Organic Pesticides
Answered by: Conrad Richter
Question from: Amanda
Posted on: October 12, 1998

I have been having problems trying to get rid of the pests on my herbs. I went in search of organic pesticides and the feedback was they are not at all effective but I really wanted all my herbs to be organic and pest-free. Do you have Derris elliptica which I have heard quite effective against red spider mites, whitefly and etc.?

Derris is a source of the organic pesticide, rotenone. Rotenone is available from many garden centres in spray and dust forms. The seeds of the herb, tephrosia, also contain rotenone, and may be used to make sprays.

Controlling pests with organic pesticides can be challenging, but not impossible. The main requirements are persistence and thoroughness and vigilance.

A key point about most organic pesticides is that they have little residual action. That means that soon after they are applied to plants, they lose their effectiveness. Unlike some chemical pesticides they do not diffuse throughout plant tissue or otherwise coat the plants with long-lasting films that kill every time an insect bites. Organic pesticides, as a rule, are ‘contact’ pesticides that need to get slathered on the target pests otherwise they will not kill.

Because of this fundamental feature, organic pesticides need to be applied thoroughly and frequently in order to kill. We often recommend dipping potted herbs in a pail full of insecticide solution (such as insecticidal soap). This ensures that the insecticide comes in contact with every above-ground part of the plant, including, especially, the undersides of the leaves. This procedure is very effective against aphids, spider mites, and even whitefly larvae.

Still, because even the dipping technique misses tiny crevices along the stem and amongst leaves, pests can persist. Eggs, too, escape the killing liquid. For this reason, it is necessary to reapply every few days until the pest problem is cleared up. We recommend twice a week, for 3-4 weeks at least. We suggest that the treatment continue at least 2 weeks past the point that pests are no longer visible.

For plants in the garden, organic sprays are much less sure in their kill, precisely because it is impossible to apply the spray thoroughly enough to cover every leaf surface. In this case, organic sprays are sill useful, but more in a delaying role where one tries to slow the population growth of pests in the hopes that natural predators can takeover and bring balance to the garden. Introducing natural predators to the garden is an option you could consider if they are available from insectaries in your area.

Natural predators may be your only natural recourse in the case of pests such as scale and mealy bugs. These are very difficult to control using pesticides, and sometimes the only solution is to throw the host plant out when the infestation is severe.

We have found that monitoring our plants regularly is the best single thing we can do to help keep plants clean of pests. If infestations are caught early, it is often possible to save a plant. It is important to be able to recognize the pests and the damage that they do.

An excellent book is "Natural Insect Control: The Ecological Gardener’s Guide to Foiling Pests" (available from Richters). This has colour pictures of the insects and good suggestions on how to control them.

There is no doubt that organic pest controls require more effort. The attraction of chemical pesticides is really borne in laziness. Gardeners and horticulturists who dismiss the organic methods are really speaking from an older mindset that thinks that one spray should get rid of everything that troubles them and their plants. Organic gardeners have come to realize that pests and plants (and humans) are part of a balancing act no one lifeform has exclusive control of.

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