chance that they will contribute to virus spread. However, if virus symptoms appear the plants should be removed from the greenhouse. It is important to know if the thrips in the greenhouse are carrying viruses (i.e. are infective), because this will lower the number of WFT that can be tolerated - if a virus-susceptible crop is being grown. This in turn will affect a thrips management program. 2. Exclusion. Western flower thrips can enter greenhouses through top vents, side vents, doorways, etc. 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. This can be done by placing sticky traps inside and outside of the greenhouse and comparing trap counts over time. Be careful here! The thrips caught outside may not be the same species as those causing problems in the greenhouse. If you do determine that problem thrips are entering from outside and want to investigate screening, contact an agricultural engineer or screening supplier for information on exclusion screening options. Research on the use of reflective aluminum shade cloth or tape around vents to repel thrips shows some promising results. In this case, no screening may be needed to significantly reduce the numbers of thrips entering the greenhouse. 3. Sanitation. Keeping weeds under control inside and outside of the greenhouse will help to reduce insect numbers. Some weeds are excellent hosts for both WFT and viruses (Photo 9). 4. Pesticides. There is no single pesticide that will provide total WFT control. Pesticide resistance is a problem. A suggested pesticide rotation program for western flower thrips (WFT) is listed below. This rotation can be started at any point. Only follow the pesticide rotation until the thrips infestation problem has been eliminated, as indicated by plant scouting and monitoring with sticky traps. There is no need to continue spraying forever! A scouting and monitoring program will track the WFT cycles in individual greenhouses and at different times of the year. Research has demonstrated that pesticide applications for WFT control will be most effective when used at the low points in the cycles. A. Apply a tank mix of abamectin (Avid @4-8 oz/100 gallons) + an azadirachtin (neem) insecticide. Azatin XL (10 to 16 oz/100 gallons) and Ornazin (8 oz/100 gallons) are products that can be used. Reapply after five to seven days. Avid is not yet registered for control of western flower thrips (or any other thrips, for that matter), but this is expected to change soon when product labels are revised. Greenhouse ornamental plants are listed, and in most states it is legal to apply a pesticide for control of a non-labeled pest as long as the plants are on the label. Adding horticultural oil (e.g. Ultra-fine Oil) at 0.5 - 1% by volume will increase the activity of Avid. In this case, it is not necessary to use a neem insecticide in the mix. B. Apply a tank mix of endosulfan (Thiodan 50WP @1 lb/100 gallons) + a Pyrethroid such as bifenthrin (Talstar) cyfluthrin. (Decathlon), or fenpropathrin (Tame) at suggested label rates. Reapply after five to seven days. As with Avid, thrips are not on the Thiodan label, but greenhouse ornamental plants are listed. As in the case of Avid, it is legal in most states to use a pesticide to control non-labeled pests on labeled crops. Thiodan can also be applied in a smoke generator formulation. In this case, thrips are on the label. C. Apply acephate (Orthene TT&O 97 @ 8 oz/100 gallons) methiocarb (Mesurol 75WP @8 to 16 oz/acre) bendiocarb (Closure 76WP @ 11 to 20 oz/100 gallons). Except for Orthene TT&O, which specifies a 14- day interval between applications, reapply after five to seven days. Orthene TR, a total release aerosol containing acephate, and Duraplex TR, a total release aerosol containing chlorpyrifos and cyfluthrin, could also be used. D. Apply spinosad (Conserve SC @ 6-22 oz/100 gallons). Repeat after five to seven days. Conserve is a new product for the greenhouse market, and has been very effective against western flower thrips. Use it late in the crop to kill thrips in flowers. Activity is very rapid. The above rotation has been effective for most growers. Different chemical families are applied each time. The products are still on the market as this is written. However, there are other older and newer products that should be considered as additions or substitutions to the rotation. Here are some other suggestions. Others may have their own favorites. Beauveria bassiana (BotaniGard or Naturalis-T&O (label rates). Make at least three applications, five to seven days apart. Use when thrips numbers are low. These products will not control severe thrips populations. A tank mix with Azatin XL, Ornazin or spray oil (0.5%) may increase effectiveness. Microbial insecticides (often called biopesticides) can be inserted anywhere in a rotation, but are most effective when thrips number are low. Do not use these products to reduce already-high thrips levels. Dichlorvos (DDVP Smoke Fumigator) This fumigant formulation may be useful for growers to use as an alternative to sprays. It is an organophosphate insecticide, so should be used as part of rotation schedule "C" above. Nicotine (Nicotine Smoke Fumigator) This fumigant formulation can be used anywhere in a pesticide rotation because it does not belong to any of the above pesticide classes. 5. Biological control. At this time on most commercial greenhouse ornamental plants there are no practical effective biological controls. Predatory mites (Photo 10) and insects, as well as insect-attacking nematodes have been and continue to be evaluated. Some growers have had success on bedding plants using biological controls for WFT. At least one grower has been able to manage WFT using predatory mites, with imidacloprid applied for aphid and whitefly control. See the articles on beneficial insects and mites in other issues of this newsletter as well as the web sites listed at the end of each article for more information. The bottom line: There are no easy answers for WFT management. But if all of the above pest management procedures are used properly there should be fewer problems with these insects.