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Thrips bug you? Bug them back!

February 7, 2012  By Julie Graesch

Western flower thrips (WFT) are one of the most prevalent and damaging thrips species in greenhouse and nursery operations.

Western flower thrips (WFT) are one of the most prevalent and damaging thrips species in greenhouse and nursery operations.

Thrips are ubiquitous greenhouse and nursery pests that require constant monitoring.



Thrips are ubiquitous greenhouse and nursery pests that require constant monitoring and a rigorous management program for commercially acceptable levels of control. Many thrips species infest crops, including avocado, bean, citrus, chili, Cuban laurel, greenhouse, gladiolus, myoporum, onion and western flower thrips. Some thrips are specialists, feeding on just one or a few host plants. Others, such as WFT, are generalists that feed on many herbaceous ornamentals, vegetables, fruits, shrubs and trees.

Regular scouting and monitoring are essential to identify the presence of WFT early in a cropping system in order to limit damage. Adult WFT are tiny (<2 mm), yellowish, slender insects with fringed wings. Female thrips are slightly larger than males and can deposit 130-230 eggs directly into plant tissues such as leaves, bracts and petals. Newly hatched larvae go through two actively feeding instars that are followed by a non-feeding pre-pupa and pupal instar.

Blue and yellow sticky cards.

Western flower thrips typically pupate in growing media when plants are small or vegetative, however, thrips also may pupate on flowering plants or plants that have dense foliage such as chrysanthemums. Under ideal conditions (20-37°C or 68-99°F), thrips may take as little as one to two weeks to complete their life cycle.

Foliar damage occurs when WFT insert their piercing, sucking mouth parts into plant cells, draining cellular contents. Western flower thrips prefer to feed on young tissue and buds, causing damaged cells to collapse and resulting in deformed plant and flower growth. Damage to mature leaves and flowers result in silvered patches and feces flecking.

In addition to feeding injury, WFT can vector two tospoviruses: the impatiens necrotic spot virus and the tomato spotted wilt virus. Transmission of tospoviruses to unaffected plants occurs as the larvae and adults feed. Indicator plants such as fava bean and petunias can be grown among susceptible plants to monitor for tospoviruses. 
Adult WFT population densities can be monitored by placing yellow or blue sticky cards (at least one per 1,000 square feet) just above the crop canopy. Action thresholds vary depending on crop species and stage, crop susceptibility, growing conditions and likelihood of tospovirus transmission. Establish thresholds by reviewing monitoring records to determine when a control measure was applied, its effectiveness, and the acceptability of the finished plant. Acceptable sticky card numbers should be at or below an economically damaging level.

Traditionally, WFT were controlled with synthetic insecticides that often provided a quick and effective knockdown. However, thrips have developed resistance to several conventional pesticides. Resistance is common with thrips because of their ability to rapidly produce hundreds of

Due to the potential for WFT to develop resistance, many growers have begun adopting integrated pest management (IPM) practices. Common IPM practices include a thorough scouting and monitoring schedule with detailed records as well as cultural, mechanical and physical control strategies. Additional practices include rotating modes of action, using insecticides with broad modes of activity, and using biological control agents (BCA).
Biological control agents (BCAs) can be used alone or in combination with compatible insecticides. Commonly used insect predators and parasitoids include mites, true bugs, beetles, fungi and insect parasitic nematodes (IPN). Amblyseius cucumeris and A. swirskii are both foliar-dwelling predatory mites that feed on larval thrips. Orius insidiosus, also known as the minute pirate bug or noseeums, targets adult and larval thrips on foliage. Hypoaspis miles, a soil-dwelling predatory mite, and Atheta coriaria, the rove beetle, target pre-pupal and pupal thrips in the media. Two additional thrips-killing BCAs are Beauvaria bassiana, a fungus, and Steinernema feltiae, an insect parasitic nematode.

thrips3 thrips4
 An adult western flower thrip on a leaf.

 A closeup of a nematode.

Insect parasitic nematodes (IPN) are microscopic unsegmented roundworms that infect insects through natural body openings, killing hosts in 24 to 48 hours. Most of the BCAs listed above are applied to either the foliage or growing media. Steinernema feltiae, on the other hand, can be applied to both the foliage and growing media. Successful programs using S. feltiae start with a drench application to the media where S. feltiae has the potential to persist for several weeks. 

Following the soil drench, weekly foliar applications should be made to target foliar-dwelling thrips. Some growers use sprench applications targeting both foliar and soil-dwelling populations. Drench and sprench applications are particularly useful for growers who also have fungus gnats, another pest controlled by S. feltiae.

When making foliar and sprench applications, IPN require at least two hours of foliar moisture in order to be effective. For the best result, use spray adjuvants, close blackout curtains and vents, and switch off artificial lighting. If concerned about fungal pathogens, IPN can be combined with compatible fungicides to minimize chances of disease. Nematode applications should be made in the early morning or late evening to minimize UV light and heat exposure to IPN as well as to target thrips when their activity is generally low.

Insect parasitic nematodes can be applied through common application equipment such as irrigation injection, boom sprayers, overhead, drip, tank sprayers, dips and backpack sprayers. However, minor equipment modification may be necessary. Filters of 50 mesh or finer and pump pressures of 300 psi (20 bar) or greater can damage IPN. Avoid using mist nozzles or foggers when applying nematodes. Finally, prevent stock tanks from settling or getting too hot (>30°C/86°F).

To enhance WFT control, growers often combine S. feltiae with chemical programs. Insect parasitic nematodes can be tank-mixed with many common insecticides, fungicides, insect growth regulators and fertilizers. Always check chemical compatibility before tank-mixing. Because of their chemical compatibility, IPN often are the first BCA used by growers when starting a biological or resistance management program.

In addition to combined biological and chemical programs, growers can also use complete biological programs that incorporate S. feltiae with other BCAs. Complete biological programs are especially desirable for food crop growers who have fewer chemical options.

In order to be successful, biological programs require an early start, targeting low insect populations. Banker plants are useful tools to help establish, support and maintain one or more BCA populations by providing pollen, plant material and insect hosts. The sweet pepper plant commonly is used as a banker plant for thrips control that provides Orius insidiosus with pollen, an essential food source.

A benefit of developing integrated control programs is reduced chemical applications. Fewer applications are desirable for workers who apply chemicals, and for consumers who want to receive plants with fewer insecticide residues. Additionally, shorter or no restricted entry intervals allow growers to have fewer interruptions of worker and customer schedules. Finally, a well-designed IPM system provides growers with a rigorous and sustainable resistance management program that can preserve longevity and effectiveness of chemical insecticides.

Western flower thrips are one of the most damaging and prolific greenhouse pests, therefore, meticulous programs incorporating chemical and biological control agents are important for long-term management of this pest.

Biological control agents, such as IPN, have become widely accepted as a viable method for controlling thrips as well as other pest species. When transitioning from an insecticide-based program to a biologically based program, know that there are resources such as books, consultants, manufacturers and distributors that will help bridge any learning gaps. Though challenging at first, biologically based IPM systems are more user-friendly, economical and sustainable than traditional chemical programs.

Learn how beneficial nematodes can help you control western flower thrips in your greenhouse operation.

Julie Graesch is a nematode field development specialist at Becker Underwood, where she is responsible for product development, as well as coordinating research opportunities with universities and third parties. She can be reached at or 515-232-5907.

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