Thrips

Available natural enemies and their potential for control

There are some predators, pathogens, and parasites available for control of thrips.

However, even when used in combination, they will provide only moderate control. Other remedies may be necessary when pest populations are high.

Order Thysanoptera: Thrips

Family Thripidae: Common thrips

Western flower thrips,

Frankliniella occidentalis

Onion thrips, Thrips tabaci

Eastern flower thrips,

Frankliniella tritici

Greenhouse thrips,

Heliothrips haemorrhoidalis

Thrips palmi

Thrips are important pests of cucumbers, peppers, and a broad range of ornamental greenhouse crops. These very small insects commonly hide in flowers, buds, and leaf axils, and often go unnoticed until damage appears. Western flower thrips (the word "thrips" is both singular and plural) is of special importance because it transmits the tomato spotted wilt virus and impatiens necrotic spot virus to many greenhouse floricultural and horticultural crops. It is replacing many of the other thrips as the predominant thrips pest in greenhouses throughout the world. Thrips palmi,native to Southeast Asia, is a severe pest of numerous vegetable and ornamental crops. It is now present in Florida and may spread to other parts of the United States.

Damage

Both larval and adult thrips have rasping mouthparts that they use to puncture the plant surface.They feed on the sap that is exuded from the resulting wound. Plants are also injured when female thrips lay their eggs in the plant tissue.The damage appears as white or silver speckles on the leaf or flower petal, arranged in streaks rather than the stipples caused by mite or aphid feeding.These spots eventually dry up and turn tan or brown on some plants but remain silver on others.Thrips can enter flowers while still in the bud stage and after color shows. Blossoms turn brown, or buds fail to open completely. Petals become distorted, develop brown edges, and stick together. As they feed, thrips excrete brown droplets on petals and leaves, and the droplets turn to black spots when they dry.

Leaf feeding reduces plant vigor and diminishes the yields of vegetable and ornamental crops. Feeding on the growing tips and blossoms of plants causes deformities in the new leaves, flowers and fruit, reducing the plant's marketability.Tomato spotted wilt virus and impatiens necrotic spot virus, transmitted by western flower thrips, cause lesions on leaf tissue and can reduce crop yields.Viral symptoms vary depending on the plant species; on cyclamen the lesions appear as ringspots. Affected ornamental plants are not marketable. Thrips will also bite people, causing a mild stinging sensation.

Description and life cycle

Adult thrips are slender insects 1/16 inch (1-2 mm) long with narrow fringed wings.They may be yellow, tan, brown, or black depending on the species or strain.Western flower thrips are yellow to brown with an orange thorax. Onion thrips are a uniform tan or brown. Greenhouse thrips are dark brown or black. T.palmi are clear yellow with black hairs. Males are typically smaller and lighter in color than females.

Thrips can reproduce year-round in greenhouses, producing 12-15 generations per year.The female thrips inserts minute,opaque eggs through holes cut into leaf, flower, or fruit tissue with her sawlike ovipositor.They lay 60-100 eggs during their lives.The nymphs hatch in 2-7 days, feed during the first two instars,then pass through two nonfeed-ing pseudopupal stages (the "prepupa" and the "pupa") in protected parts of the plant such as leaves or flowers, in leaf litter, or in the soil. Although thrips have a simple metamorphosis, and thus don't actually pupate, these pseudopupal stages are similar to the pupal stage of insects with a complete metamorphosis. Therefore, thrips nymphs are often referred to as larvae.They do not feed or move unless disturbed during the 2-5 day "pupal"stage.

Thrips often remain together in large numbers on a single leaf or flower. Onion thrips often feed along prominent veins.Western flower thrips are usually found within developing terminal foliage or at the base of flower petals where they feed on pollen. Greenhouse thrips are found primarily on developed leaves. T.palmi feed along leaf midribs.

Monitoring

Effective population monitoring is essential for timing the application of biological controls and for gauging their effect. Early visual detection of thrips is difficult because they are small and often hide within young, terminal foliage and developing flowers. Let previous experience guide you in deciding when to begin monitoring. Using sticky traps and sampling plant parts, including flowers, can give you some idea of the thrips population in your greenhouse.

Yellow or blue sticky traps are most useful for detecting the first infestation of thrips and determining where they are invading the greenhouse.The traps, which are commercially available, are also useful for monitoring population trends within a greenhouse. Begin monitoring early in the cropping cycle, especially before flower buds form. It is easier to see dark-colored thrips on yellow traps, but blue traps are more attractive to some species of thrips and therefore will detect lower populations.The blue traps will highlight light-colored species. Traps should be placed so that the bottom of the trap hangs 1-2 inches above the crop canopy. In mature commercial cucumber crops, place traps 8 ft above the floor to catch the most western flower thrips. Be aware that there is no correlation between trap catch and actual plant damage or the number of thrips in flowers.This is because thrips catches may be influenced by the presence and color of flowers, light intensity, placement of traps relative to crops, and crop type. These factors can make population counts inaccurate.

