Variable diets and changing taste in plant-insect relationships

The host ranges of phytophagous insects are determined to a large degree by plant chemistry. Specialist insects are often closely associated with plants that produce characteristic chemicals, which may act as attractants or stimulants to aid in finding or recognizing a host. Generalist insects are generally believed to rely on the presence of repellents or deterrents to ensure avoidance of unsuitable plants. However, the chemistry of any plant can be highly variable, as a result of growth characteristics, genetic variation, or environmental factors. Such variable chemistry may provide windows of opportunity for nonadapted insects to utilize a plant or for a plant to become resistant to a normally adapted herbivore. Differences in insect responses to plant constituents may also result from genetic variation or environmental factors. In particular, dietary experience has been found to influence the ability of insects to taste plant chemicals that may serve as signals of suitability or unsuitability. Certain dietary constituents appear to suppress the development of taste sensitivity to deterrents in an insect, whereas the presence of specific stimulants in the diet may result in the development of dependence on these compounds. These findings further emphasize the fact that the dynamics of plant biochemistry along with plasticity in the sensory system of insects might be expected to play a major role in the evolution of new plant–insect relationships.