Thrips

Thrips are tiny, slender insects about 1/50 to 1/16 inch (0.5 to 1.5 mm) long. Adults have four wings that at rest are folded over the abdomen. The wings appear featherlike in flight. They have numerous, microscopic hairs along the front and rear margins that allow thrips to be carried easily on air currents. If it is uncertain whether the thrips found are a species that can damage crops several can be collected, such as with an aspirator, and taken to the local office of the county department of agriculture or University of California Cooperative Extension for identification, which requires expert preparation and examination of microscopic characters as illustrated here for flower thrips versus onion thrips.

Thrips have six life stages: egg, first instar and second instar (called nymphs or larvae), third instar (prepupa, or propupa), fourth instar (pupa), and adult. They have multiple generations per year. Adult females either insert eggs into plant tissue (western flower thrips), lay eggs openly on foliage (greenhouse thrips), or oviposit on tissue within folds distorted by their feeding (Cuban laurel thrips, myoporum thrips). Only adults and the first two instars feed.

Western flower thrips is the most economically important pest thrips and the most common of the Frankliniella spp., all of which are commonly called flower thrips. Western flower thrips feeds on numerous hosts, causing direct feeding damage. It also transmits a complex of tospoviruses including Impatiens Necrotic Spot Virus and Tomato Spotted Wilt Virus, which can severely damage or kill numerous herbaceous plant species. Adults have pale wings and body coloration that varies greatly from entirely black, orange, yellow, or whitish to a mix of pale-colored head and thorax with a dark abdomen. First instars are white. Older instars commonly are orange to yellow. During the prepupal and pupal stages eye spots and wing pads become apparent.

During winter and spring dark forms predominate outdoors; paler forms predominate during summer and fall or throughout the year in heated greenhouses with supplemental light. The presence of dark forms in greenhouses indicates thrips are probably migrating into growing areas.

Western flower thrips adults and nymphs are fast-moving and commonly feed in crevices of tissues such as in buds, flowers, and growing tips, but may also feed on the surface of leaves of some hosts. Damage may appear as pale scarring or stunting. Buds feed upon before they open develop into distorted and necrotic tissue as they grow. Western flower thrips also feeds on pollen and spider mites. The prepupa and pupa occur in growing media or soil beneath infested plants. Females lay male eggs if unmated and female eggs after mating. Development from egg to adult ranges from about 11 days at 77° to 87°F to 44 days at 50° to 60°F.

Greenhouse thrips are slow-moving insects that feed openly in groups on foliage, commonly on the underside. Adult greenhouse thrips are black with translucent to whitish wings. Legs of adults and nymphs are yellow to whitish. Nymphs are yellow or whitish overall, but blackish gut contents commonly are visible through the body surface. Adults and feeding-stage nymphs commonly hold a droplet of dark feces at the tip of their abdomen.

Cuban laurel thrips infests Ficus spp., especially Indian laurel fig. Its presence is easy to recognize from the tightly rolled and podlike terminals enclosing the thrips. Note that ficus leaf-rolling psyllid causes similar damage. Cuban laurel thrips are most abundant outdoors from about October through December. Feeding over the winter results in newly galled foliage apparent by midsummer.

Cuban laurel thrips adults are black and nymphs are yellow, closely resembling weeping fig thrips. Eggs are oval, beige to whitish, and laid on the leaf surface commonly within galled tissue.

Myoporum thrips adults are black with whitish wings and nymphs are orangish. Feeding causes myoporum terminals to become galled i.e., tightly bunched, and twisted. All life stages can be found within the galled terminal growth where the thrips reproduce and feed. Terminal distortion can occur year-round, but new growth in the spring is especially susceptible.

Weeping fig thrips adults are black and nymphs are translucent to pale tan or yellow. This thrips and its damage closely resemble those of Cuban laurel thrips. Note that ficus leaf-rolling psyllid causes similar damage. Eggs of weeping fig thrips are oval, whitish, and laid on the leaf surface within galled tissue. It infests Ficus benjamina, but not Ficus microcarpa.

Thrips pierce plant cells and suck out the contents. Cuban laurel thrips, myoporum thrips, and weeping fig thrips also discolor and distort (gall) foliage. These thrips do not kill hosts, but their aesthetic damage can make plants unmarketable.

Greenhouse thrips feeding leaves patches of brownish, yellow, or whitish leaf tissue. Their black specks of feces foul leaf surfaces. Stippling on foliage of numerous field and greenhouse crops from greenhouse thrips can be confused with damage from spider mites. However, unlike with mites, greenhouse thrips feeding and sometimes that of western flower thrips and other thrips species is accompanied by black, varnishlike specks of excrement on foliage. Spider mite damage is often accompanied by silken webbing and cast skins. In both cases the culprits are often found on close examination.

Western flower thrips on many hosts primarily feeds on pollen and can be found in the flowers of virtually any species of dicot. On other plants it may feed on immature flower petals and leaves, causing damage that becomes apparent as tissue grows. Crops susceptible to tospoviruses vectored by Frankliniella spp. and infected when young can die prematurely or fail to develop to a marketable growth stage.

Western flower thrips direct-feeding damage includes blossom streaking or color break, discolored patches of foliage, leaf spotting, and tissue distortion. Feeding damage may cause premature senescence of flowers, such as African violets where blossoms may be prematurely pollinated. On orchids, western flower thrips feeding and egg laying will leave translucent, pimplelike spots on leaves and petals and buds may fail to open and may drop prematurely (e.g., on Phalaenopsis). Tospovirus infections can result in a wide variety of symptoms (and occasionally be asymptomatic) including brown and yellow necrotic leaves, distorted leaves, ring spots on foliage, gray or whitish lesions, and various others.

