Host-related fitness trade-offs in a presumed generalist parasitoid, Diaeretiella rapae (Hymenoptera: Aphidiidae)

1. Host ranges of parasitoid wasps are mediated by behavioural responses to hosts and their environment (infectivity), and development in hosts (virulence). Determinants of host range were measured in Diaeretiella rapae (Hymenoptera: Aphidiidae), which has been described as a generalist that attacks more than 60 species. 2. In northern Colorado, this wasp mainly attacks two hosts: cabbage aphid (Brevicoryne brassicae) and Russian wheat aphid (Diuraphis noxia). Here, laboratory experiments are described in which D. rapae originating from these two hosts were offered several hosts for oviposition. Both infectivity and virulence were measured. 3. Infectivity included host acceptance and handling time, while virulence was measured as productivity (number of progeny), survival of immatures within hosts, development time, and sex ratio. 4. Wasps had higher productivity and survival when attacking ‘home’ hosts than ‘alternate’ hosts, and trade-offs were found by quantitative genetic analyses to be genetically determined. Sex ratio and development times also showed tradeoffs, but mainly related to the host environment in which females were reared. 5. In previous genetic studies in northern Colorado, populations were genetically subdivided on the scale of 1 km. The fitness differences described here could be strong enough to create populations adapted to different hosts, but it appears that gene flow is sufficient to prevent formation of separate lineages on the two hosts. 6. Rather than being a generalist with a broad host range, D. rapae is a serial specialist, attacking particular hosts according to availability in different seasons or in different geographical areas.