Oviposition-deterring pheromone deposited on blueberry fruit by the parasitic wasp, Diachasma alloeum

Insect parasitoids are known to deposit chemical signals on utilized hosts following oviposition. It is believed that these chemical signals alert future conspecifics of an exploited and thus sub-optimal host alleviating potential suffering among brood that would otherwise compete over a limited resource. Diachasma alloeum (Muesebeck) is a braconid wasp that specifically attacks two species of fruit-parasitic flies in the genus Rhagoletis. Female wasps lay a single egg into a second or third instar fly maggot developing in blueberry, hawthorn, or apple fruit. Following oviposition, female wasps press and drag their ovipositor across the fruit surface depositing a clear liquid; this has been termed ‘excreting’ behaviour. In this report, we describe excreting behaviour in a field population of D. alloeum attacking the blueberry maggot fly, Rhagoletis mendax Curran. Subsequently, we demonstrate in a series of laboratory assays that D. alloeum females deposit a substance on blueberry fruit directly following egg laying that deters subsequent naïve females from ovipositing into the marked fruit. Marking fruit with this putative oviposition-deterring pheromone without associated egg laying was sufficient to induce the deterring effect, while oviposition alone without subsequent marking had no effect. The oviposition-deterring effect was removed by rinsing fruit with a solution of 50% ethanol in water. Spraying unmarked fruit with an ethanol-water rinsate of previouslymarked berries induced the oviposition-deterring effect. Significant deterrence of oviposition lasted up to 7 days after marking. To our knowledge, this is the first demonstration of a hymenopteran oviposition-deterring pheromone that is deposited externally on the surface of the fruit skin rather than on the surface of the parasitized larva. Female D. alloeum maximally parasitize second-instar R. mendax larvae during a short 4-5 d window. The length of