Download Greenhouse Management of Western Flower Thrips and Tomato

ORNAMENTALS
NORTHWEST
ARCHIVES
Nov.-Dec. 1988
Vol.12, Issue 6
Pages 7-11
Jack DeAngelis, Extension Specialist,
Department of Entomology, OSU
GREENHOUSE MANAGEMENT OF WESTERN FLOWER THRIPS AND TOMATO
SPOTTED WILT VIRUS
The Western flower thrips (WFT), Frankliniella occidentalis (Pergande), has become a serious
insect pest in the commercial greenhouse industry throughout the US and Canada. Some consider
WFT to be the most serious insect threat to the industry today. The reasons that this insect poses
such an important threat industry-wide are: (1) WFT carries a plant disease called tomato spotted
wilt virus (TSWV) that can potentially infect nearly every commercially-grown greenhouse
plant; and, (2) in many parts of the country WFT populations have developed resistance to
insecticides commonly used for its control. The extent and distribution of resistant populations in
the Northwest is not yet known; however, California has reported populations resistant to
organophosphates, pyrethoids and carbamates.
The virus. Tomato spotted wilt virus was first described in the 1930's as a disease of tomato.
It is currently recognized as a major disease of lettuce, pepper, tobacco, peanut, and floral crops
in addition to tomato. The southeastern US currently is experiencing severe outbreaks of the
virus in peanuts, Hawaii has lost 100% of the lettuce crop in some areas and in California it is
considered to be among the most serious greenhouse diseases. In 1988, at least three states have
reported field infections of tomato spotted wilt virus in tomatoes and peppers which originated
from infected greenhouses; this was the first time that field infections of tomato spotted wilt
virus had been reported from any the three states (Jones and Baker, 1989).
Six different species of thrips carry the virus.
As far as is known thrips feeding is the only way a plant can become naturally infected;
however, seeds from an infected plant may contain the virus and develop into an infected plant.
TSWV is the only virus that is known to be transmitted by thrips.
There appears to be at least several host-specific forms (strains) of the virus, and each causes
different disease symptoms. Infected plants may appear chlorotic, splotchy at first with wilting
and necrotic areas forming later on; ring-shaped discolorations on leaves or flowers are good
indicators of TSWV infection. The only way to be sure, however, is to have the plant analyzed
using several virus detection methods: Apparently, not all strains of the tomato spotted wilt virus
are detected by the serum commonly used in ELISA (a laboratory procedure that tests for the
presence of a particular virus); virus-indexing on susceptible indicator plants and electron
microscopy may be necessary to detect some TSWV strains.
Figure 1. WESTERN FLOWER THRIPS (WFT). A, Top view of egg in plant tissue. B, Egg. C,
Larvae oozing out of plant tissue just as they hatch. D, First larva. E, Second larva. F, Prepupa.
G, Pupa. H, Adult female. [From: Jones, R.KX and J.R. Baker. 1989. Tomato spotted wilt virus.
N. C. Flower Growers Bulletin 34(l):12-15.]
One characteristic of this disease is that infected plants tend to show the most obvious symptoms
when plant growth has been slow or checked, for example by cool temperatures, low light, or
other adverse cultural conditions. If the plants survive the initial severe shock phase of the
disease, new growth can emerge symptomless, but still carry the virus: New growth on an
infected plant may not display symptoms at all when growing rapidly, for example during warm,
high light periods, but symptoms may again appear when growth rate is checked. This can make
detection of infested material difficult.
The vector. Western flower thrips (note that the name "thrips" is always plural; one thrips, or
many thrips) is a tiny insect that feeds on leaves and flowers. All feeding stages probe plant
tissue with a knife-like structure of the mouth. Cell sap from broken cells is then sucked up
leaving behind empty, air-filled cells that form a feeding scar that imparts a silvery appearance to
the plant surface. The adult female lays eggs into tiny slits in the plant tissues. These hatch in 2-4
days into translucent larvae that begin to feed. The larvae molt to a second stage and after 2-4
days again molt to the prepupal stage. This stage leaves the plant and seeks shelter in the soil or
leaf litter at the base of the plant to pupate to the adult. The generation time, depending on
temperature, is 7-13 days. The adult female may live 30 to 45 days and lay 150-300 eggs during
her life time. Mating is not necessary for egg production; unfertilized eggs develop into males.
