Organic gardeners prefer not to use any kind of spray to control pests, relying instead on resistant plant varieties and good cultivation, but there are certain times when this approach is just not enough. Organic gardeners then have to resort to something more drastic – and there are in fact several pesticides that they can use. Here are four of the most effective:
Derris powder
This is derived from the roots of the derris plant and is extremely effective against plant lice, blackfly and other aphids, as well as flea beetles, raspberry beetles and sawfly larvae. It is harmful to some beneficial insects, such as ladybugs and lacewings, though not to bees.
Pyrethrum
An extract of the daisy-like pyrethrum flower, pyrethrum is harmful to all aphids – and many beneficial insects, although its effect is not long lasting. Use it in the evening, when most insects are not flying around.
Rapeseed oil
A quick spray of this oil will control aphids, greenhouse and brassica whitefly, thrips, scale insects and red spider mites by suffocating them. Seedlings and some plants, such as fuchsias and begonias, are damaged by rapeseed oil, so it should never be sprayed in areas where these are growing.
BT (Bacillus Thuringiensis)
These are bacterial spores that produce a protein which is toxic to some insects. BT, usually applied as a drench, kills caterpillars by paralyzing their mouthparts and gut. It is often used as a last, organic resort to control cabbage-white caterpillars. It should not be used near the host plants of non-harmful butterfly larvae such as nettles.
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When good cultivation is not enough to keep pests away, organic gardeners can resort to organic pesticides.
