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Integrated Pest Management Program

Department of Plant Science and Landscape Architecture, Department of Extension

Fact Sheets > Greenhouse > Insect and Mite Pests

Managing Western Flower Thrips on Greenhouse Crops

Introduction

More than one dozen species of thrips feed on greenhouse grown crops.  One of the most important and difficult- to-control species is the western flower thrips (WFT), Frankliniella occidentalis.  The greenhouse thrips (Heliothrips haemorrhoidalis), and the onion thrips (Thrips tabaci) may also be found on greenhouse crops. The chilli thrips, (Scirtothrips dorsalis) a foliage feeder, occurs in Florida and Texas and may also be introduced into greenhouses on plant material. 

Western flower thrips have spread throughout the horticulture industry on plugs, cuttings and small plants. Their small size (1/16 inch) and tendency to remain hidden in flower buds makes it difficult to detect the thrips before severe feeding damage is evident. Their broad host range, high reproductive capacity, rapid life cycle, and resistance to insecticides make them difficult to control.

Feeding Damage

Thrips feed by piercing plant cells with their mouthparts and feeding on the exuded plant juices. This collapse of plant cells results in deformed flowers, leaves and shoots. Silvery flecked scars or small black "fecal" spots may be seen on the expanded leaves. (See photos). Western flower thrips have a broad host range and may feed upon ornamentals, vegetables and herbs including butterfly weed, garden impatiens, fuchsia, chrysanthemum, ivy geraniums, lupine, marigolds, gerbera daisy, hibiscus, tarragon, peppers, petunia and verbena.

In addition to direct feeding injury, WFT may vector (spread) two closely related tospoviruses; impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV), to uninfected plants.  The tospoviruses have one of the widest host ranges of any known virus. Over 600 plant species in 62 families are confirmed hosts. Significant losses have occurred for growers of garden impatiens, New Guinea impatiens, cyclamen, begonia, primula and gloxinia. Almost all greenhouse crops with the exception of roses and poinsettias are susceptible.  Tospovirus symptoms resemble those caused by fungi, bacteria, or chemical spray injury. Infected plants may also show a wide range of symptoms depending upon the species or cultivar infected. Some of the more generic symptoms of tospoviruses include leaf spots, necrotic areas, mottling and ringspots.  Young plants may be especially vulnerable to infection. There is no cure. As soon as tospoviruses are detected, rogueing of infected plant material must be combined with strict thrips management.

Virus Transmission

Only the first instar larval thrips acquire the virus. If infected as larvae, the adults can transmit the virus in their salvia as they feed upon plants. The winged adults are primarily responsible for viral spread, however, due to a midgut barrier; the adults cannot acquire the virus. The virus persists in the adult thrips for their entire life.  Fortunately, the adults do not transmit the virus to their young. There is no transmission from adult to egg. Overlapping generations of thrips within a greenhouse may result in continuous or sporadic virus spread.

Life Cycle of Western Flower Thrips

Their life cycle consists of eggs, 2 nymphal stages, two pupal stages and adults. Adult females may live for approximately 30 to 45 days, feeding primarily on pollen. Females insert their saw-like ovipositors into plant leaves to lay eggs. During their lifetime, female thrips lay from 150 to 300 eggs that hatch in about one week. Eggs may be laid on the upper surface or lower leaf surface depending upon the plant species. Eggs tend to be laid on the upper leaf surface on chrysanthemum but may be laid on the lower leaf surface on pepper plants.  The first two larval stages remain protected in the tender young growth.  They resemble adults but are wingless (See photo). After the 2nd instar larvae stops feeding, it drops to the soil or growing media to pupate.  Thrips may also pupate in open flowers. Adults may then emerge in about 6 days, depending upon temperature. Adults are weak flyers but are dispersed throughout the greenhouse on air currents.

The thrips life cycle is dependent upon temperature with development occurring between 50 F and 90˚ F. Thrips can survive cooler temperatures then 50˚ F, however, there is no development at that temperature. Their life cycle varies from seven to 14 days at fluctuating temperatures between 68 to 98˚ F that may be more common in the greenhouse environment (Table 1).

