Plant
Doctor Archive
Plague
of thrips
 A
friend of mine says my plants have thrips! What are these and how
can I get rid of them?
 November
is the height of the thrips season. And if you think you´re
never likely to meet a thrips, look again (closely) next time you´re
walking in your garden. The chances are that you´ll find tiny
dark insects on your light-coloured flowers. And don´t become
complacent because you only see a few of these pests. There are
bound to be many more you haven´t noticed, because thrips
always come in huge numbers!
It´s significant
that there is no such word as "thrip". Whether we´re
talking one, or several thousand, the word is thrips, with a definite
"s" on the end. The very nature of thrips, however, is
that we almost never encounter just one. Thrips always seem to come
in huge numbers and, as spring weather gradually warms up, their
numbers increase even further.
The thrips that are most
noticeable at this time of the year are called plague thrips, and,
just like an old-fashioned infectious disease, they can multiply
rapidly when conditions are to their liking. Female thrips lay lots
more eggs when weather is warm, and these eggs hatch in just a few
days.
Plague thrips are particularly
attracted to light-coloured flowers but they often get confused
about what exactly is a flower. They sometime mistake white nappies
— or clothes that are pegged on the line — for big white
blooms. And babies tend not to appreciate little insects infesting
their nappies!
Thrips can cause problems
for flowers that go on to produce fruit or vegetables by feeding
on the pollen, causing problems with fruit set. They also infest
popular garden ornamentals, such as gardenias, rhododendrons and
azaleas, and they share our enthusiasm for roses.
Thrips can really spoil
a flower display. Affected petals turn brown or silver and become
spotted with dark blobs of excrement. For the novice gardener, these
symptoms can be really puzzling. The insects are difficult to see
and so it´s hard to pick the main culprit. And, without identifying
the pest, it´s even harder to fix the problem. However, if
you sit a flower face down on a piece of white paper for a few minutes,
the paper will soon become speckled with the tiny insects.
One way of reducing or
discouraging a population of thrips is by watering over leaves or
flowers. Like many other sucking insects, thrips prefer to be in
dry conditions.
But watering doesn´t
help much when the insects are protected inside the layers of a
double flower. There they can stay dry and warm, even if it´s
quite wet on the outside of the petals.
A very effective way
to control thrips on a range of flowers and ornamentals is by spraying
with the synthetic pyrethroid, Baythroid. Baythroid is available
in a ready-to-use aerosol formulation or as an economical concentrate.
It is of low toxicity to mammals, and its active constituent is
effective at very low rates. As well as being a fast-acting contact
insecticide, Baythroid has another important quality — it has
a flushing action that tends to encourage insects to move on.
Other insecticides to
use are Maldison, Mavrik or Orthene. For organic control use fatty
acids or Neem oil.
 Advice
by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor
of Resource Management.
Reproduced
with permission from NZOOM Home and Garden content,
from the previous
website of 
The views expressed here are not necessarily those of the RNZIH
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