Voracious thrips can be managed

Thrips are well-known insect pests of greenhouse-grown ornamental crops but a number of thrips species including the western flower thrips, Frankliniella occidentalis, feed on a wide variety of greenhouse-grown vegetable and fruit crops. Vegetable and fruit crops susceptible to attack from thrips include tomato, pepper, cucumber, eggplant, and strawberry. Although this article focuses on the western flower thrips, most of the information also pertains to other thrips species. This article describes the damage, and primary management strategies that producers can implement in order to alleviate western flower thrips problems on greenhouse-grown vegetables and fruits.
 

Damage: direct and indirect

Western flower thrips (referred to as “thrips” the remainder of the article) cause direct damage by feeding on plant leaves and pollen. They are also responsible for causing indirect damage to certain vegetables and fruits by vectoring the tospoviruses; impatiens necrotic spot virus and tomato spotted wilt virus. For more information on the specific details regarding thrips and virus association, refer to the article in this month’s issue of Greenhouse Management.

In general, adults feed on pollen whereas the nymphs (larvae) feed on plant leaves, although adults may also feed extensively on plant leaves if no pollen is present. Damage to plant leaves may also occur when females, using their sharp ovipositor, insert eggs into plant tissue. Any wounds created during feeding or oviposition may serve as entry sites for plant pathogens such as fungi. Black fecal deposits may be present on leaf undersides.
 

Management strategies

When growing vegetables or fruits in greenhouses, these practices can help to avoid problems with thrips.

Scouting or monitoring is important to determine the numbers of thrips present in the greenhouse. In addition, scouting will detect seasonal trends in populations throughout the growing season (or year), and allow greenhouse producers to assess the effectiveness of management tactics that have been implemented.

The main technique used to scout for adult thrips is to place either blue or yellow colored sticky cards just above or among the crop canopy. Cards are counted weekly and the numbers of adults are recorded. Visual inspection by shaking plant leaves over a white sheet of paper is another method that may be used to scout for both nymphs and adults, though it is important to remember to kill the thrips that landed on the white sheet of paper. There is some psychological satisfaction in doing this.

Plant material from suppliers should be inspected before introducing into the main crop. However, in reality, this may be too time consuming and may not be possible during spring through fall business hours, especially after receiving large quantities of plant material.

Sanitation practices such as removing weeds, old plant material, and growing medium debris are the first-line-of-defense in minimizing problems with thrips on vegetable and fruit crops.

Certain weeds, particularly those in the Compositae (chrysanthemum) and Solanaceae (potato) families and those with yellow flowers will attract adults, and may serve as reservoirs for thrips populations, and the viruses that can be transmitted (vectored) by adults. It is critical to remove all weeds from both inside and around the greenhouse perimeter.

Also, be sure to immediately remove plant material debris from the greenhouse or place into containers with tight-sealing lids as thrips adults may abandon desiccating plant material and migrate onto the main vegetable and fruit crops. Finally, plants that are heavily-infested with thrips should be disposed of immediately.

Avoiding over-fertilization of plants is important because thrips – especially western flower thrips – may attack plants with elevated concentrations of nitrogen because of an abundance of amino acids and proteins. Furthermore, female egg production tends to be higher after feeding on plants containing abundant levels of amino acids.

Screening greenhouse openings such as vents and sidewalls will reduce thrips populations entering greenhouses from outside or migrating into other greenhouses. The appropriate screen size or mesh for thrips is 192 microns (0.037 millimeters squared) or 100 mesh. This may alleviate problems with thrips possibly moving from weeds and/or field-grown crops, such as corn or soybean, and vegetables into greenhouses.

Trapping plants (especially those with flowers) that are highly attractive to thrips, such as yellow flowering marigold and transvaal daisy, may be used to protect vegetable and fruit crops, especially prior to blooming. Trap plants may be randomly dispersed among the main vegetable and fruit crops and then either sprayed with an insecticide, routinely removed from the greenhouse (and replaced with a fresh trap plant), or inoculated with biological control agents, such as predatory mites or predatory bugs, that will feed on the nymph and/or adult life stages residing either on the leaves or in the flowers.

Alternative non-insecticidal management strategies include the use of ultraviolet-absorbing plastic films, which may influence adult thrips flight behavior by reducing the levels of ultraviolet light entering greenhouses, and aluminized reflective fabrics may inhibit or repel adults from entering greenhouses.

Others include the placement of a weed- fabric barrier underneath benches, which may prevent thrips from entering the soil to pupate and mechanical blowers that can then be used to distribute plant and/or growing medium debris (along with thrips pupae) into concentrated areas, which can then be collected, and disposed of promptly to reduce pupae numbers.

Insecticides require users to do three things first: 1) read the label…2) read the label and…3) read the label. The key to managing thrips with insecticides is to initiate applications when populations are “low,” which avoids dealing with different age structures or life stages including eggs, nymphs, pupae, and adults simultaneously over the course of the growing season. Once thrips populations reach “high” levels, then more frequent applications will be required.

Furthermore, insecticides must be applied prior to thrips entering terminal or flower buds. Three factors influence the success in suppressing thrips populations with insecticides: timing, coverage, and frequency. Timing is associated with making applications when a high proportion of the most susceptible life stages (nymphs and adults) are present.

Coverage refers to thoroughly covering all plant parts with insecticide sprays in order to maximize effectiveness. Frequency means that insecticide applications need to be applied at regular intervals in order to keep thrips populations from building-up.

Biological control can be an effective means for controlling thrips without affecting vegetable and fruit yields. There are a number of biological control agents commercially available including the predatory mites, Neoseiulus (Amblyseius) cucumeris and Amblyseius swirskii; and the minute pirate bug, Orius insidiosus.

It is important to release biological control agents early and use appropriate release rates in order to continually suppress thrips populations during the growing season.

However, the use of biological controls may not be feasible for certain vegetable crops, such as tomatoes, because the plants possess hairs (trichomes) that may disrupt biological control agents for example, predatory mites, from effectively suppressing thrips populations.

The use of entomopathogenic fungi such as Beauveria bassiana (BotaniGard by BioWorks), Isaria fumosorosea (NoFly by Natural Industries and Preferal by Sepro), and Metarhizium anisopliae (Met52 by Novozymes) may be another option.

Be sure to read the label to make sure it is legal to use these products on greenhouse-grown vegetables and fruits, and determine the appropriate environmental conditions (e.g., temperature and relative humidity) that need to be maintained in order to enhance effectiveness. Biological control has advantages including minimizing worker exposure to insecticide residues, avoiding plant phytotoxicity, and mitigating the prospect of resistance developing in thrips populations.
 

Diligence pays

In conclusion, thrips management in greenhouse-grown vegetables and fruits requires diligence in order to prevent substantial feeding damage and yield losses. This involves establishing an appropriate scouting program, maintaining proper sanitation practices, avoiding over-fertilizing plants, using trap plants, properly applying insecticides, and/or initiating early releases of biological control agents.

Taking these steps and maintaining a level of consistency throughout the growing season will help ensure profitable yields.

Raymond A. Cloyd is a professor and extension specialist in Horticultural Entomology/Integrated Pest Management at Kansas State University - Department of Entomology.