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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 Damage
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.
Virus Transmission
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).
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 over wintered 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 over wintering 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.
Prevention
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 and 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.
Sanitation
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.
Monitoring
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.
Indicator Plants
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.
Biological Control
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.
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
References
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.
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