This system of reproduction, called arrenotoky, is found in spider mites and other small insects
and mites that depend on the wind for dispersal. (See Figure 1 illustrating the stages of the WFT
life cycle).
Figure 2. The symptoms of
tomato spotted wilt virus
on gloxinia include stunted
plants with chlorotic to
necrotic line patterns and
ring spots on leaves and
flower sepals, malformed
plants, delayed flowering,
white ring or line patterns
on red or blue flowers
(color breaking), and
flower distortion. Symptom
expression is most severe
within 5-10 days after
infection and is particularly
severe on young plants. If
the plants survive the
initial, severe shock phase
of the disease, new growth
can emerge symptomless,
but stunted, and still carry
the virus. (photos courtesy
R.K Jones and J.R. Baker, North
Carolina State University).
Both WFT and TSWV have extremely wide host ranges. Virtually every flowering plant is
susceptible to WFT. The host range of TSWV is more limited but includes most flowering plants
used in the ornamentals industry as well as many weeds associated with greenhouses. To date
over 160 hosts of TSWV have been confirmed.
Management. Control of WFT and TSWV in the greenhouse is a subject that every grower must
be concerned with even if you have not yet experienced a problem. Cases of the disease are
increasing in the Pacific Northwest as well as nationwide. Direct feeding injury accounts for a
portion of the economic impact of WFT on the ornamentals industry. Far more important,
however, is the thrips ability to transmit TSWV. Not every WFT carries the virus, in fact
probably only a small proportion are infective under normal circumstances. The real concern is
that once TSWV is introduced into the greenhouse, perhaps by a single thrips or by infected
propagation material, there is no treatment that will eradicate it short of destroying every infected
plant and infective thrips.
.
The key then is to prevent the introduction
of the virus in the first place.
Barriers. You should suspect that all purchased propagation material is infected, and if possible
isolate it until you are sure it is virus-free. If you are not able to isolate new material, watch it
closely for symptoms and be prepared to rogue out any plants that show signs of TSWV,
especially if WFT also are present.
A large open range where many different varieties are grown at many different stages is the
worst possible situation in terms of WFT and TSWV management. Once the virus is introduced
it becomes virtually impossible to eradicate it short of removing and destroying all plants and
then fumigating. Therefore, if possible, partition the range with barriers of plastic sheets or
plastic screen. Segregate plants arriving from elsewhere, preferably in a separate house, until you
are sure they are healthy.
Thrips enter houses mainly through doors and vents, on plant material and workers. Vents should
be screened to exclude adult thrips. Doorways should be fitted with a double entry constructed of
the same screening material. Some greenhouse operators have found that screening that is
adequate to exclude thrips reduces air circulation so much that additional fan power is required
to draw air through the screened pads and vents. Finally, since thrips are attracted to light colors,
especially yellow, blue and white, avoid wearing clothes of these colors while in the range
because thrips can be carried on clothing from points of infestation into clean areas.
Monitor. An effective thrips monitoring program, conducted year-around, may be the most
important single element of your control program. A good monitoring program will (1) alert
greenhouse managers to the presence of WFT so that control measures can be taken before
populations build to the point where control is difficult, (2) locate WFT within the range so that
spot treatments may be applied, and (3) "trap out" many adult WFT, especially those migrating
into the range through doors and vents.
Thrips can be monitored with sticky card traps. Yellow sticky traps work well. But, recent
research has shown that light blue ("Fixall Rust Resistant Spray Enamel - Light Blue") or white
traps ("Fixall Rust Resistant Spray Enamel - Flat White) sticky traps coated with one layer of
Seabright Limited Stikem polybuthene adhesive are more effective, especially at low thrips
densities (Jones and Baker, 1989). Place 1 trap per 400 - 2000 sq. ft. just above the plants.
