Greenhouse Pest 101: Western Flower Thrips
Western flower thrips (Frankliniella occidentalis) are one of the most, if not the most, serious pests of greenhouse floricultural crops. Although the insects’ actual feeding causes damage to leaves and flowers, the most serious consequence of an infestation is that thrips can spead the tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV). Both of these viruses are serious problems for a greenhouse grower and can cause large-scale losses of plants. Western flower thrips (WTF) feed on most major greenhouse ornamentals, including chrysanthemum, gerbera, fuchsia, petunia, impatiens and roses, as well as hundreds of other plants species.
There are six stages in the thrips’ lifecycle: eggs, which are deposited within the leaves, flowers or bracts, two larval stages, prepupae, pupae and adult. The second-stage larvae are quite active, jumping and crawling on the surface of the leaves.
Monitoring For Thrips
Monitoring for WTF is essential. By the time damage is visible, populations are usually quite high. Sticky cards placed throughout the greenhouse just above the tops of plants are the first line of defense. Both yellow and blue sticky traps will work; blue is attractive to thrips and less attractive to other insects. Tapping the leaves or flowers of plants against white paper will also reveal the presence of thrips. The tiny insects (only 1/25th of an inch long) are knocked off onto the paper where they are visible to the naked eye. Black specks of feces deposited on the undersides of the leaves can also be seen.
Thrips have piercing, sucking mouthparts and damage appears as silvery patches and specks on the leaves and flowers. If feeding occurs within buds, the resulting leaves and flowers will be deformed.
Use of indicator plants, such as petunia, that are especially attractive to thrips is an additional way to monitor for their presence. These plants are affected first when there is an infestation, giving the grower time to respond with controls to protect the actual crop.
Control methods range from sanitation to mechanical methods (screening over vents and air intakes) to insecticides and biological controls.
Weed control and removal of dead plant debris and old soil is important. Weeds can be hosts of both INSV and TSWV as well as of the thrips themselves, and thrips pupate in soil as part of their lifecycle.
Chemical controls are often used, and it is important to use long-term rotations of several classes of insecticides to reduce chances of resistance. Timing is important, because eggs and pupae are not susceptible to pesticides. Chemicals must be applied at three- to seven-day intervals. In addition, if biological controls are being used, the compatibility of the insecticides with the biological controls must be considered.
Biological controls may include parasitic wasps, predacious mites, pathogenic fungi and nematodes.
Frank, Steven D. 2012. Western Flower Thrips in the Greenhouse. North Carolina State University Cooperative Extension. http://www.ces.ncsu.edu/depts/ent/notes/O&T/flowers/ort072e/ort072e.htm
McDonough, Michael J., Daniel Gerace and Mark E. Ascerno. 2006. Western Flower Thrips in Commercial Greenhouses. University of Minnesota Extension Service. http://www.entomology.umn.edu/cues/extpubs/7374WFT/DG7374.html.
Van Driesche, Roy. 2012. Biological Control of Western Flower Thrips. University of California Riverside. http://biocontrol.ucr.edu/wft.html