N O V E M B E R 2 0 0 7
P R I M E F A C T 7 1 3
Western flower thrips and tomato
spotted wilt virus
Dr Sandra McDougall
Industry leader (vegetables), Vegetable Industry
Industry leader (protected cropping), EMAI
Western flower thrips
Western flower thrips (WFT), Franklienella
occidentalis is primarily a pest as a vector of
tomato spotted wilt virus (TSWV) in tomatoes,
lettuce, potatoes, and capsicum. Its feeding can
cause scarring and deformations on leaves and
Figure 2. TSWV-infected lettuce
fruit, with seedlings and soft tissue particularly
prone to feeding damage. Capsicums, cucumbers
and beans are susceptible to fruit scarring.
Immature thrips are pale yellow, thin, wingless and
up to 1 mm in length. Adults are also thin, with
yellowish head and darker abdomen. They are
about 1.5-2 mm in length, with two feathery wings.
Figure 3. Juvenile WFT
Figure 1. WFT scarring on capsicums
WFT is more of a problem than other thrips species
because it develops resistance to pesticides easily,
hence there are few chemical options to control it.
WFT was first found in Western Australia in 1993
and has spread to all states and most production
areas since. It originates from western USA.
Figure 4. Adult WFT
The life cycle of WFT is illustrated in Figure 5. Eggs
TSWV causes significant damage to solanaceous
are laid into soft plant tissue. Within a few days
vegetables such as tomatoes, potatoes and
eggs hatch into a wingless juvenile or larval stage.
capsicums, but also to lettuce and a wide range of
Thrips have two feeding larval stages followed by
herbs and ornamental crops. Cucumber infections
non-feeding pre-pupal and then pupal stages which
tend to hide in soil crevices or within foliage.
Winged adults emerge from the pupae to mate and
feed. The length of the life cycle and life
expectancy of the adults depend on temperature
Many hundreds of plants (>900) are TSWV hosts,
and food quality. At 30°C the life cycle is
most being in the Solanaceae, Asteraceae or
approximately 12 days while at 20°C it is 19 days.
Legumaceae. Some show symptoms and some do
Common weed hosts of both WFT and TSWV
include amaranth, cape weed, pigweed, mallows,
blue heliotrope, fat hen, purple top, shepherd’s
purse, nightshades, Scotch thistle and sow thistle.
Figure 5. WFT life cycle
Figure 6. Cape weed showing TSWV symptoms
Western flower thrips breeds on a wide range of
flowering plants including weeds, vegetable crops
and fruit trees.
Tomato spotted wilt virus
Tomato spotted wilt virus (TSWV) is a tospovirus
transmitted by some species of thrips. WFT, tomato
thrips (Franklienella schultzei) and onion thrips
(Thrips tabaci) are common vectors for TSWV in
vegetables. Melon thrips (Thrips palmi) is also a
vector for TSWV but is not widespread in NSW.
Once a plant is infected with TSWV it cannot be
cured, so prevention or use of tolerant varieties, if
Figure 7. Sow thistle
available, are the only management options.
How do thrips acquire TSWV?
Thrips larvae must feed on a TSWV-infected plant
Tomato spotted wilt virus is one of the most wide-
to acquire the virus. Uninfected adult thrips cannot
spread and damaging viruses affecting vegetable
acquire the virus. Plague thrips and other non-host
crops in Australia. TSWV was first described in
thrips cannot acquire the virus, nor can other
Australia in 1915 and has been a sporadic problem
insects such as aphids. TSWV is not spread in
since then. The arrival of the very efficient vector
seed or via mechanical damage although it can be
WFT has seen an increase in the seriousness of
spread through cuttings used for plant propagation.
the disease, particularly in hydroponic and covered
PRIMEF ACT 713, WESTERN FLOWER THRIPS AN D TOM ATO SPOTTED WILT VIRUS 2
How do thrips transmit TSWV?
