Two different life-history strategies determine the competitive outcome between Dirhinus giffardii (Chalcididae) and Pachycrepoideus vindemmiae (Pteromalidae), ectoparasitoids of cyclorrhaphous Diptera.

Dirhinus giffardii Silvestri and Pachycrepoideus vindemmiae Rondani are solitary parasitoids attacking puparia of many cyclorrhaphous flies. They are not typical ectoparasitoids, as they feed on host pupae within puparia that develop from the exoskeleton of host larvae. Dirhinus giffardii did not kill its host until the parasitoid egg developed into a larva, while P. vindemmiae permanently paralysed its host at the time of oviposition. As a result, ovipositing into a young host puparium (< 1 day old) in which the host pupa has not yet fully formed resulted in complete death of offspring in P. vindemmiae, but D. giffardii, although suffering higher mortality than in older host puparia, still showed a level of successful development. In a choice experiment, both parasitoids preferred to attack 2- to 3-day-old puparia in which the host pupae had fully formed, rather than 1-day-old host puparia. Pachycrepoideus vindemmiae always prevailed in competition because it injected venom that not only paralysed the host, but also caused the death of D. giffardii larvae in multi-parasitized hosts. Dirhinus giffardii preferred to attack unparasitized hosts rather than hosts previously parasitized by P. vindemmiae, while P. vindemmiae did not show a preference between unparasitized hosts and hosts previously parasitized by D. giffardii.