Table 1. Life Cycle of Western Flower Thrips (Robb, 1988)

Stage

Approximate duration at temperatures between 68° and 98°F

Egg

2-4 days

1st instar (immature)

1-2 days

2nd instar

2-4 days

Prepupal

1-2 days

Pupal

1-3 days

Adult

30-35 days

In the greenhouse, thrips can be found year-round whenever temperatures are favorable for their development and host plants (including weeds) are available for food. Many common greenhouse weeds such as redroot pigweed, chickweed, lambsquarters, bindweed, thistle, oxeye daisy, galinsoga and pineapple weed are suitable hosts for thrips to lay eggs and may carry  tospoviruses and showing, few, if any symptoms.

Prevention

  • Inspect incoming plants or cuttings.
  • If possible, keep thrips-infected plants isolated in a separate area to avoid the spread of thrips. 
  • Remove weeds and growing media debris.
  • A weed-free barrier of at least 10 feet around the greenhouse may help to discourage thrips entry. When outdoor weeds desiccate or when weedy areas are mowed, thrips may enter the greenhouse to search for new hosts.
  • Remove “pet plants” that may serve as a breeding ground for thrips.
  • Dispose of plant debris in tightly sealed containers. Do not allow open garbage bins in the greenhouse, as thrips may disperse from the plant material unto the crop.

Scouting

Early detection of western flower thrips is difficult due to WFT's high reproduction rate, rapid developmental time and tendency to hide in flowers and buds. In spite of this difficulty, early thrips detection is essential to limit thrips feeding damage and possible transmission of tospoviruses to uninfected plants.

Use sticky cards (yellow or blue) to monitor for the adult thrips.  A 10-20x-hand lens is needed to distinguish the adult thrips from grains of peat moss or other debris. Weekly counts of thrips adults on sticky cards helps determine population trends and the effectiveness of pest management tactics. Tolerance levels depend upon the crop, its stage of growth and the customer’s tolerance of pest damage.  Some growers use a tolerance level of between five to 10 thrips per card per week on their spring crops. If plants become infected with the virus, the tolerance level for thrips is zero and strict thrips control is then needed.

Growers may also gently tap plant foliage or flowers over a sheet of white paper to dislodge thrips. Gently blowing into open flowers agitates the thrips so they are easier to see.

Biological Control

Commercially available natural enemies include predatory mites (Hypoaspis miles (= Stratiolaelaps scimitus), Neoseilus cucumeris, Amblyseius swirskii) and predatory bugs (Orius species).  Hypoaspis miles is a soil dwelling predatory mite that is primarily used against fungus gnat larvae in propagation houses. However, this generalist predatory mite may also feed on thrips pupae in the growing media. 

N. cucumeris primarily feeds on the young thrips larvae but may survive on pollen and spider mites in the absence of thrips.  Because it only feeds on the first instar thrips larvae, it is important to begin releases preventively at planting. It is sold in bulk with a bran carrier for broadcast release, or for use in breeder piles. N. cucumeris is also available in sachets or mini sachets that are often used in hanging basket crops. The predatory mites emerge from these sachets over a four to six week period. N. cucumeris is most effective at temperatures greater than 70˚ F with an optimum range between 50 to 85˚ F and relative humidities between 60 and 85%.

Amblyseius swirskii is a predatory mite that feeds upon both thrips and whiteflies. It may also feed upon broad mites, two-spotted spider mites and pollen in the absence of thrips and whiteflies. However, this predatory mite prefers warmer temperatures than N. cucumeris (77- 82˚ F) and a relative humidity of 70%. It is available in breeding sachets or in bulk releases. 

Different species of predatory bugs (Orius spp.) are generalist predatory that feed upon thrips, spider mites and pollen. Both adults and larvae are predatory and can feed upon both the larval and adult thrips.  They are most effective on long term greenhouse vegetable crops such as peppers due to the presence of pollen.  Growers have experimented with using thrips banker plants (“Black pearl” Ornamental Peppers) and habitat plants to help encourage their establishment in the greenhouse.  Minute pirate bugs may also undergo a resting or diapause period during short days.

