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Euseius hibisci feeding on citrus thrips.

Neoseiulus barkeri. This phytoseiid mite—formerly called Amblyseius mckenziei—is still listed by some commercial suppliers, even though this species is now considered to be a strain of N. cucumeris.

Neoseiulus (=Amblyseius) cucumeris.

This predatory phytoseiid mite, previously known as Typhlodromus thripsi, feeds on young thrips.The pear-shaped adults are a pale brown and are noticeably smaller and flatter than the spider mite predator Phytoseiulus persimilis. Females lay an average of two small white eggs per day over a period of 20 days, often attaching them to plant hairs. Adults live up to 30 days, consuming three to six immature thrips per day. Although they prefer first-instar thrips, they will feed on spider mites and their eggs, broad mite adults, and pollen of various plants if thrips are not available. When no food is available they will resort to cannibalism.This mite will coexist with Phytoseiulus, although both prey on each other to some extent. Some strains of the mite enter diapause during the winter months in response to shorter photoperiod (less than 12.5 hours daylight) and temperatures below 69°F. Both diapausing and nondiapaus-ing strains are available commercially. Neoseiulus cucumeris prefers humidity levels above 65%; eggs will not hatch at lower humidity levels.

Neoseiulus (=Amblyseius) degenerans. This species is very similar to N. cucumeris in appearance and biology, but it does not enter diapause.The dark brown adults are found in flowers and on the underside of leaves. Larvae have a brown X shape on the back.This species can tolerate lower humidities than N. cucumeris can. It is commercially available.

Other predators

Dicyphus tamaninii. This mirid bug is an important predator of whiteflies, but it has also been shown to be effective against western flower thrips in cucumber greenhouses. (See "Whiteflies" for more information.)

Macrolophus caliginosus. This mirid bug is primarily a predator of whiteflies, but will consume thrips and other pests. (See "Whiteflies"for more information.)

Orius spp. These minute pirate bugs are common in gardens and many agricultural crops, and are often found in flowers.The black, 1/i6-3/i6 inch (2-5 mm) adults are ovoid and somewhat flat, with distinctively patterned black and white wings.The eggs are laid in leaf tissues with one end barely sticking out.The nymphs are pinkish-yellow to light brown. First-instar nymphs are about the same size as thrips larvae and are found in the same places. Both minute pirate bug nymphs and adults are very active and feed on all active stages of thrips.They consume 5-20 thrips per day, living up to 25 days as an adult. Because they are found in flowers, they are effective predators of western flower thrips.They also feed on spider mites, aphids, whiteflies, and caterpillar eggs,as well as pollen and plant juices. Several species of Orius are available commercially.

The predatory mite Neosiulus cucumeris.
Adult minute pirate bug, Orius sp., feeding on thrips.
Orius nymph feeding on western flower thrips larva.

Pathogens

Several fungal pathogens and some nematodes infect thrips naturally. Onion thrips are susceptible to Beauveria bassiana, Metarhizium anisopliae, Paecilomyces fumosoroseus, Verticillium lecanii, and several species of Entomophthora.However, natural fungal infections do not provide effective control despite high rates of mortality because epizootics occur when thrips populations have already grown large and damaging.These pathogens are useful only if the active stages of the fungi can be introduced early and under environmental conditions suitable for infection.Beauveria is the only fungus currently commercially available for control of western flower thrips.

Fungi

Beauveria bassiana.This fungus attacks many insects, including thrips (see "Aphids"for more information). At least two strains of the fungus are commercially available.

Entomophthora parvispora. This fungus causes significant thrips mortality outdoors in Europe, but in greenhouses it usually changes quickly to the resting spore stage that does not infect or spread. Shortly after the thrips is infected by a conidial spore, the body becomes filled with fungal growth that kills the host insect in 3-6 days.Active spore preparations could be useful for greenhouse applications, but the fungus would not spread beyond the initial application. It is not commercially available.