Exclusion and sanitation are key strategies for preventing thrips damage. Releasing commercially available natural enemies may be effective if done early and combined with other compatible practices. Insecticide resistance is a major problem with western flower thrips, so rotate applications among modes of action of known effective insecticides and do not rely on insecticides alone. Rogue and dispose of galled plants and those otherwise with obvious thrips damage, then treat all the nearby plants with an effective insecticide. For plant-galling species (Cuban laurel thrips, myoporum thrips, weeping fig thrips), a preventive application of insecticide that kills on contact or through ingestion can be warranted while highly susceptible succulent terminals are present. Consider growing cultivars resistant to thrips; for example, Ficus microcarpa ‘Green Gem' is less extensively damaged by Cuban laurel thrips than other F. microcarpa cultivars.

In coastal locations a high proportion of greenhouse thrips nymphs can be parasitized by a tiny wasp, Thripobius semiluteus.Resident populations of parasitic wasps and predaceous minute pirate bugs, mites, and thrips prey on plant-feeding thrips, but naturally occurring parasites and predators cannot provide satisfactory pest control in greenhouses and nurseries.

Certain predators are commercially available for thrips control; combine their use with compatible cultural controls and selective pesticides. For biological control to be effective, avoid application of broad-spectrum, persistent insecticides and miticides for all pests in the crop and consider whether environmental conditions are favorable for the natural enemies. Several species of minute pirate bugs (e.g., Orius spp.) and predatory mites (Neoseiulus spp., Stratiolaelaps scimitus =Hypoaspis miles) can be purchased and released. Minute pirate bugs also feed on aphids, small caterpillars, mites, and various other pests. General recommendations are to release Orius at a rate of 2,000 to 4,000 per acre several times during early crop growth when hosts are most susceptible to thrips damage. Neoseiulus cucumeris are released weekly at a rate of 10 to 50 mites per plant. Neoseiulus also feed on spider mite eggs and pollen. See Protecting Natural Enemies and Pollinators for information on how to improve biological control.

Stratiolaelaps scimitus is a soil-dwelling predator that feeds on thrips prepupae and pupae, larvae and pupae of fungus gnats, and certain other small arthropods. Stratiolaelaps are generally released on soil soon after planting and are most effective when there is plant-to-plant contact that facilitates movement of the predators. For more information, see Biological Control, Natural Enemy Releases for Biological Control of Crop Pests, and Relative Toxicities of Pesticides Used in Floriculture and Nurseries to Natural Enemies and Honey Bees and Miticides.

Carefully inspect new plants and propagation stock to ensure they are pest free. Greenhouse thrips and western flower thrips have numerous hosts including some weeds, so keep production areas free of weeds that may serve as reservoirs. Screens covering greenhouse vents and other openings can exclude thrips, but the material must be of sufficiently small pore size (145 microns, 0.145 mm, about 1/200 inch). Some larger fine-mesh screens can exclude most thrips. When retrofitting fine-mesh screens be sure to provide sufficient surface area for adequate air movement to ensure the ventilation system can accommodate the reduced flow caused by fine screening; or modify the system as needed. Install double doors with positive ventilation (air constantly flowing outwards) to reduce pest movement into growing enclosures.

Reflective mulch

For field-grown crops, applying reflective mulch in row middles or entirely covering the soil surface and planting through holes in the mulch can greatly reduce the extent of thrips infestation of young plants. Reflective mulch also reduces the infection from insect-vectored viruses when crops are young and most susceptible to these plant pathogens. Reflective mulch can increase crop growth and cut-flower yield while reducing need for irrigation by conserving soil moisture. See Reflective Mulches for more information.

Biological and cultural controls are organically acceptable management methods. Certain formulations of the botanicals azadirachtin (Azatin), neem oil, and pyrethrins without piperonyl butoxide (PyGanic), the microbial Beauveria bassiana and spinosad (Entrust Naturalyte, Entrust SC), certain narrow-range oils (Organic JMS Stylet-Oil), and potassium salts of fatty acids (insecticidal soap) are acceptable for organic production.

Blue sticky cards are most attractive to western flower thrips. But yellow cards are generally recommended because they attract and capture greenhouse thrips, western flower thrips, and the adults of various other pest species, and insects are easier to identify and count on the yellow surface. Place yellow sticky cards vertically in the crop canopy with the lower one-third of the trap below the top level of leaves and the upper two-thirds above the top of the leaf canopy. Raise traps as the crop grows to maintain them at the proper height, which is optimal for capturing thrips. For more information, see Monitoring with Sticky Traps and Sticky Trap Monitoring of Insect Pests.

Research in California greenhouse roses suggests that three yellow sticky traps per cultivar is adequate. In greenhouses with many different cultivars, place traps among the most susceptible varieties. In large greenhouses of the same or similar cultivars, deploy at least 8 traps per 100,000 sq. ft. In other crops, place at least one card per 10,000 sq. ft.

Consider applying an effective insecticide if an average of 5 to 10 greenhouse thrips plus western flower thrips per card per week are trapped. Most effective insecticides must be applied at least two times about 5 to 7 days apart to control western flower thrips. Note there may be several species of thrips present on a sticky card, but these suggested thresholds apply only to western flower thrips and greenhouse thrips counted together. There are no research-based thresholds for Cuban laurel thrips, myoporum thrips, or weeping fig thrips in nurseries. If these species are trapped in growing areas containing their hosts, a preventive insecticide application may be warranted. A general application strategy is to apply foliar insecticides that kill thrips early in the cropping cycle, before crop damage is observed. When crop damage is observed, cull affected plants and apply a systemic insecticide (e.g., acephate or a neonicotinoid) to the remaining crop. See Establishing Action Thresholds for more information.

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