Beneficial insect killing nematodes (Steinernema feltiae) that are used against fungus gnat larvae may also be effective against thrips pupae in the growing media.

The insect killing fungus Beauveria bassiania requires a relative humidity greater than 70% and temperatures between 65 and 75˚ F for 8 to 10 hours in order to be effective. Thorough spray coverage is also needed.  Isaria (= Paecilomyces) requires a relative humidity of between 68 and 100% and temperatures between 72 and 86 ˚F.

Chemical Controls

Insecticides with contact or translaminar activity are generally used against thrips. Systemic insecticides do not move into the flowers, so only suppress thrips on the foliage.  Resistance has been reported to organophosphates, carbamite, pyrethroid, and macrocyclic lactone chemical classes.  To delay the onset of insecticide resistance, growers need to rotate between insecticides with different modes of action every two to three weeks, or after one generation (depending upon temperature). Repeated applications two to three times every 3 to 5 days (depending upon temperature) may be needed to reduce thrips numbers.

Growers often ask about the addition of sugar to their sprays against western flower thrips. Either thrips adults or larvae are attracted to sugar based compounds, so the addition of brown or white sugar to a spray is not going to increase its effectiveness.  In addition, the sugar may provide a growing media for the growth of black sooty mold fungus.

Consult the most recent edition of the New England Floricultural Crop Pest Management and Growth Regulations Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators for more specific guidelines. Available from Northeast Greenhouse Conference and Expo and the UCONN CANR Communications Resource Center.

References

Blumthal, M.R., R. A. Cloyd, L. A. Spomer, and D. F. Warnock. 2005. Flower Color Preferences of Western Flower Thrips. Hort Technology. 15 (4): 846-853.

Casey, C. (ed). 1997. Integrated Pest Management for Bedding Plants. A Scouting and Pest Management Guide. Cornell Cooperative Extension Publication No. 407. 109 pp.

Cloyd , R. and C. S. Sadof. 2003.  Seasonal Abundance and Use of an Action Threshold for Western Flower Thrips in a Cut Carnation Greenhouse. Hort Technology. 13(3) 497- 500.

Cloyd, R. 2010. Western Flower Thrips Management on Greenhouse Grown Crops. K State University Fact sheet MF-2922. 8 pp.

Cloyd, R. A. and J. D. Gillespie.  2012.  Effect of Sugar Based Compounds in Enhancing the Efficacy of Insecticides against the Western Flower Thrips.  Hort Technology. 22(2): 177-184.

Daughtrey, M., R. Jones, J. Moyer, M. Daub, and J. Baker. 1997. Tospoviruses Strike the Greenhouse Industry. Plant Disease 81(11):1220-1230.

Immaraju, J.A., T.D. Paine, J.A, Bethke, K.L. Robb, and J.P. Newman. 1992. Western Flower Thrips (Thysanoptera: Thripidae)  Resistance to insecticides in Coastal California Greenhouses, Journal of Economic Entomology. 85(1) 9-14.

Robb, K.L. 1988. Analysis of Frankliniella occidentalis (Pergande) as a Pest of Floricultural Crops in California Greenhouses. P.h.D. dissertation, University of California, Riverside.

Sanderson, J.P. 1990. Western Flower Thrips Biology and Control. Long Island Horticulture News. August 1990 1-3 pp.

Sether, D.M. and J.D. DeAngelis. 1992. Tomato Spotted Wilt Virus Host List and Bibliography. Agricultural Experiment Station Oregon State University Special Report 888.

Stobbs, L.W., A.B. Broadbent, A.B. Allen, and A.L. Stirling. 1992. Transmission of Tomato Spotted Wilt Virus by the Western Flower Thrips to Weeds and Native Plants in Southern Ontario. Plant Disease 76:23-29.

Prepared by Leanne S. Pundt, Extension Educator, University of Connecticut.   1995 Revised 2013

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.