Western flower thrips (WFT) are major insect pests in greenhouse crop-production systems. These insects attack a wide range of horticultural crops, preferring plants with blue, purple, yellow, orange, and white flowers. Western flower thrips damage plants directly by feeding on leaves and flowers, and indirectly by vectoring the tospoviruses, tomato spotted wilt virus, and impatiens necrotic spot virus. Thrips are difficult to control in greenhouses because of their high reproductive capacity, short development time, a cryptic habit (they hide in unopened buds), resistance to many insecticides, and a high immigration rate.
The life cycle consists of an egg stage, two larval (immature) stages, two pupal stages, and the adult stage. Adult western flower thrips are 2.0 mm long, and light yellow to brown in color. Development from egg to adult is temperature dependent. If temperatures are normal for June through August, the life cycle can be completed in 7 to 13 days. In general, the life cycle takes two to three weeks. Females live approximately 45 days and can lay up to 300 eggs during their lifetime. Eggs are inserted into plant leaves where they hatch into larvae that feed on plant foliage. The first larval stage lasts one to two days, and the second stage lasts two to four days. Western flower thrips then pupate in growing medium, leaf litter, or flowers. Adults emerge from pupae after approximately six days. They are generally found in flowers as they are heavy pollen feeders. The females require pollen for reproduction (egg laying).
Management of WFT involves a combination of diligently applied strategies. Scouting crops early helps detect populations before they get out of hand. Use either yellow or blue sticky cards. I prefer yellow cards because WFT are easier to see on them. Place the cards above the crop canopy to catch winged adults. This will help determine the effectiveness of your management strategies. When scouting, pay particular attention to plants located near openings such as doors, vents, and side walls. Concentrate scouting efforts on plants with blue, purple, yellow, orange, and white flowers. Greenhouse operations with gravel or soil floors should have sticky cards placed beneath benches to help detect thrips that may be pupating in the gravel or soil.
Proper sanitation is important in managing WFT. Remove all plant debris and old growing medium because these provide sites for thrips to hide and pupate. Control all weeds inside and outside the greenhouse. An Indiana grower and I demonstrated that thrips could easily immigrate into the greenhouse from weeds just outside vents. Removing flowers from plants that are not yet ready for sale will also help minimize thrips problems.
Many insecticides are used to control WFT (Table 1); these materials must be applied, however, before thrips enter unopened flower buds. After that point, it is very difficult to control them. Knowing this, you will want to control thrips before plants enter the bud stage. Clifford S. Sadof and I conducted research over the last three years at Purdue University that showed that Conserve is very effective in controlling WFT. However, donít base your thrips control program on one insecticide. It is important to rotate different chemical classes with different modes of action to minimize the potential for resistance. Three examples of possible rotation schemes follow.
Remember that the organophosphates (Orthene and Duraguard) and carbamates (Dycarb and Mesurol) have similar modes of action, so spraying with Orthene, then switching to Mesurol is not a rotation scheme. Frequency of application is very important as most of the insecticides have no activity on the eggs and pupae stages. When generations overlap, more frequent applications, such as spraying every three to five days, may be necessary.
Biological control of WFT is another strategy, but it is difficult due to the characteristics mentioned. It is possible, however, under certain crop-production situations. I will discuss biological control of WFT in a future issue of this newsletter.
Table 1. Materials used to control western flower thrips.
Diazinon (Knox Out)
Chlorpyrifos + Cyfluthrin (Duraplex)