Some plants contain large amounts of the juvenile hormone of predatory insects, causing them to remain in the larval stage and ultimately to die without reproducing. Other plants do the opposite, producing substances which overcome the effects of juvenile hormones, with the result that the insects skip some vital larval stages and become adults too early. Yet other plants contain compounds which interfere with the moulting process. A similar technique is used to deter mammalian herbivores by a group of plants containing substances which imitate the effects of oestrogen , thus lowering the mammals' fertility rates. Only a few compounds from rainforest plants have been studied for their useful potential, but of those that have, the results have been impressive.
Plants produce fungicides , insect repellents, and also pesticides to deter predators.
Some have been found which, unlike many man-made chemicals, are effective against insects but harmless to mammals. One of the most obvious examples, rubber, evolved by the plant to protect itself, is of great economic importance to many tropical countries and has never been matched by man-made substances. Defensive chemicals are the active ingredients in practically all medicines that come from plants. These medicines currently make up forty per cent of the drugs prescribed in the West. The compounds evolved by rainforest plants could, by careful breeding and genetic manipulation, be used to strengthen food plants against disease and predators in areas of the world where people are struggling to grow food.