The closely related sulfur butterflies, Colias eurytheme and C. philodice, coexist over much of North America. Although they are capable of hybridizing, the females of each species normally prevent this by evaluating both visual and chemical (i.e., pheromonal) information provided by courting males. Taylor and colleagues now have evidence that the male pheromones within a species are quite variable both in quantity and quality and that unmated females often reject some courting males of their own species while accepting others. Preliminary data on C. eurytheme generally support the hypothesis that an otherwise suitable courting male may be rejected or accepted by a female based on the qualities or quantities of the chemicals making up its pheromone. Although a vast amount of work has been done on the identification of pheromones of many insect species, relatively little is known about the extent or kind of within-species pheromone variation, and practically nothing is known about what causes this variation or its effects on mating behavior. Colias eurytheme and C. philodice are exceptionally suited to serve as model species for examining the role of within-species variation in insect pheromones. The courtship system is well understood both behaviorally and genetically, the pheromone components have been identified, and these species can be mass-reared in the laboratory making them susceptible to genetic manipulation and behavioral experimentation. Modern methods of pest-management often include the use of the pest-species' pheromone either for sampling or for actual attempts at suppression. The result of this study should make clear that pheromone variation can be considerable and cannot always be ignored.