mesh and actively monitoring for WFT and TSWV
will greatly reduce your chances of bringing them
Once the thrips larva has acquired the virus, it will
onto your farm.
multiply within the larva. When an infected larva
reaches adulthood it can fly to a new plant,
Given that both WFT and TSWV have very large
transmitting the virus as it pierces the plant cells
host ranges, management of weeds within and
and sucks the contents. The virus does not pass
surrounding your crops is essential. In areas where
through the egg stage so each succeeding
both WFT and TSWV populations are high, putting
generation of thrips must re-acquire the virus as
pressure on local councils and neighbours to also
larvae feeding on TSWV-infected plants.
control roadside or paddock weeds, particularly
broadleaf weeds, can greatly assist in reducing the
overall regional pressure.
Trials in South Australia using native plants,
particularly saltbushes and grasses that are poor
hosts for WFT and TSWV, around greenhouses
and along road verges can reduce the incidence of
virus in nearby crops. The added benefit is that the
plantings reduce the need for weed management
around the structures.
If TSWV is noticed in crops, removing the plants
showing symptoms will reduce the within-crop
infection. Early detection and removal can greatly
reduce final crop losses due to TSWV.
Chipping plants and leaving them on the ground is
not sufficient; the infected plants must be removed
completely from the farm, bagged or buried. If a
planting is heavily infected with TSWV it may be
much better to remove the whole crop to avoid
infection of younger plantings.
Figure 8. Diagram of thrips–TSWV transmission
Crops grown in greenhouses can be protected from
WFT using thrips-grade mesh covering all openings
and double doors to avoid thrips entering.
A TSWV-infected plant cannot be cured.
Management options focus on preventing infection
and minimising spread.
Some varieties of capsicums and tomatoes are
resistant to TSWV although strains of TSWV that
break the resistance can develop in areas of high
− avoiding contaminated or infected seedlings
TSWV pressure. Therefore, if you do use resistant
− weed management within and surrounding
varieties it is still important to reduce the virus
pressure through weed management and other
roguing and destroying infected plants
− removing or cultivating in crops after harvest
Monitoring WFT and TSWV
− screening greenhouses with thrips-grade
Sticky traps, visual monitoring and vacuum
sampling are all used to monitor WFT. Blue traps
• TSWV resistant varieties (capsicums)
may be preferable for monitoring WFT because
• Monitoring crops for WFT and TSWV
they are less attractive to other non-thrips species.
Tapping flowers over a white tray is recommended
Biologically based Integrated Pest Management
when susceptible crops are flowering. As part of
• Foliar insecticide sprays to control WFT.
monitoring the crop for a range of pests a small
proportion of thrips will be picked up in a vacuum
sample, although this method of sampling is not
recommended if primarily assessing for thrips
The most important management option is to
reduce the populations of WFT and TSWV. Avoid
bringing in seedlings contaminated with WFT or
Not all plants that are infected by TSWV will show
already infected with TSWV. Sourcing seedlings
symptoms. Crops that are susceptible will tend to
from nurseries that are screened with thrips-grade
show symptoms on the new developing foliage
PRIMEF ACT 713, WESTERN FLOWER THRIPS AN D TOM ATO SPOTTED WILT VIRUS 3
Some field trials are also assessing the potential
for using soil amendments to raise soil organic
carbon as a means of encouraging native
predatory mites, particularly Pergmasus sp.
Another promising area is the use of
entomopathogenic (insect infecting) fungi. Two
groups have developed strains of Beauvaria and
Metarhizium fungi that are effective against thrips
and some other sap-sucking insects. They are
currently working with a commercial producer and
the APMVA on registration.
Figure 9. Sticky traps for thrips monitoring
Insecticide applications targeting WFT can reduce
after infection. Test kits are available for testing for
WFT populations and reduce within-crop
transmission of TSWV. Given that WFT is
Alternatively, plant or thrips samples can be sent to
notoriously effective at developing resistance to
state department diagnostic laboratories.
chemicals used against it, is important to adhere to
the recommended resistance management
strategy (see ‘Further Information’).