Entomophthora thripidum. Fungal epizootics in onion thrips in European greenhouses are usually caused by this fungus.Where it occurs, it can almost eliminate a thrips population within a few weeks.The fungus completes its life cycle in the insect host in 4 days. Only the abdominal organs are infected, so even after the fungus breaks through abdominal coverings, the thrips remains alive as long as the fungus continues to sporulate. Infected thrips often move to an elevated part of the leaf before the fungus breaks through to facilitate spore release. Sporulation occurs on cloudy days when the humidity is greater than 80%, but ceases in bright sunshine. It is not commercially available.

Metarhizium anisopliae. This fungus attacks many insects, including thrips (see "Aphids"for more information). All mobile stages of thrips are susceptible to this fungus, although the immature stages are more resistant to infection. This fungus is not registered for use in the United States on any greenhouse crops.

Paecilomyces fumosoroseus. This fungus has been investigated in Florida for control of western flower thrips on greenhouse ornamentals (see "Aphids" for more information).This fungus requires humidity over 90% for infection, which limits its utility. It is commercially available only in Europe.

Verticillium lecanii. This fungus controls aphids and whiteflies and has strong potential for controlling both onion thrips and western flower thrips. Although it is commercially available in Europe, this product is not expected to be available in North America for use against any greenhouse pests in the near future.

Nematodes

Heterorhabditis bacteriophora (=helio-thidis). This nematode has been shown to reduce adult emergence of western flower thrips by 40%. In addition, infection may reduce the number of progeny produced by adults that survive, and reduce the ability of thrips to transmit virus diseases.H.bacteriophora has good searching capabilities and attacks thrips pupae deeper in the soil profile than do other nematodes, such as Steinernema feltiae.Rates necessary to control thrips are extremely high and may not be economically feasible. It is commercially available.

Thripinema nicklewoodii. This nematode infects both larval and adult thrips.The infective stage of the nematode bores into the host thrips' body and multiplies in its abdominal cavity.The mature nematode burrows into the thrips'diges-tive tract, then passes out with its frass. Nematode infection in adult female thrips prevents egg production. Up to 20% of western flower thrips in California are parasitized by indigenous populations of this nematode. However, T. nicklewoodii has not been investigated for use in greenhouses and is not commercially available.

The fungus Verticillium lecanii infecting a thrips larva.

Possibilities for effective biological control

Most of the research on the biological control of thrips has focused on vegetable crops, especially pepper and cucumber. Biological control can be used on other greenhouse crops for thrips control, but it may not suffice for ornamentals that cannot tolerate cosmetic damage or the potential for viral transmission. In vegetable crops, success often depends on the individual grower's tolerance for foliar damage during the initial control phase— damage that will not affect fruit production. Other limitations to the biological control of thrips include reinfestation from surrounding outdoor plants during warm weather, the propensity of western flower thrips to hide in flowers or buds, and the difficulty of protecting crops from tomato spotted wilt virus and impatiens necrotic spot virus if present in a thrips population. Nevertheless, if selected and released appropriately for your thrips problem, predatory mites, minute pirate bugs and soil mites may control thrips.They may work best when used in combination, as detailed below.

The most commonly used natural enemies for thrips control are the two phytoseiid predatory mites Neoseiulus cucumeris and N. degenerans. N. cuc-umeris is effective against both the western flower thrips and onion thrips, especially on cucumber. Most strains of N. cucumeris do not reproduce during short winter days with night temperatures below 65°F. By contrast, N.degener-ans does not enter diapause and will continue to develop under cool condi-tions.The eggs of this species are also better able to tolerate low humidity, which can severely reduce hatching of N. cucumeris eggs. Both species are efficient at low thrips densities and feed on spider mites, broad mites, and pollen in the absence of thrips.The two species together may provide control over the range of conditions characteristic of greenhouses.

The greenhouse crops themselves influence the effectiveness of predatory mites. On peppers, N. cucumeris tends to establish easily, even without thrips. Therefore, it can be introduced to peppers in low numbers before thrips are detected for earlier control. However, N. cucumeris is not as efficient a predator on cucumber, perhaps because the tri-chomes on cucumber leaves interfere with the mite's ability to search. Repeated releases are often necessary for successful establishment on this crop. Pollen is critical for optimal reproduction of N. cucumeris, so it is not as successful on nonflowering plants and those that do not produce sufficient pollen.Thrips reproduction also can vary on different cultivars, and even a slight decrease in thrips reproduction may allow the mites to prevail.

