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Any good garden shop should sell derris
and pyrethrum.
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There are many makes of both and mixtures
of the two are stronger than either separately, capable of killing
caterpillars and a range of pests including aphides of all types listed
on the tin or bottle.
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They are sometimes mixed with lindane and
other organo-chlorine compounds and these should be refused.
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Derris is available as a dust that is
most effective against the fleabeetle which eats holes in the leaves of
Brassica plants and radishes.
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As it began as a Malayan fish poison, do
not let it trickle into the goldfish pond.
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If you keep bees and have to spray
anything in flower, use pyrethrum in the evening and it will have
killed your aphides and be harmless by the time your bees start work in
the morning.