Plant protection chemical herbivores

Examples of compounds belonging to these subgroups are shown in Table 17.1. These nitriles release cyanide under mild conditions, and are used by plants as chemical protective agents against casual herbivores. 

This global trend is in line with the Darwinian rationale for xenobiotic metabolism, which is believed to have evolved in an animal-plant "warfare," with herbivores, adaptating to the emergence of protective chemicals (e.g., alkaloids) in plants (180). 

Each of these classes of compounds is used by plants as protection against predators. Tropical plants seem to contain more toxic alkaloids than do temperate species (Molles, 1999). Despite this, tropical herbivores remove approximately ll%-48% of leaf biomass in tropical forests, while in temperate forests they remove only about 8%. This higher level of grazing in tropical areas has apparently produced more intense selection pressure for plants with chemical defenses (see Section 6.17.1). 

Host plants play a key role in the production and use of sex pheromones by herbivorous insects through larval or adult sequestration of chemically active compounds and pheromone precursors [210]. One of the best examples of sequestration of plant chemicals by larvae and their subsequent use by adult males in sex attraction or courtship interactions is shown in Utetheisa ornatrix (Arctiidae), whose courtship pheromone derives from pyrrolizidine alkaloids (PAs) ingested at the larval stage from the host plant Crotalaria spectabilis [211]. U. omatrix larvae sequester PAs (e.g. monocrotaline) and retain the alkaloids through metamorphosis into the adult stage to provide egg protection for the next generation. 

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