Biological controls have not been considered
Not all effective chemicals are available for all
important for WFT control in Australia until recently.
crops so it is similarly important to use only the
Thrips are highly mobile and small and many of the
chemicals specifically registered or available via
common generalist predators do not feed on them.
Permit for your crop.
Predatory mites are common but rarely appear to
Correct chemical application can be extremely
adequately control WFT numbers. Predatory mites
important for WFT; for example, make sure your
such as Amblyseius montdorensis and Amblyseius
spray equipment is properly calibrated, use
cucumeris are, however, proving to be effective in
insecticide rated nozzles, use the appropriate water
greenhouses that use only ‘selective’ chemistry and
rates, and spray only in suitable weather
have adequate ventilation. Commercial rearing
conditions. For more information pick up the Spray
systems are being developed by NSW DPI and
Sense information sheets from your nearest NSW
existing commercial producers of beneficial
DPI office or from the web site.
Integrated pest management
Integrated pest management (IPM) is a strategy
that draws on a range of management tools with
the goal of using the least ecologically disruptive
techniques to manage pests to within economically
acceptable levels. For TSWV and WFT
management, an IPM approach concentrates on
reducing the source of both WFT and TSWV.
Where it is possible, the use of TSWV tolerant
varieties are recommended. Monitor crops for both
WFT and TSWV levels and when TSWV is
Figure 10. Predatory mites (A. montdorensis and
observed rogue out and destroy the infected plants.
If growing in a greenhouse use thrips-grade mesh
on all openings and use double doors.
If WFT numbers are high enough to be causing
physical damage at vulnerable stages or TSWV is
present, then insecticide sprays may be needed.
Use recommended rates, adhere to the resistance
management strategy and apply sprays to
maximise spray coverage.
Figure 11. Predatory thrips larva (Haplothrips)
PRIMEF ACT 713, WESTERN FLOWER THRIPS AN D TOM ATO SPOTTED WILT VIRUS 4
Sending samples for identification
Management strategies are most effective when
Identification Guide to thrips associated with crops
accurate identification of both thrips and viruses is
in Australia (1997), CSIRO/NSW DPI, L. Mound
and P. Gillespie
Adult thrips can be identified either from specimens
Managing WFT and TSWV in Vegetables (2003),
collected into a vial or container, or from sticky
[CD] DPI VIC, anon
traps. If it is likely to be some time before you can
deliver or send the sample then, if possible, collect
Pest thrips of the world (2004), [CD] ACIAR, Mortiz
adults into a small quantity of rubbing alcohol
(available at the chemist). Collect at least 10
Spray Sense – information for users of chemicals.
specimens as handling and transportation may
damage important diagnostic features used to
identify the insects. Sticky traps can be covered in
Thrips and tospovirus: a management guide
plastic cling wrap before removing from the field or
(2007), QDPIF, D. Persley et al.
greenhouse. Try to avoid squashing sticky traps as
accurate identification is difficult if specimens are
Western Flower Thrips (1994), [Information guide],
DPI VIC, Hill
For plants suspected of being infected with TSWV,
Western flower thrips (WFT) insecticide resistance
collect a number of plants showing the range of
management plan (2005), NSW DPI, G. Herron et
symptoms. Wrap plant material in damp but not wet
paper, place in a plastic bag, keep cool and send or
deliver as quickly as possible.
Samples can be left at any NSW DPI office or sent
Which thrips is that? A guide to the key species
transmitting TSWV in NSW (2005), [Information
sheet] NSW DPI, M. Steiner et al.
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based on knowledge and understanding at the time of writing
(November 2007). However, because of advances in
Elizabeth Macarthur Agricultural Institute
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PRIMEF ACT 713, WESTERN FLOWER THRIPS AN D TOM ATO SPOTTED WILT VIRUS 5