PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Plague thrips
A native thrips, which is a key pest in NSW,
Vic, SA and WA, minor pest in NT.
Scientific name
Thrips imaginis (Order Thysanoptera).
Host range
Wide range of plants.
Ornamentals, eg carnation, dahlia, marigold, roses.
Native flowers, eg Acacia victoriae, Atriplex
suberecta, Eucalyptus tetragona. Fruit, eg apple,
citrus, stone fruits, grape, raspberry, strawberry.
Vegetables, eg cucurbits. Pasture, eg grasses.
Field crops, eg lucerne. Weeds, eg capeweed.
Thrips can invade white washing on clotheslines.
Description & damage
Damage is caused by nymphs and adults rasping
the plant surface and sucking sap and by the egglaying
of female thrips. Plague thrips is mainly a
blossom feeder, but may attack young foliage.
Other species of thrips also feed in blossoms, eg at
least 2 species are found in rose flowers.
Adult females are narrow-bodied, light brown
or gray and about 1-2 mm long. Males are smaller
and yellow. Both sexes have 2 pairs of narrow
delicate wings, fringed with long hairs, which lie
along the back when not in use. 1 st stage nymphs
are yellow with red eyes, 2 nd stage nymphs change
to orange-red. 2mm long. Nymphs are similar in
shape, pale to orange-yellow, wingless and smaller.
Damage.
Flowers. Nymphs usually feed on stamens and
pistils but can also feed on the petals, adults mainly
feed on petals. If flowers are heavily infested adults
can be easily seen with the naked eye. Thrips feeding
causes anthers, petals and pistil to brown,
shrivel and fall prematurely. Where thrips enter
unopened blossoms, normal opening may be
adversely affected. Petals of infested roses brown,
dark drops of faeces disfigure light-coloured blooms.
Leaves. Egg laying in young rose tissue may
cause the tissue around the eggs to die and fall
out, leaving small irregularly-shaped ‘shotholes’.
Damage is barely detectable when leaves mature.
Young leaves of some hosts, eg citrus, stone
fruit may become spotted yellow and scarred
with tiny blisters due to egg laying.
Fruit. Apples, pears, peaches and plums may be
heavily infested; injured blossoms turn brown
and fall prematurely. In apples, egg laying and
feeding by thrips causes blossoms to wither and
reduces fruit set. Note reduced fruit setting in
apples may also be caused by late frosts, unusual
heat waves during blossoming, a dry spell before
flowering and an absence of bees as well as
thrips injury or a combination of any of these.
Diagnostics. Thrips can be detected by shaking
flowers upside down over a sheet of white paper or
handkerchief. Alternatively they can be made to run
around the side of flowers by gently breathing warm
air into the flower.
It can be difficult to distinguish one species from
another. Home gardeners need to identify the
problem only as thrips.
As other thrips species also feed in flowers, eg
WFT (Frankliniella occidentalis), onion thrips
(F. schultzii), commercial growers need to
identify the species of thrips, eg plague thrips,
western flower thrips (WFT) (page 138).
Lucid key Thrips ID key www.lucidcentral.com/
Pest cycle
There is a gradual metamorphosis (egg, larval
stages (2), pre-pupal, pupal and adult stage) with at
least 12 generations each year. The adult female
inserts minute, transparent eggs in the tissue of all
parts of the flower, eg petals, sepals, blossom stems,
stamens, pistils, calyx cups, and in the young leaves
adjacent to the blossoms. As many as 150 eggs
have been found in a single blossom stalk. The tiny
nymphs that emerge cluster mainly inside blooms
where they mostly feed on the pistils, stamens and
petals, but may also feed on the young leaves.
When fully grown they crawl down the plant, enter
the soil to a depth of about 5 cm, change to prepupae
and then pupae. The emerging females are
yellow but they begin to turn brown in 2-3 days.
The life cycle from egg to adult, varies from about
10-30 days depending on temperature.
136 Insects and allied pests - Thysanoptera (Thrips)
Fig. 99. Plague thrips (Thrips imaginis).
PhotoNSW Dept of Industry and Investment (E.H.Zeck).
Enlarged x35
1. Egg
2. 1 st stage nymph
3. 2 nd stage nymph
4. Pre-pupa
5. Pupa
6. Adult winged thrips
Actual size
7. Thrips on petals