Exact placement of sticky traps within the range is critical and may be difficult to determine at
first. Thrips are weak fliers. They tend to "fall out" at places where air flow changes abruptly; i.e.
at windbreaks or obstacles within the range. Trial and error or visually monitoring air currents by
vertically hanging colored polyfilm ribbons (surveyor's ribbon) throughout the range will locate
these areas in the range. Place additional traps at doors and vents. Traps should be checked at
least every 5 days. Number each one so that accurate records can be kept of location and what is
caught. Parrella, et al (1989) indicated that one or two thrips per trap per week is not necessarily
a problem, "But 20 or more per week per trap should be investigated and an insecticide
application considered."
"Thrips can also be sampled by tapping infested foliage over white paper. Thrips falling off the
plant are easily seen against a white background with a hand lens" (Parrella, et al, 1989).
Another important monitoring tool is the use of "indicator plants". These are varieties that are
both highly susceptible to TSWV and display symptoms quickly after infection. A good indicator
plant in this case is gloxinia. Place a minimum of 1 pot per bench and inspect these "indicator
plants" during sticky card trap inspections. (See Figure 2 illustrating visual symptoms of TSWV
on Gloxinia).
Chemical Controls. WFT has reportedly developed resistance to a range of insecticides in many
parts of the country.
The source of your thrips and what they have been sprayed with will largely determine their
susceptibility to various insecticides.
If you experience a control failure, try switching to an insecticide from a different chemical
group. According to Parrella, et al (1989) "Insecticide efficacy varies considerably from location
to location depending on the level of insecticide resistance in each thrips population."
Spot treat areas of the range where WFT has been trapped or observed. It is a waste of time and
money to treat an entire range if the population of WFT is confined to a small area. Over
treatment is likely to promote resistance as well. WFT will not normally migrate unless its host
plant begins to deteriorate. You should repeat treatments with the same material at least every 35 days until WFT is under control in areas where it has been detected (usually 3 treatments).
Insecticidal soap is an excellent material to use as a spot treatment or tank mixed with other
materials. It is unlikely that thrips will develop resistance to soaps. Bear in mind, however, that
soaps have little or no residual activity so repeat applications are needed.
Chemical Control of WFT in Greenhouses
Material
Common
Brand Name
avermectin B
Avid
bifenthrin
Talstar
pyrethroid
chlorpyrifos
Dursban ME2
organophosphate med ium
dibrom
Naled fumigant organophosphate high
fluvalinate
Mavrik
fo rmet anate
Ca rzol
insecticidal
soap
S a f e r 's S o a p
oxamyl
1
Vydat e
Chemical
Group
Toxicity
avermectin
low
me d iu m
pyrethroid
low
formamidine
high
soap
very low
carbamate
high
1
Thrips not currently on the Avid label but some growers have reported good
control.
2
Microencapsulated; check availability and label for specific uses.
NOTE. The table above was constricted from a variety of sources. It is not
intended to be an exhaustive list of all available materials; materials were
selected according to effectiveness and safety. Always read and follow label
instructions. The use of common brand names is for simplicity and should not be
construed as an endorsement of one product over similarly labelled products.
Sanitation. Weeds and discarded, deteriorating plants are important sources of WFT and TSWV.
Do not leave discarded plants or soil mix in the range.
As plants or soil deteriorate and dry out thrips will leave them and move to fresh plant material.
It is an excellent idea, if possible, to occasionally empty a house of all plant material, refuse and
weeds and then fumigate to control any WFT that may be lying in wait for the new plants. A
regular weed control program in and around the range, and under benches, is important as well.
References:
Jones, R.K. and J.R. Baker. 1989. Tomato spotted wilt virus: prospects for 1989. N.C. Flower
Growers Bulletin 34(1):12-15, February 1989.
Parrella, M., K. Robb, and D.R. Smiley. 1989. Western flower thrips: detection and
management. PPGA News 20(3):6, March 1989.
Green, J.L. 1988. Tomato spotted wilt virus in greenhouse crops. Ornamentals Northwest
Newsletter 1.2(3):10, May-June 1988.
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