Kairomonal Effect of Aphid Alarm Pheromones and Analogs on the Parasitoid Diaeretiella rapae

SIMPLE SUMMARY: Aphid alarm pheromones, as important semiochemicals, not only mediate the behavioral response of aphids, but can also act as kairomones to attract their natural enemies. The sesquiterpene (E)-β-farnesene (EβF), the major alarm pheromone component of most aphid species, has been shown to have a kairomonal effect on the predators of aphids, but other alarm pheromone components, especially the monoterpenes and analogs, are rarely investigated. In this study, we examined the kairomonal effects of four alarm pheromone components and two EβF analogs on the aphid parasitoid Diaeretiella rapae. We found that the blend of aphid alarm pheromone components is more preferred by the parasitoid than are the individual components. Even if individual pheromone components showed no kairomonal activity at lower concentrations, the blending of these components elicited a response. This study contributes to our understanding of the mechanisms involved in the regulation of parasitoid behaviors by kairomones and provides a promising opportunity for designing kairomones for the natural enemies of aphids to mediate aphid populations in the field. ABSTRACT: Aphid alarm pheromones, as important semiochemicals, not only mediate behavioral response of aphids, but can also act as kairomones to attract their natural enemies. The sesquiterpene (E)-β-farnesene (EβF), the major alarm pheromone component of most aphid species, has been shown to have a kairomonal effect on the predators of aphids, but other alarm pheromone components, especially the monoterpenes and analogs, are rarely investigated. Here, two EβF analogs were successfully synthesized via the nucleophilic substitution reaction, and we then examined the kairomonal effects of four alarm pheromone components and two EβF analogs on the aphid parasitoid, Diaeretiella rapae. In olfactory bioassays, D. rapae females generally showed no significant behavioral response to these alarm pheromone components and analogs under low concentrations (0.1 μg/μL). Nevertheless, their olfactory response to these compounds gradually enhanced with increasing concentrations. Among the four pheromone components, EβF showed the highest attractive activity, but the parasitoid preferred blends over single compounds. Moreover, the response time decreased as the concentration increased. We confirmed the kairomonal effect of monoterpene alarm pheromone components and their blends, in addition to EβF, on the natural enemies of aphids. This is the first report that the blend of alarm pheromone components and their analogs has a stronger kairomonal effect than do the single components on the natural enemies of aphids. This study contributes to our understanding of the mechanisms involved in the regulation of parasitoid behaviors by kairomones and provides a promising opportunity for designing kairomones for the aphid parasitoid to mediate aphid populations in the field.