Chemical Espionage, Anti-Aphrodisiacs and Hitchhiking…all in a Day’s Work for a Parasitoid Wasp!
Evolution has resulted in a remarkable array of reproductive strategies in the animal kingdom. After all, if one is unsuccessful in passing on one’s genetic blueprints there was really not much point in being alive in the first place. Several invertebrate organisms employ a ‘polyandrous’ sexual system, wherein a female mates with several males. It is in the females’ best interest to shop around for the best sperm out there; however, it’s in a males’ best interest to prevent a female from further mating attempts once he has made his genetic deposit. Cabbage white butterfly (Pieris sp.) males solve this problem by depositing an anti-aphrodesiac chemical along with their sperm. The anti-aphrodesiac functions to prevent any further mating by the females, hence ensuring the paternity of her previous partner. Sounds so simple doesn’t it? Kind of like a little butterfly-like chastity belt.
Everything would be just peachy (according to the male cabbage butterfly) if it weren’t for the presence of Trichogramma parasitoid wasps. These parasitoids deposit their eggs within the fertilized eggs of the butterflies, and the larvae of the former grow up by eating (and killing) the fertilized eggs of their butterfly hosts. Parasitoids themselves have evolved a remarkable array of ways in which to find their hosts. Recently, it has been discovered that the anti-aphrodesiac pheromone benzyl cyanide is utilized by the wasps to find the butterflies. In other words, the chemical signal that the male butterflies use to ensure paternity over a clutch of eggs is instead providing a clear signal to parasitoids that ultimately kill the entire clutch. After finding the ‘marked’ females, parasitoids hitch a ride with them to their egg laying sites. This remarkable strategy is officiallly termed ‘espionage and ride behavior’ by the authors of the study.
The astonishing abilities of the parasitoids do not end with the simple recognition of benzyl cyanide. Laboratory experiments revealed that the wasps do not respond to the presence of this compound unless it is accompanied by the ‘complete mated-female odor blend’ (soon to be available at a consmetics counter near you J). In addition, the two parasitoid species utilized in this study exhibit a difference in the acquisition of espionage behavior. In T. brassicae, which specializes on parasitizing the cabbage butterfly, the espionage and ride behavior is innate. However, in T. evanescens, which is a generalist opportunistic parasitoid, the behavior is learned.
Overall, the findings of this study indicate that the cabbage butterflies are under strong selective pressure to minimize the use of an anti-aphrodesiac signal. Looks like the males will have to rely on their genetic superiority alone in order to successfully sire a clutch of eggs…the parasitoids have played the anti-aphrodesiac trump card.
Huigens, M., Woelke, J., Pashalidou, F., Bukovinszky, T., Smid, H., & Fatouros, N. (2010). Chemical espionage on species-specific butterfly anti-aphrodisiacs by hitchhiking Trichogramma wasps Behavioral Ecology, 21 (3), 470-478 DOI: 10.1093/beheco/arq007