Observations of parasitism and aggression by female Enoggera nassaui, an egg parasitoid of the eucalypt pest, Paropsis charybdis

Background: Female parasitoids that display aggressive behaviour towards conspecifics in defence of host resources must make a trade-off between time spent on aggression and time spent on parasitism. Such a trade-off not only affects the fitness of individual females but also may affect the overall rate of parasitism in the host population. Enoggera nassaui is an egg parasitoid of the eucalypt pest, Paropsis charybdis, with females that defend batches of host eggs from conspecific females by chasing, pushing and sometimes biting their opponent. Methods: The behaviour of female E. nassaui was observed when alone (solo) and in pairs (owner-intruder) in response to a single batch of P. charybdis eggs. The first female in a pair to contact the host eggs was designated the owner and the second female the intruder. Both parasitism-related behaviour for solo and paired females and aggressive interactions between paired females were described from direct observation. Parasitism rates for solo and paired females were also measured. Results: Solo females had a consistent pattern of parasitism-related behaviour, spending 80 % of their time in contact with the host eggs. Paired females showed a similar pattern of parasitism-related behaviour when not interrupted by their competitor. Owners spent 6 % and intruders spent 17 % of their time on aggressive interactions. Intruders completed parasitism faster than solo females because intruders reduced the duration of specific parasitism-related behaviours. Yet from the host’s perspective, parasitism by solo females (69 %) was similar to that achieved by both owners and intruders together (66 %). Conclusions: By completing parasitism faster in the presence of a competitor, intruder females compensated to some extent for time spent on aggressive interactions. If E. nassaui has a similar response to intraspecific competitors under natural conditions, then aggression between conspecific females is unlikely to harm pest suppression by these parasitoids.