The western flower thrips (WFT), Frankliniella occidentalis, is of increasing concern to greenhouse growers in New England. Western flower thrips have spread throughout the horticulture industry on plugs, cuttings and small plants. WFT's 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.
Thrips feed by piercing plant cells with their mouthparts and feeding on the exuded plant juices. This collapse of plant cells may result in deformed flowers, leaves and shoots. Silvery flecked scars or small black "fecal" spots may be seen on the expanded leaves. Western flower thrips have a broad host range and may feed upon impatiens, fuchsia, chrysanthemum, carnation, ivy geraniums, marigolds, hibiscus, verbena, rose and petunia.
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.
TSWV is primarily of concern for field production of tomatoes,
peppers and peanuts. INSV is of primary concern to greenhouse
growers. 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 cineraria,
ranunculus, impatiens, New Guinea impatiens, cyclamen, exacum,
begonia, primula and gloxinia. Almost all greenhouse crops with
the exception of roses and poinsettias are susceptible. Symptoms
may 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 TSWV/INSV include leaf spots, necrotic areas,
mottling and ringspots. Young plants may be especially vulnerable
to infection. There is no cure for this virus. As soon as INSV/TSWV
is detected, rogueing of infected plant material must be combined
with strict thrips management to prevent serious losses.
Only the first instar larval thrips may acquire the virus. If infected as larvae, the adults can transmit the virus in their salvia as they feed. The winged adults are primarily responsible for viral transmission, however, due to a midgut barrier, the adults cannot acquire the virus. The virus persists in the adult thrips for their entire life. In addition, INSV/TSWV multiplies within the adult thrips. However, 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. In a recent survey of 72 different Massachusetts greenhouses, 42 greenhouses had plants present that tested positive for INSV.
Life Cycle of Thrips
Most of the thrips in a greenhouse are females. Adult females may live for approximately 30 to 45 days. During their lifetime, female thrips may lay from 150 to 300 eggs that are inserted into plant tissue. 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. Therefore, in some cases, it may be important to target spray applications to the lower leaf surface to directly contact the small yellow thrips larvae. The first two larval stages remain protected in the tender young growth. After the 2nd instar larvae stops feeding, it drops to the soil or substrate to pupate. Adults may then emerge in two to five days, depending upon temperature.
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. Robb found that their life cycle varied 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 68o and 98oF|
|1st instar (immature)||1-2 days|
|2nd instar||2-4 days|
In the greenhouse, WFT can be found year-round whenever temperatures are favorable for their development and host plants (including weeds) are available for food. Recently, adult thrips have overwintered outdoors in nectarine orchards in southeastern Pennsylvania. This spring capture followed high WFT population densities in the fall and a milder than normal winter. Thrips have not yet been found overwintering in New England; however, more research needs to be conducted on this issue.
The potential for thrips to overwinter may result in the possible spread of INSV/TSWV to perennial weed hosts. In one study, Canadian researchers collected weeds both in and near greenhouses where there was a history of INSV/TSWV. Many weeds were found to be both susceptible to TSWV and suitable hosts for thrips to lay eggs. Fifty species, including such common greenhouse weeds as redroot pigweed, chickweed, lambsquarters, bindweed, thistle, oxeye daisy, galinsoga and pineapple weed, may be susceptible to both WFT/TSWV.
The first step in thrips management is to prevent thrips entry into the greenhouse. It is important to monitor for adult and larval thrips as soon as young plants or cuttings are received. If possible, keep thrips-infected plants isolated in a separate area to avoid the spread of thrips. Winged adults are weak fliers but may be carried on air currents employees clothing. Avoid wearing highly attractive colors such as pink, blue, yellow, white or green. Consider worker movements so that highly infested greenhouses are entered last.
As discussed earlier, weeds may serve as a host for both WFT and INSV. Therefore, weed control both inside and outside the greenhouse is a critical part of thrips management. A weed-free barrier of at least 15 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. Avoid keeping over "bridge plants" such as holiday cactus, stephanotis, Swedish ivy and cyclamen that may be infected with INSV.
