The case against the widespread use of chemical pesticides in gardens is overwhelming from both an environmental point of view, and as an effective long-term strategy of pest control. Indeed, the approach that is generally accepted by most landscaping professionals today is known as Integrated Pest Management (IPM), which advocates the application of pesticides only when no realistic alternative exists.

The antipathy to poisons and an acceptance of an "organic" approach to weed and pest management should not be confused however with an extremist rejection of synthetic pesticides under any circumstances. There are such gardeners of course, and they are perfectly entitled to their opinion, especially when one considers that pesticides are tested on laboratory animals before entering the marketplace.

There may be a strong moral case for the outright rejection of chemical poisons, but does that coincide with a strong practical case for such a position? Here are three examples that show in that in certain circumstances, the use of chemical pesticides, including weed killers, is both necessary and justified.

*The first case concerns the Pine Processing Moth, (Thaumetopoea wilkinsoni) from the Eastern Mediterranean. This insect builds its silky nest on the tips of pine trees in the autumn. At the onset of spring, the caterpillars march down the tree in a strictly regimented fashion, before burying themselves in the ground, to emerge as adults at the end of the summer.

The moth is less of a problem to the pine trees themselves, at least to mature specimens, but extremely unpleasant to people. The caterpillar's tiny hairs, are highly toxic, and are liable to cause severe skin and eye infections. In rare cases, they have caused blindness. While there are a number of non-poisonous means of control, such as spraying BT from the air, (the bacteria that are harmless to animals) this is clearly impractical in urban situations. The solution commonly applied, is for professional exterminators to drill some holes into the tree's bark, and then inject a systemic poison into the tree.

*The second example shows how the temporary use of a pesticide has saved forestation projects from collapse. Stands of Cedar are liable to be attacked by a deadly aphid known as Cinara. While the population levels of this pest are controlled in natural eco --systems by a parasitic wasp called Pauesia; the Cedars in planted forests are highly vulnerable to the aphid, in the absence of this natural enemy.

This is what happened in Israel, where Cinara aphids were decimating Atlantic Cedar plantations (Cedrus atlantica). It was decided to introduce the species of Pauesia that parasites on the aphid, as a means of biological pest control, but the wasp had to be imported from Morocco, thereby entailing long quarantine procedures. In the meantime, a systemic pesticide was applied to save the precious cedars, until the Pauesia wasp could be safely introduced.

*Finally, let's look at invasive plants that "escape" from fields or parks and gardens, and establish themselves in natural areas. This problem of "invasive alien species" is an ecological question of the first order, because the invasive plants are liable to eliminate the natural flora, and by so doing, devastate the fauna (from bacteria to reptiles and mammals) which associates with the vegetation.

In many places, it has been found that the only way of getting rid of the invasive species is to fell them and then paint the stumps with a suitable herbicide. In other words, chemical poisons are used in order allow for the reclamation and re-establishment of natural eco-systems.

My name is Jonathan Ya'akobi. I've been gardening in a professional capacity since 1984. I am the former head gardener of the Jerusalem Botanical Garden, but now concentrate on building gardens for private home owners. I also teach horticulture to students on training courses. I'd love to help you get the very best from your garden, so you're welcome to visit me on http://www.dryclimategardening.com or contact me at jonathan@dryclimategardening.com Contact the author, Jonathan Ya'akobi , at Jonathan@dryclimategardening.com