Department of Entomology and Division of Neurobiology, University of Arizona, Tucson, AZ 85721, USA.
Insect behavior is affected by secondary compounds in plants, either directly through the chemical senses or indirectly via post-ingestive effects. Behavioral avoidance of a plant due to its secondary compounds is not necessarily an adaptation to avoid toxicity. Many of the compounds are not toxic at naturally occurring concentrations, and may even be phagostimulatory, especially to polyphagous species. Further, the gustatory receptors responding to secondary compounds are usually sensitive to structurally diverse chemicals, including many to which the species or its ancestors cannot have been exposed. Receptor molecules for such compounds in cell membranes are much more ancient than insects so it should not be supposed that their occurrence is necessarily a specific insect adaptation. Some secondary compounds are toxic to herbivorous insects but are not tasted. Insects may nevertheless avoid such compounds because the neurons that respond to nutrient compounds inducing feeding are inhibited. The taste sensitivity of insects to secondary compounds makes it possible for the insects to use them as indicators of plant quality. This is a learned response, facilitating the ability of at least some herbivorous insects to balance their nutrient intake and avoid plants containing unusable compounds. On the other hand, polyphagous species may habituate to the taste of chemicals. This behavioral response allows time for the insect’s detoxification mechanisms to be mobilized (induced) so that foods initially rejected can be used to provide adequate nutrient resources. Finally, the use of secondary chemicals as sign stimuli has facilitated host-plant specialization by many insect species.