The greenhouse conditions into which predatory mites are released also influence the mites'effectiveness.Thrips that migrate from outdoors in summer can overwhelm the ability of the mites to get the thrips under control. Environmental conditions such as relative humidity and temperature also affect predator efficiency and egg survival. Optimal conditions for predatory mites include moderate temperatures (around 86°F) and high humidity (80-90%).Thrips thrive at 70-90% humidity, but their mortality increases above that humidity level.The mites will also be more efficacious if plant leaves are touching, so that the mites are able to easily move from plant to plant.

Predatory mites are shipped in a carrier or packing material. If you receive the mites in loose packing material, sprinkle the material (often cereal bran) onto plants. If the material is in slow-release packets (sachets), hang them on plants. Because some strains of N. cucumeris diapause during the winter months, when the temperature drops below 70°F and daylight lasts less than 12.5 hours, the mite is not usually released in greenhouses in the upper Midwest until March or April. However, warm night temperatures in the greenhouse, even during winter, will prevent diapause in a large proportion of the mites. N.degen-erans can be released during fall and winter.

Success with these predators depends on releasing them as soon as thrips are detected on sticky traps or plants. For cucumbers, releases of 50 predatory mites per plant plus an additional 100 per infested leaf are recommended. For peppers, recommended release rates are 10 predatory mites per plant plus an additional 25 per infested leaf.The initial pest density does not seem to be important as long as introductory rates are high. Repeated releases of predatory mites are recommended for good protection of peppers or cucumbers. Make weekly releases until there is one predatory mite for every two thrips.The mites only attack thrips in their first and second instars, so it may take several months for biological control to work.

Making regular releases of predatory mites is also the only way to control thrips on ornamental crops, such as chrysanthemum.You should release the mites weekly, biweekly or monthly, depending on the crop and the thrips pressure. Before thrips are present, introduce the mites at 10-50 per ft2, depending on the crop. Good results have also been obtained on chrysanthemum using N.cucumeris at rates of 100 mites per individual cutting. Additional releases of three mites per leaf are sufficient where adult thrips pressure is intense. Under less pressure, you can gain control with fewer mites per leaf, but regular releases are still necessary. Remember, your own experience is your best guide to the timing and rates of natural enemy releases.

Minute pirate bugs (Orius spp.) may be more effective than mites for control of thrips in some situations.Mites feed on thrips for only a small portion of their lives.The nymphal and adult minute pirate bugs prey on both adults and immature stages of the thrips.They also feed in flowers, where female western flower thrips, which are directly responsible for population growth, concentrate. Minute pirate bugs will also feed on other pests, such as spider mites, as well as pollen and plant juices, so they can exist for long periods at low thrips densities. Orius are most effective when they reproduce in the crop, but the adults do not lay eggs equally well on all plants. O. insidiosus reproduces best on chrysanthemum, gerbera, cucumber, and peppers, but oviposition is so poor on roses and carnations that the bugs provide no control when thrips are present in the flowers. O.laevigatus and O.albidipennis are able to control western flower thrips on peppers and strawberries, but not on cucumbers, due to the absence of pollen and the numerous hairs on the cucumber leaves which inhibit movement.Other Orius species may have different plant preferences.

Minute pirate bugs are shipped as adults in a carrier or packing material, such as bran, rice hulls, or vermiculite, along with a food source. Shake the packing material onto the plants, and the bugs will readily disperse and locate prey. Distribute the insects evenly throughout the greenhouse, but place extra bugs in thrips hot spots, such as open flowers. Release the bugs in the early morning or late afternoon, or on a cloudy day, to reduce the chances they will fly out of the greenhouse. Keep the vents closed or screened to prevent the bugs from leaving the greenhouse.

