Taking control of thrips

Reflective mulch barriers can help deter thrips from entering the greenhouse. Courtesy of Robert C. Hochmuth

Thrips can be detrimental to several vegetable crops, especially those in the Solanaceae family, which includes tomato, pepper and eggplant. There are roughly 5,000 described species of thrips worldwide, and fortunately, most live in leaf litter or on dead wood.

Thrips are very small, ranging from 1/32 inch to 1/8 inch in length, and they can be yellow, brown or black in color. (Note: Even if there is just one, it would still be called a thrips.)

Lifecycle and species
Adult male thrips develop from unfertilized eggs and have half the number of chromosomes (haploid) as adult female thrips (diploid), making them haplodiploid organisms. A thrips’ lifecycle includes an egg stage, two larval instars, two pupal stages and an adult stage. Several species of thrips are common in vegetable production. They include the following:

Flower thrips. Adult flower thrips are yellow with brown splotches, and larvae are lemon yellow in color and wingless. The Florida flower thrips (Frankliniella bispinosa) and western flower thrips (Frankliniella occidentalis) attack hundreds of plant species, including oaks, roses, citrus and vegetables. Feeding and damage on buds, flowers and leaves result in discoloration and premature dropping.

Western flower thrips are responsible for vectoring tomato spotted wilt virus (TSWV). The vector cycle begins with thrips feeding in the larval stage, eventually multiplying the virus in their salivary glands. The virus is then transmitted to an uninfected plant during feeding by adults.

TSWV is most common on Solanaceae crops, in which 168 species are susceptible worldwide. In tomatoes, the virus causes small dark brown spots to appear on young leaves, stems and leaf petioles, eventually causing plants to exhibit one-sided growth. The immature fruit of infected plants will display mottled, light-green rings with raised centers. Mature fruit will display unique orange and red discoloration. TSWV has also been found to infect natural vegetation (serving as an alternate host) found growing near susceptible crops.

Gladiolus thrips. Gladiolus thrips (Thrips simplex) are dark brown with a gray band on their folded wings, and immatures are pale yellow with red eyes. They are pests of gladiolus, but they can also be found feeding in tomatoes. Gladiolus thrips may be seen on foliage on overcast days, but they hide in flower buds and beneath leaves in sunny conditions. Flowers with severe damage fail to open.

Melon thrips. Melon thrips (Thrips palmi) are light yellow or whitish with numerous hair-like bristles on their bodies. Immatures resemble adults in body form but lack wings and are smaller. Melon thrips are most commonly pests of Cucurbitaceae and Solanaceae crops. They can also be found feeding on beans, cabbage and okra. Melon thrips damage most often occurs on foliage; damaged leaves become yellow, white or brown and then begin to crinkle and die, oftentimes causing a bronzing color across the entire crop.

Monitoring techniques
A few flowers should periodically be sampled by placing them in small containers with 70 percent alcohol (flowers can also be shaken on white paper). The container can then be shaken to eject the thrips, which can then be examined under a microscope with at least 40X magnification to determine the species (your county agent can help with this process). Several small, medium and large fruits should be examined directly per acre. It is important to examine small, developing fruits frequently because eggs are generally laid during the flower stage, and larvae on small fruits is the first indication of a problem.

Adult flower thrips are yellow with brown splotches. Cortesy of Sizanne Wainwright-Evans.

White sticky traps are preferred for thrips collection because yellow traps attract a large percentage of beneficial insects, and it is difficult to see thrips against a blue background. Traps should be placed 1 to 2 feet below the tops of plants and should be monitored and changed weekly.

Thrips management
A well thought-out integrated pest-management program is the most effective way to manage thrips. This includes the incorporation of cultural practices, biological controls and reduced-risk insecticides. With the risk of potential problems associated with the use of chemicals in a greenhouse environment, growers should count on exclusion as their first line of defense.

Insect management should certainly be considered when designing a greenhouse. Insect screening is available for openings of ventilation, but reduced airflow should be a consideration when using this strategy because many screens that exclude thrips may reduce airflow by 50 percent or more. The use of ultraviolet-absorbing plastics can reduce insect populations for plastic-covered greenhouses. The UV-free light produced inside the greenhouse disrupts insect landing and feeding behavior, greatly reducing the spread of insect-vectored viruses.

Insect exclusion can be achieved through, but not limited to, four basic strategies: Sanitation, air-lock entrance, insect screening, and/or reflective mulch. A series of short videos on these topics can be found at the UF/IFAS Extension site (bit.ly/XDD9u5).

Sanitation. The area around a greenhouse should be maintained relatively free of weeds and plant material that could harbor thrips. You can achieve this with nursery cloth or a ground cover in at least a 10-foot-wide barrier around the structure. Preferably, a turf area should be maintained beyond this area. Other plant material in the general vicinity should be evaluated for their pest-harboring ability.

Placing tomato flowers in vials with 70% alcohol for sampling. Courtesy of Joe Funderburk, Professor, Entomology and Nematology Dept, North Florida Rec, Quincy, Florida

Air-lock entrance. Thrips often prefer to use the front entrance of a greenhouse. An air-lock entrance is critical in greenhouses with fan and pad-type ventilation systems. An air-lock can be achieved by simply installing a room enclosing the entry doorway of the greenhouse. This extra room allows workers to close the outside door behind them, reducing the chance of pests being pulled in by the ventilation section. The air-lock entrance room can also be utilized to house a footbath, hand washing area and any other sanitation stations.

Insect screening. Insect screening allows for adequate ventilation while still maintaining exclusion properties. Screens can be used in both passively ventilated and fan and pad greenhouses by either installing a screen box around the outside fan system or by simply installing material to roof vents. It is important to keep in mind that any size screen mesh will reduce the air flow in the greenhouse. Improper sizing has the potential to burn fan motors or reduce cooling. It is important to follow recommendations from the manufacturer with regard to screen size and surface area covered.

Reflective mulch. Ultraviolet, reflective mulch has been used in field-grown production for quite some time. A similar approach can be used in greenhouse production to help deter and confuse pests from entering through the ventilation system. Research has shown that reflective mulch should form at least a 20-foot wide, continuous barrier from the ventilation air intake end wall. Mulch along the sidewalls may also help repel insect pests. It is recommended that reflective mulch be used in conjunction with screening for an optimal effect.

Chemical and biological control. There are a limited number of registered insecticides for thrips control in greenhouses. Some azadirachtin- and spinosad-containing products are labeled, but they are susceptible to resistance. However, various biological controls are available. Amblyseius cucmeris and Amblyseius swirskii are very effective at controlling thrips larvae, but not adults. It is important to note that A. cucumeris should be released over a period of weeks to be effective.

When the crop begins to flower, minute pirate bugs (Orius spp.) can be released to control adult thrips. Insect-parasitic nematodes in the genus Thripinema have been found to parasitize some thrips species. It is important to note that biological controls cannot be used in conjunction with most insecticides.

Thrips cause significant damage to foliage, blossoms and fruit, as well as through disease vectoring. It is important to develop an effective IPM strategy that involves exclusion as a first line of defense. Very few management options are available once thrips enter the greenhouse.

Matthew Lollar is a Multi County Extension Agent, University of Florida/IFAS Extension at Seminole County. Matt received his B.S. and M.S. in Horticulture from Auburn University. Robert C. Hochmuth is a Multi County Extension Agent, University of Florida/IFAS Extension — Suwannee Valley Agricultural Extension Center. Bob received his B.S. in Extension Education and M.S. in Entomology from the University of Maryland.