PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Western flower thrips (WFT)
Scientific name
WFT (Frankliniella occidentalis, Order
Thysanoptera) is one of the world’s most
important horticultural pests due to:
Its resistance to many insecticides.
Efficiently spreading tomato spotted wilt virus
(TSWV) and impatiens necrotic spot virus (INSV).
Feeding in unopened growth or flower buds.
Its tiny size, rapid life cycle (13 days at 30 o C),
and high reproductive capacity (an adult female
can live for 30-45 days and lay 150-300 eggs).
Host range
WFT has been recorded on more than 250 plant
species including weeds and greenhouse crops.
Ornamentals, eg chrysanthemum, gerbera,
gypsophila and roses; cut flowers, native plants.
Fruit, eg strawberry, stone fruits, soft fruits.
Vegetables, eg capsicum, cucumber, lettuce,
potato, tomato, various herbs. Field crops, eg
peanut. Weeds, eg capeweed, flowering white
clover, redflowered mallow, sow thistle, wild
mustard, Paterson’s curse.
Description & damage
Adults and nymphs damage plants by rasping or
scraping surface cells and sucking cell sap.
Adults are 1-2 mm long and just visible to the
naked eye. They have 2 pairs of feathery long
narrow wings with a fringe of long fine hairs along
the margin. Wings are held parallel along the back
when at rest. Nymphs are wingless, white, straw
yellow or brown in color.
Damage.
Flowers, new buds and young leaves
– Damage is not always obvious after feeding
but becomes more obvious as affected flowers,
leaves or fruit grow. Crops show silvering,
flecking or deformation of flowers, growing
tips, young foliage, stems and fruit. WFT does
not generally infest foliage, but when it does,
drops of excreta may disfigure leaves.
– Most weeds are symptomless.
Strawberry. Thrips feed between prominent
seeds in green fruit causing surface bronzing,
reducing shelf life and marketability.
Pollen removal. Thrips are attracted to most
plants in flower that produce copious quantities of
pollen, eg Asteraceae, legumes. Flowers of African
violets become covered with pollen.
Transmission of virus diseases, eg tomato
spotted wilt virus (TSWV), impatiens yellow spot
virus (INSV) and other viruses are the main cause of
crops losses. Symptoms of TSWV include stunting,
distortion and color variation in the leaves (page
286). Test kits are available for testing for TSWV.
General. The presence of TSWV does not mean
that WFT is in your crops, other thrips, eg onion
thrips (Thrips tabaci), tomato thrips (F. schultzei)
also spread TSWV. If thrips numbers are high their
feeding can damage crops regardless of whether
they have TSWV or not. Damage to native plants is
confined to pollen removal. Thrips are attracted to
white washing and pale coloured clothes.
Diagnostics.
Several species of thrips can infest flowers.
WFT is the most serious. Diagnosis is difficult
because to the naked eye most thrips found in
flowers look alike. Other thrips commonly
caught in traps in greenhouses are onion thrips
(Thrips tabaci) and tomato thrips (F. schultzei).
Thrips can be identified as thrips using a
hand lens, but it is very difficult to tell one
species from another. A qualified taxonomist is
required to identify them accurately. Thrips can
be caught on a yellow or blue sticky trap or be
shaken out of flowers onto white paper or a
sticky trap. Place in cling wrap and send it to
your nearest diagnostic service (page xiv).
Keys. There are many keys available, even ones
for identifying thrips on particular crops, eg
cotton, strawberries. There are also several Lucid
Keys, eg ID Thrips, AQIS Identification Guide –
Thysanoptera www.lucidcentral.com/
Commercial growers. If thrips are a recurring
problem on crops, growers should learn how to
distinguish one species from another using a
high power microscope. WFT has a pair of long
hairs at each corner of the thorax.
DNA finger-printing indicates that there are at
least 2 different WFT populations in Australia.
Home gardeners. Because damage by thrips
may be mistaken for damage caused by mites or
other insects, eg leafhoppers, white flies or on
some hosts, lace bugs, the main thing is for them
to identify the problem as thrips and not as
something else.
Pest cycle
There is a gradual metamorphosis (egg, larval
stages, pre-pupal, pupal and adult stage) with many
generations each year. The WFT life cycle is
mostly continuous and all stages can be found
throughout the year. Female thrips live for
4-5 weeks and insert 150-300 eggs into flower
parts and are protected from sprays. Nymphs have
a pupal stage in the soil from which adults emerge.
At 10-20 o C the length of the life cycle is
25-35 days. At 20-30 o C the life cycle is 15-25 days.
‘Overwintering’
On infested crops, stock plants, cuttings, weed
hosts, crop and weed debris. Especially critical in
spring and early summer when a major source of
WFT is likely to be greenhouse crops.
Fig. 100. WFT (Frankliniella occidentalis).
Left: Nymph. Centre: Adult (1-2 mm long).
Right Natural size.
138 Insects and allied pests - Thysanoptera (Thrips)