One minute pirate bug per plant was sufficient to control western flower thrips within 5 weeks in small-scale experimental trials on greenhouse cucumbers. Release rates of one Orius insidiosus per 20 ft2 provided control of western flower thrips on peppers in some trials in European greenhouses, but higher rates may be necessary on other crops. On peppers an initial introduction of three to five Orius per 10 ft2 is suggested in areas of pest activity.This should be followed by an introduction of two Orius per 10 ft2 throughout the greenhouse 2-4 weeks later. On cucumber and eggplant, use 5-10 Orius per 10 ft2 in specific areas of pest activity, followed 2-4 weeks later by a general introduction of one to two per 10 ft2.Additional research on a larger scale is necessary to make recommendations for release rates on other crops in commercial greenhouses. Minute pirate bugs can also be used in conjunction with predatory mites, although the bugs will eat the mites if insufficient food (thrips) is available.

Minute pirate bugs enter a reproductive diapause in late fall, so more regular releases become necessary after late October to keep populations high enough for thrips control.The use of supplemental blue light has been shown to reduce the number of O. insidiosus entering diapause, which may allow these bugs to suppress western flower thrips in short-day flowering crops such as chrysanthemums.

The predatory bug Dicyphus tamaninii was effective at controlling western flower thrips in a Mediterranean cucumber greenhouse. A ratio of three late-instar nymphs to 10 pests kept thrips under the economic injury level. A lower ratio was successful only when thrips populations were low. Releasing late-instar nymphs would not be as practical in a commercial greenhouse as releasing adults which could disperse to find the thrips, but might also leave the greenhouse.

The soil-dwelling predatory mite Hypoaspis, which is not a phytoseiid, will attack thrips pupae in the soil when placed around the roots of each plant. These mites work best when the soil or medium is moist, with an open structure, and soil temperature is at least 59°F. It cannot be relied on as a sole source of control for thrips in a commercial greenhouse, but it could enhance biological control by predators that feed on immature thrips and adults on leaves. In small-scale experiments this mite reduced adult thrips by about 70%.

These mites are also shipped in a carrier, such as vermiculite, and are distributed by shaking the contents onto the soil or potting medium.H.miles does not need to be applied to every pot and/or flat of plants because it will disperse some distance on its own. It must be applied early in the growing season to allow establishment before thrips begin pupating in the soil.

Using phytoseiid mites, minute pirate bugs, and Hypoaspis together should offer the best control of thrips.The predatory mites consume immature thrips but do not kill adult thrips, which can continue to damage the plants for several months until they die. Minute pirate bugs will prey on adult thrips, as well as immatures on the foliage. Hypoaspis will feed on thrips in the soil, where the other predators do not occur. Releases of Hypoaspis and phytoseiids together provided excellent control on chrysanthemum in Canadian greenhouses. Release the mites before thrips are detected, and release Orius in hot spots when thrips are found. If additional releases are necessary, alternate weekly between phytoseiids and Orius. The phytoseiid mites perform best under humid conditions and often die out when humidity declines. Orius tolerates drier conditions, so the two are complementary.

The parasite Thripobius semiluteus can be used for control of greenhouse thrips only. Because it only accepts this species of thrips it is important to know what species you have before contemplating releasing this insect.The wasps are shipped in the pupal stage and should be held at 65°-75°F until adults emerge. The highly mobile adults will disperse throughout the greenhouse to find greenhouse thrips. Release rates vary considerably depending on the crop, thrips population levels, and other natural enemies being used at the time of release. Minimum rates begin at one wasp per 20 ft2.Two releases should be made 2-3 weeks apart. Monitor for wasp reproduction and establishment by observing immature thrips. Parasitized thrips have a milky appearance. Once the parasite pupates inside the thrips, the host body turns black and hard, but still remains attached to the plant surface.

Pathogens can also be used to suppress thrips populations. Adult western flower thrips appears to be most susceptible to fungi possibly because they generally occur in flowers where humidity is higher and conditions more favorable for infection.The higher humidity in the flowers increases the potential for the fungus to sporulate and infect additional thrips.The commercially available Beauveria bassiana suppresses western flower thrips on rose, gerbera, carnation, and other flowers as well as currently registered insecticides do. However, none of these materials provide satisfactory control of western flower thrips.The fungus and predatory mites or Orius need to be used together to achieve acceptable control.

