Western flower thrips feed on hundreds of plant species, including weeds, grasses, fruit, vegetable, and field crops, and most ornamental plants, including propagation material. They have a single mandible they use to scrape and puncture plant tissue and feed on sap that seeps from the wounds. They also consume pollen and spend much of their time in flower heads, if available. In flowers, thrips feed on pollen and on the petals.
Facts about Thrips Photo by Frank Peairs, Colorado
Look on the underside of leaves for the fast-moving larvae and fecal matter. Tap flower heads onto a sheet of paper and look for thrips that are dislodged.
Indicator plants can also be useful to monitor for thrips and tospoviruses such as INSV (impatiens necrotic spot virus). Petunias make good indicator plants. In addition, petunias have a hyper-sensitive response to tospovirus infection resulting in rapid, visible death of tissue around the infection site. This provides a visual cue for growers that infected thrips are present in the greenhouse.
Lifecycle: Western flower thrips have six developmental stages: egg, two larval stages, prepupae, pupae and adult. Western flower thrips females can lay 150 to 300 eggs in her lifetime. Eggs are delicate, cylindrical, slightly kidney-shaped, smooth and translucent white. These are inserted into leaf, bract, or petal tissue and very difficult to detect. First stage larvae are very tiny, almost worm-like insects that are translucent white. Second stage larvae are also translucent white but are similar to the adult in size and shape. They crawl and jump quickly on the surface of leaves. Both instars have red eyes. Prepupae are similar to second stage larvae except that the wing buds are externally visible. They pupate in flowers or in soil. Pupae do not feed and have longer wing buds and the antennae are folded back over the head. At 86°F, development from egg to adult takes 13 days or less. Adults live about 28 days.
Damage: Feeding from Western flower thrips causes leaf and flower discoloration and distortion.
Control: Sanitation is essential for thrips management. Thrips will reproduce on weeds that are present in greenhouses. Weeds also serve as a reservoir for tospoviruses and other pests. Thrips can pupate in potting soil or other debris on the floor and benches. Denying these resources can help interrupt the lifecycle of western flower thrips and reduce the rate of population growth. Keep grass and flowering weeds outside the greenhouse mowed short, also. If there are multiple greenhouses, workers should avoid yellow or bright colored clothing, or provide them with lab coats to switch out when moving from one greenhouse to another. In high risk or high value areas, a vestibule, outside "chamber" could be built through which to pass first before entering the greenhouse itself.
Perhaps the best way to manage western flower thrips is to prevent them from entering greenhouses in the first place. This requires the use of thrips screen over fans, vents, and other openings. It also helps to quarantine and inspect new plant material before moving it into the greenhouse with existing plants.
Biological control agents include predacious mites (Neoseilus cucmeris and N. degenerans ), minute pirate bug (Orius insidiosus ), entomopathogenic fungi and nematodes.
Pesticides kill active stages of thrips that they contact but will not kill eggs or pupae. Just days after a pesticide application, new larvae and adults will emerge. Therefore, a second application is recommended three to seven days later. All plants may need to be treated, not just the severely infested ones. To avoid resistance issues, use several insecticides from different IRAC Mode of Action groups.
