Comparing the attraction of two parasitoids to herbivore-induced volatiles of maize and its wild ancestors, the teosintes

Artificial selection of crop plants for desired traits such as increased yield and improved seed or fruit quality has been hypothesized to have had a cost for other potentially useful traits, including resistance to herbivores. Besides direct defences, such as the production of toxins, plants may also indirectly protect themselves by emitting volatile organic compounds (VOCs) that attract the natural enemies of herbivores. Parasitoid wasps are known to use these VOCs to localize hosts for their offspring. However, domestication and selective breeding of crop plants have reportedly led to the loss of such signals. The aim of this study was to identify possible differences in the attraction of parasitoid wasps by modern maize and its wild ancestors, the teosintes. In a six-arm olfactometer, we compared the capacity of teosintes and maize to attract the parasitoid wasps Cotesia marginiventris (Hymenoptera: Braconidae) and Campoletis sonorensis (Hymenoptera: Ichneumonidae). We studied the attractiveness of plants in which VOC emission was induced by the application of artificial damage and caterpillar regurgitant, as well as the attractiveness of extracts of volatiles that we collected from plants exposed to herbivory. C. sonorensis did not distinguish between the odours of maize and teosintes, whereas C. marginiventris showed a significant preference for the odours of teosintes over the odours of maize. The fact that we obtained very similar results with extracts of collected volatiles implies that we could use these extracts to identify the key compounds that are responsible for wasp attraction. Restoring and/or enhancing such key parasitoid attractants in cultivated plants could be an effective way to increase natural pest control.