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.
Sticky cards may be used to monitor for the adult thrips. Blue sticky cards are more effective in detecting thrips than yellow sticky cards. Therefore, blue cards are best used in thrips-sensitive crops such as roses or African violets. Yellow sticky cards are best used for general pest monitoring because they are attractive to whiteflies, winged aphids, leafminers, fungus gnats and shoreflies in addition to thrips. A 10-20x-hand lens is needed to distinguish the adult thrips from grains of peat moss or other debris. Place cards on a clothespin attached to a stake so that the cards may raised as the plants grow. It is best to place the cards one to two inches above the crop canopy. Place cards in an even pattern throughout the entire greenhouse. Cards may be placed at the rate of at least one card per 1,000-sq. ft. with additional cards placed near the doors, vents and thrips-sensitive plants. Thrips will move on air currents in the greenhouse. Place twice the number of recommended cards for several weeks to detect hot spots of thrips activity. Place cards in those locations. Thrips populations will be highest when the plants have flowers present.
Tolerance levels will vary depending upon the crop's susceptibility to damage and stage of growth. Tolerance levels will depend upon the crop, its stage of growth and the tolerance of pest damage. (Because thrips are pollen feeders, it is important to treat for thrips early before extensive damage to the flower can be seen.) Keep track of thrips counts on the cards and correlate this to the damage you see on your crops. Some growers use a tolerance level of between five to 10 thrips per card per week. Propagators will have a lower threshold for thrips, than finished plant growers will. If INSV/TSWV is present, the tolerance levels for thrips is zero and strict thrips control is then needed.
Growers may consider the use of indicator plants to help determine if thrips are carrying the virus. Certain varieties of petunia or fava bean are highly attractive to thrips, especially if a blue nonsticky card is placed just above the foliage. Remove the flowers so that the thrips feed upon the leaves.
A small, localized brown to black lesion may be seen if the feeding thrips are carrying the virus. Because the virus infection is localized, the petunia indicator plants do not serve as a source of virus inoculum. Growers may simply remove the infected petunia leaves and continue to use the petunia indicator plants. However, fava bean indicator plants should be removed to avoid the possibility of a systemic infection.
The use of natural enemies against thrips has been used in greenhouse vegetable production. The potential exists for biological control to be part of an integrated strategy toward thrips control in ornamentals. To be successful, it is important to attempt biological control strategies on a small scale after becoming familiar with monitoring and the use of the more environmentally "friendly" materials such as insect growth regulators, and botanicals.
The following natural enemies are commercially available. On-going research is evaluating how to best use the predatory mites in ornamentals. Neoseilus cucumeris is a small, predatory mite that may be sold in slow release packs or suspended in bran. Adult females consume an average of one thrips per day over their 30-day life span. N. cucumeris primarily feeds on the young thrips larvae but may survive on pollen and spider mites in the absence of thrips. Iphiseius degenerans is a predatory mite that have been successfully used in sweet pepper production, where there is a significant source of pollen. Orius insidious is a minute pirate bug, which feeds on both the larval and adult thrips. However, Orius goes into diapause in response to short days, so should not be used between April and August.
Allen, W.R. and J.A. Matteoni. 1991. Petunia as an Indicator Plant for Use by Growers to Monitor for Thrips Carrying the Tomato Spotted Wilt Virus in Greenhouses. Plant Disease 75:78-82.
Anonymous. 1992. Western Flower Thrips Overwinters. Regulatory Horticulture, 8-16. Spring 1992 issue.
Casey, C. (ed). 1997. Integrated Pest Management for Bedding Plants. A Scouting and Pest Management Guide. Cornell Cooperative Extension Publication No. 407. 109 pp.
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.
Matteoni, J. Elliott, D. Cahn, D. Ravensberg, W. and M. Dissevelt. 1993. Chemical effects on greenhouse biological control agents. GrowerTalks 57:82-85.
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, Commercial Horticulture
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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