PSI’s most wanted: Western Flower Thrip

Meet the perp

Bio: Western flower thrips (Frankliniella occidentalis) are still the most destructive insect pest of greenhouse-grown horticultural crops, the reason being that western flower thrips cause both direct feeding damage to leaves and flowers, and indirect damage by vectoring viruses. This results in a very low tolerance for this insect pest.

Description: Western flower thrips are approximately 2.0 mm in length and light brown in color. The life cycle consists of an egg stage, two nymphal stages, two pupal stages, and an adult. Western flower thrips is extremely polyphagous, feeding on a wide-variety of greenhouse-grown horticultural crops including annuals, perennials, herbs, vegetables, and orchids. Both adults and nymphs feed on leaves and flowers.

Known locations: Commonly found residing in un-opened flower or terminal buds, which provide protection from either insecticides or natural enemies.

How they hurt plants: Western flower thrips cause direct plant damage by feeding on plant leaves and flowers with their piercing-sucking mouthparts. They do not feed in the phloem sieve tubes like aphids and whiteflies but instead feed on the mesophyll and epidermal cells of leaf tissues. Symptoms of feeding include leaf scarring, distorted growth, sunken tissues on leaf undersides, and deformed flowers. Leaves and flowers exhibit a characteristic “silvery” appearance due to the influx of air after the removal of plant fluids. Furthermore, black fecal deposits may be present on leaf undersides. Damage to plant leaves may also occur when females, using their sharp ovipositor, insert eggs into leaf tissue. Any wounds created during feeding or egg-laying may serve as entry sites for plant pathogens such as fungi.
 

Scouting for the perp

Monitoring or scouting is important to determine the numbers of western flower thrips present in the greenhouse throughout the growing season. Monitoring will determine seasonal trends in populations and assess the effectiveness of plant protection strategies. The primary technique used to monitor for western flower thrips adults is to place blue or yellow sticky cards just above the crop canopy. These cards should be counted weekly and the number of adults recorded. Visual inspections such as looking into open flowers, and/or shaking open flowers and leaves over a white sheet of paper are additional ways to determine the presence of adults and nymphs.
 

Preventing damage

Sanitation is the best way to avoid problems with western flower thrips. Practices like removing weeds, old plant material and growing medium debris will help avoid or minimize problems with western flower thrips. It is also important to immediately remove heavily-infested plants and plant material debris from the greenhouse or place into containers with tight-sealing lids as western flower thrips adults may leave desiccating plant material and migrate onto the main crop. If economically feasible, installing insect (specifically thrips) screening over greenhouse openings such as vents and sidewalls will reduce adult populations from entering the greenhouse from outside.
 

Treatment

Insecticides are still primary used to suppress western flower thrips populations. However, it is critical to rotate insecticides with different modes of action in order to avoid the potential for resistance developing. The same mode of action should be used within a generation, approximately two weeks, before using another insecticide with a different mode of action for the same time period. Natural enemies such as predatory mites (Neoseiulus cucumeris and Amblyseius swirskii) and bugs (Orius spp.) may be used to regulate western flower thrips populations; however, natural enemies must be released before high numbers of western flower thrips are present. The use of products that contain entomopathogenic (beneficial) fungi including Beauveria bassiana (BotaniGard) should also be considered. If used early in the crop production cycle these products may be effective in suppressing populations of western flower thrips.

Western flower thrips are one of the most damaging greenhouse insect pests. The best way to avoid infestations is to practice proper sanitation, including removing weeds, old material and growing medium debris.