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die of starvation. This type of self-defense could conceivably reduce or negate the need for using pesticides on cultivated potatoes (Gibson, 1979; Harlan, 1984).
Formerly one of the most important timber trees of western coastal Ecuador, Persea theobromifolia, also called Caryodaphnopsis theobromifolia (family Lauraceae) has been pushed to the very brink of extinction by overexploitation. When it was finally described in 1979, it was found to be a relative of the common avocado (Persea americana) and might one day prove useful as rot-resistant root graft stock for the cultivated species (Gentry and Wettach, 1986).
Many tropical plants have developed chemical defenses to deter predation by herbivorous animals. Tropical people possess a sophisticated knowledge of these plants, often using them as medicines or poisons. The calabar bean (Physostigma venenosum) was traditionally used as an ordeal poison in West Africa, and studies of the active principle of this species led to the development of methyl carbamate insecticides. World trade in daisy flowers (Chrysanthemum cinerariaefolium), the source of insecticidal pyrethrum extracts, is a multimillion dollar business (Oldfield, 1984). This plant was first discovered because of its use by African tribal peoples to control insect pests.
South American Indians use Lonchocarpus, a forest vine, as a poison to stun fish. Today we import the roots of this plant as a source of rotenone, a biodegradable pesticide. Other plants used by tribal people as fish poisons have yet to be evaluated for their potential as pesticides. Plants used to make arrow poisons or curares also bear looking into, since one such species, Chondrodendron tomentosum already provides us with d-tubocurarine—an anesthetic administered during abdominal surgery. Not only do we need to investigate the individual components used in the manufacture of the many different types of curare but we must also study the interactions among different species that are sometimes used together. In the northeast Amazon, the preparation of an arrow poison may involve the mixing of seven different species, and the Indians insist that each plant changes and amplifies the toxicity of the poison.
Yet another category of potentially useful natural pesticides are allelochemicals. These are chemicals produced by plants that inhibit the growth of other plants and of soil microorganisms. Allelochemicals include a number of different types of chemicals and may one day be used directly or serve as models for seminatural or wholly synthetic compounds (Balandrin et al., 1985).
Species that might prove useful as sources of biodegradable pesticides in the future include the following:
Piquiá (Caryocar spp.; family Caryocaraceae). One Amazonian species of Caryocar produces a compound that seems to be toxic to the dreaded leaf-cutter ant (Atta spp.). This insect is the scourge of South American agriculture, causing millions of dollars of damage each year.
Guaraná (Paullinia cupana; family Sapindaceae). This woody vine is native to central Brazil. It is grown on plantations near Manaus for use in Brazilian soft