Thrips

Enemies

Ceranisus adult nymph -

Thripobius

Hypoaspis

Phytoseiid mites

Dicyphus

Macrolophus

Minute pirate bugs

Pathogens

Heterorhabditis

Thripinema adult egg nymph -

Several brands and formulations of B. bassiana are available for use in green-houses.The fungus takes 3-7 days to kill an insect, so it will take some time to suppress the thrips population when using these products.Thorough spray coverage is essential because fungal spores must contact the insect for infection to occur.This fungus is susceptible to some fungicides, so chemical fungicide applications should not be made within 48 hours of B. bassiana applications.Multiple applications are usually necessary to achieve control.

The whitefly strain of V. lecanii also affects thrips.When humidity is high and temperatures are 64°-77°F,this fungus can cause considerable mortality of thrips. In experiments on cucumbers, V. lecanii killed more than 80% of the thrips within 4-6 days. In comparative assays,Metarhizium anisopliae caused more adult mortality of western flower thrips than V.lecanii.The efficacy of M. anisopliae increases as temperatures increase, with an optimal range of 75°-82°F. Paecilomyces fumosoroseus has also been shown to be effective against thrips in some tests.

These pathogens are applied as spores suspended in water for spraying. High humidity is necessary for infection. When humidity is low,the performance of the fungus is unpredictable. Humidity can be increased by dampening the plants with water sprays. Late afternoon applications reduce spore injury by ultraviolet light and desiccation, since the greenhouse is more humid at night. However, these fungi may be practical for use only in humid areas with moderate temperatures, such as rooting benches and shade-cloth covered areas used to induce inflorescence in chrysanthemums. Repeated applications will be'"*-necessary if humidity is not high enough to allow continuous infection. Since these fungi do not impair natural enemies, their use can be integrated with predatory mites or bugs.

Alternative control methods

Sanitation

This is the most important cultural practice for preventing thrips problems. Remove weeds that can act as a refuge for thrips or harbor viruses from inside and outside the greenhouse. Remove and immediately destroy crop residues after harvest. In addition, remove any soil debris which could contain thrips pupae. Avoid wearing yellow and blue clothing in the greenhouse because they attract thrips and facilitate the spread of the pest.

Insect screens

Thrips migration into greenhouses can be a continual problem. Monitor the greenhouse with sticky traps to determine where thrips are entering, and install screening to exclude them. Screens on greenhouse vents and doors will help block thrips' entry from outside. Screening just the side of the greenhouse that faces into the prevailing winds can also be effective at reducing thrips, even for passive air intake systems.The maximum hole size to exclude thrips is 192 p.Thrips can easily get through commercial whitefly-proof screens or unwoven polyester filters.

Soil sterilization (pasteurization)

Steam sterilizing the soil or the used planting medium between crops will kill immature thrips that are pupating in the soil.

Environmental control

Misting to increase relative humidity near 90% will reduce thrips populations on cucumber. Soil that is kept wet and frequently waterlogged will drown thrips pupae or allow fungal epizootics to kill them. However, wet soil may also increase plant disease, fungus gnat, and shore fly problems.

Traps

Yellow or blue sticky traps can be used for mass trapping of thrips. For western flower thrips, which pupate mainly in the soil, place the traps near the soil to catch large numbers of newly emerged thrips before they move into the crop. Blue traps may be more effective for mass trapping than yellow traps.

Resistant cultivars

Certain chrysanthemum cultivars appear to be more tolerant of western flower thrips than others. Dutch researchers have found differences in susceptibility of cucumber cultivars. If possible, select varieties that do not show as much injury from thrips feeding.

Chemical control

Spinosad is an insecticide derived from natural metabolites produced under fermentation conditions by the actinomycete Saccharopolyspora spinosa.It has a high level of contact and oral activity and rapid speed of action. It is very effective against thrips but has low to moderate impact on beneficials. It is registered for control of many pests on landscape ornamentals and can be used on ornamentals in shade houses and lath-houses.

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