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Thrips are small
cylindrical insects known as thunder flies, 3-4mm long, the adults
are winged. More than 150 species occur on plants in the UK. Colour
varies from white through to black. They are difficult to spot on
chrysanthemum foliage. By the time their feeding marks become
apparent on the foliage there will be quite a colony present. They
are much more difficult to control than aphids because many species
like the Western Flower Thrips, Frankliniella occidentalis
have become resistant to the pesticides available to amateur
gardeners, and they can also pupate in the soil and re-infect plants
when they become adults. The most widely known Thrips is Onion
Thrips - Thrips tabaci. |
thrips tabaci |
Impact of Thrips
The
method of operation is such that the mouthparts are driven into leaves
and petals, the head then rocks backwards and forwards to scrape the
tissue. The resulting semi-liquid mass can then be sucked out by
putting the mouthparts together to act as a drinking straw. The result
is a white patch on the leaf or petal. Brownish blobs of thrips
excreta subsequently turn black and give the white patch a
characteristic speckled appearance. There is often distortion to plant
growth associated with thrips feeding on young growth but thrips do
not feed on plant veins, an important point because this means that
they are not controlled by insecticides acting systemically.
They are the vector for the Tomato Spotted Wilt Virus which can affect
over 250 different plants in the UK. Therefore it is not just the
danger of spreading virus from chrysanthemum to chrysanthemum but
common plants like Impatiens, petunias, tomatoes and cucumbers can
carry the disease.
The symptoms of the
virus disease are stunting, distortion and leaf mottling, some
varieties being so susceptible that they die. Chrysanthemum cultivars
also vary in their susceptibility to direct attack, some having brown,
shrivelled flowers and extensively “silvered” foliage.
The virus has different symptoms on
different plants so it is hard to spot. Thrips like flowers of all
kinds, chrysanthemums are no exception. A female Thrips lives for
about a month and during this time lays about 100 eggs, quite often
these are inserted into plant tissue with its ovipositor. They have a
toxic affect on the plant and especially the petals, so it's not just
the feeding damage but petals distort badly from the toxins during a
severe attack.
Odd Thrips arriving on plants just
before the blooms mature can leave bleached marks on the tips of one
or two petals. Thrips thrive in dry hot conditions; periods of rain
will help to keep them under control. There is a biological control, a
predatory mite Amblyeius cucumeris, but it is only
possible to use this when the plants are in the greenhouse.
Control
Bifenthrin will often control the common Thrips but Western Flower
Thrips is a different matter.
Commercial products containing -
abamectin, teflubenzuron and thiacloprid have proven to give good
control.
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