The names of many insecticidal plants seem to come from a cookbook: mint, rosemary, thyme and clove, and they are proven safe for mankind and the environment.
Insecticidal plants (cont from last month)
However, all-natural does not always mean safe for all. Nicotine is poisonous in concentrated form, and ricin, another deadly poison, is also from a plant, Ricinis communis. Deadly nightshade is a weed but has that name for a reason.
But because botanicals break down quickly, even these risky compounds have no long-term impact in the environment, and this is a key benefit to risk exposure.
Botanicals are derived from plants in several different forms: powdered seeds, roots and flowers, chemical extracts and essential oils.
Oils such as cedar, cinnamon, citronella, citrus, clove, garlic, mint and rosemary have long been used since antiquity as insect repellents, but studies in the 1990s have demonstrated they are effective as contact insecticides and fumigants as well.
Some of these natural compounds have been present in the food system for more than 300 years, a very long history of positive usage. Not only in the flavour industry, but also in cosmetics and fragrances that are applied to the skin. There’s a great deal of human exposure already — they are ubiquitous in the environment.
Compounds target insect-only receptors
How are oils that are toxic to insects so safe for humans? It is all in the mode of action.
Peppermint oil is an example. This essential oil compound targets a nerve receptor system that only insects use. So it doesn’t have the same effect on mammals as it does on insects.
When applied to an insect, the chemistry of the essential oil compound is almost identical to the chemistry of the octopamine neurotransmitter that insects use. The botanical compound binds to the octopamine receptor (instead of the actual neurotransmitter) and shuts down the signal pathway.
The essential oils have broad-spectrum activity because insects have receptors for this neurotransmitter. Humans don’t have those same receptors in our bodies, whereas all the conventional synthetics target a nerve receptor system that humans share with insects.
(cont next month)
See this article in Tree Fruit April 2018
