Sympatric speciation mechanisms are often invoked yet they are still not fully understood. Host-associated differentiation (HAD) has been proposed as a potential mechanism explaining insect diversity. However, it is still unclear how common HAD is as well as the factors that could promote it. Most of the literature on HAD deals with herbivorous insects. Thus, not much is known about HAD at higher trophic levels such as predators and parasitoids. One of the characteristics that has been postulated as predisposing insects to HAD is parthenogenesis. In this study, we used AFLP markers to determine if HAD is present on two parthenogenetic egg parasitoids attacking the same herbivore species on two related tree species. The egg parasitoids species are Ooencyrtus pityocampae Mercet (Hymenoptera: Encyrtidae) and Baryscapus servadeii Domenichini (Hymenoptera: Eulophidae). The former is a generalist and the latter a specialist parasitoid of one of the main European pine forest pests, the pine processionary moth, Thaumetopoea pityocampa Denis & Schiffermüller (Lepidoptera: Notodontidae). The study was conducted in natural forest of two of the main T. pityocampa host trees in Italy (i.e. Pinus nigra and Pinus sylvestris). 

Results/Conclusions

In both parasitoid species individuals were clustered in two distinct genetic groups which could related to the kind of reproduction experienced by this individuals (i.e., strict parthenogenesis versus parthenogenesis with occasional sexual reproduction). Within each of the two clusters found in each parasitoid species, there was further sub-structure. Partial HAD was detected only in the specialist B. servadeii, which populations were not geographically structured. In contrast, population structure was found in the generalist O. pityocampae but not evidence of HAD was detected. We suggest that the differences found between these two parasitoid species may respond to differences in these parasitoids’ host range (i.e., specialist versus generalist). In conclusion, in this study we suggest that occurrence of HAD in parasitoids could not always be the result of host insect differentiation, but it may be due to the direct interaction of  parasitoids with their host plants. Further, we contribute with evidence for parthenogenesis as one of the factors that could promote HAD.  Finally, the kind of parthenogenesis (defined by the frequency of sexual reproduction or by its complete absence) seems also to play a role in shaping the genetic population structure of parasitoid species, together with the different strategies adopted by generalist versus specialists to locate their insect hosts.