Although sticky traps are very useful for detecting the presence of thrips, plant part sampling can track thrips populations more accurately. For immature thrips, counting the number found on leaf samples is the best indicator of population trends. Sample leaves from the middle of the plant to save time without reducing accuracy. Monitoring immatures is still more costly and time consuming than adult monitoring.

Blossom sampling is the best method for monitoring populations of adult western flower thrips, which concentrate in flowers.To sample a flower, place it into a 25 ml (0.85 oz) plastic vial filled with 70% alcohol and seal it immediately. Pour the vial contents through a funnel lined with filter paper.Then count the flower thrips by examining the filter paper under a dissecting micro-scope.The flower may need to be dissected to find all the thrips. Although not as accurate, a quick visual assessment of the number of thrips per flower can be done by merely blowing into open flowers.The carbon dioxide agitates thrips and causes them to move around so they can be counted. A combination of sticky traps, blowing into flowers, and flower monitoring allows quick detection of potential problem areas.

Natural enemies

A few parasites and pathogens attack thrips, and so do many general predators, including mites, minute pirate bugs, lacewings, syrphid larvae, and other predacious thrips. Predatory mites are the most commonly used natural enemies to control thrips in greenhouses.

Parasites

A few trichogrammatid and mymarid wasps parasitize thrips eggs. Most of the parasitic wasps that have been found to attack thrips larvae are tropical or subtropical species in the family Eulophidae. Few are currently commercially available.

Ceranisus (=Thripoctenus) spp.These eulophid wasps develop inside thrips larvae. After female wasps lay single eggs in young thrips larvae, the thrips continue to appear and behave normally while the wasps develop inside them.The thrips die when the parasites

Hang blue sticky traps 1-2 inches above the crop canopy to detect thrips.

pupate inside their hosts.The adult wasps also kill young thrips by feeding on body fluids from holes made with the ovipositor. Several other species have potential as biological control agents of thrips.C.brui occurs in Japan, Indonesia, Europe, and the Caribbean. It was introduced and established in Hawaii in 1933-34 for control of onion thrips, the most important vector of yellow spot virus of pineapple, but has not been evaluated as a biological control agent for thrips in greenhouses. C.menes is a solitary endoparasitoid that attacks the first instar of western flower thrips. It occurs worldwide, and has been found parasitizing western flower thrips on alfalfa and roses in California. It is being investigated for western flower thrips control in European greenhouses. Thrips feeding on exposed plant parts are likely to be attacked by this para-sitoid, but it may not be able to find thrips that enter flower and/or terminal buds. Researchers at the University of Florida are studying another species of Ceranisus from Thailand for possible importation as a biological control agent of T.palmi in subtropical areas of the United States. None are commercially available.

Thripobius semiluteus. This parthenogenic wasp was introduced in 1987 from Brazil and Australia as a possible control agent for greenhouse thrips in avocado orchards in southern California. Females lay 15-20 eggs singly in thrips larvae and the developing wasp kills the thrips before it reaches the pupal stage.The adults also host feed on immature thrips, in addition to feeding on nectar and honeydew. Under optimal conditions (65°-75°F and 50-60% relative humidity) development from egg to adult takes about 3 weeks. It enters diapause when temperatures fall below 40°F.This species is commercially available.

Predators

Many generalist predators will feed on thrips in addition to other insects. Numerous species of predatory mites and a few bugs are more specific predators of thrips. Phytoseiid mites are the most important predators.

Predatory mites

Euseius spp. Several phytoseiids in the genus Euseius are commercially available in Europe, but have not been evaluated for use on greenhouse crops. E.del-hiensis (=rubini) will feed on thrips, spider mites, broad mites, and whitefly eggs. E.hibisci is predatory on many small insects, including thrips. It has been used successfully in inoculative field releases in California to control citrus thrips. It also feeds on various mites and sweetpotato whitefly eggs. E.scutalis is widely distributed in the warm climates of North Africa and the Middle East, where it occurs mainly on trees and shrubs. It prefers to feed on pollen and spider mite eggs but will attack thrips, spider mites, and whitefly eggs as well.

Hypoaspis (=Geolaelaps) miles. This soil-dwelling laelapid mite is a native of North America. It feeds on many soil-inhabiting arthropods, including thrips pupating in the soil. Adults can kill 15-30 prey per day, targeting more small prey than large prey. Females deposit one to three eggs per day.The mites complete a life cycle in 13-22 days, during which each nymph consumes 16-33 prey. It does not diapause in the winter.This mite is available commercially.

Two thrips larvae; the larva on the left has been parasitized by Thripobius semiluteus, the other is unparasitized.

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