Toxic Plant Substances In Insects

In their constant evolutionary struggle against insects that attack them, plants have evolved many kinds of toxins to defend themselves. While most plant-eating insects have to avoid these toxins, some insects, in turn, have evolved the ability to overcome them, and some even to store them in their bodies as their own defence against predators (other insects, arthropods or higher animals). Butterflies and moths, followed by beetles, stand out prominently as insect sequesterers of plant chemicals. Day-flying, brightly coloured butterflies and moths would otherwise be particularly attractive prey. The plant substances may be stored unchanged, or changed slightly or metabolized so much that their connection with the plant may not be immediately apparent. Insects use a variety of enzymes to degrade or modify the plant products, including oxidases, reductases, hydrolases, esterases and transferases. The insect may be able to absorb selectively certain compounds and reject others or control the level of toxin they store. The ingestion of toxic compounds for protection is sometimes called pharmacophagy, and the insects are called pharmacophagous species.

The earlier chapters of this book have been organized by fundamental bio synthetic route. No system can be totally logical. It might be argued that the section on sterols and the carotenes in Chapter 7 and the whole of Chapter 8 on aromatic compounds should be included here, but all insects modify dietary sterols to make moulting hormones and all animals modify the aromatic amino-acids to make important compounds. Here we are considering small groups of insects and essentially compounds normally toxic to insects. It is, moreover, a compilation, and cannot be expected to cover all known examples.

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