The generalist parasitoid Pteromalus apum (Hymenoptera: Pteromalidae) can be found ovipositing in the Glanville fritillary butterfly, Melitaea cinxia, and in the Heath fritillary butterfly, Melitaea athalia, (both Lepidoptera: Nymphalidae) in the Åland Islands (located between south-western Finland and eastern Sweden). In contrast, Hyposoter horticola (Hymenoptera: Ichneumonidae) is a specialist parasitoid of M. cinxia. In this study we investigated the geographic population structure of H. horticola (the specialist) and P. apum (the generalist) in the Åland Islands using AFLP markers. We hypothesized that in the specialist parasitoid, the need to search for specific insect hosts will generate a panmictic population. In contrast, the generalist parasitoid will have a more structured population as a result of its ability to oviposit in more than one host species. Ovipositing on more than one insect host may allow the generalist parasitoid to remain in more localized areas than the specialist parasitoid.
Molecular markers indicate that Hyposoter horticola in the Åland Islands lacks geographically based population structure. Behavioral observations suggest that the specialist, H. horticola, has a relatively high rate of dispersal when compared with the generalist P. apum. This difference in dispersal may be explained by insect host availability and may translate in differences in the level of gene flow in generalist versus specialist parasitoids. The existence of population structure in P.apum was assessed using the same molecular markers used for H. horticola. Understanding the nature of population structure in parasitoids is crucial for designing conservation and biocontrol strategies.