Body size cascades from Neodiprion sawfly hosts to Perilampid parasitoids: An investigation of sequential divergence

Ecological speciation drives diversity in plant-eating insect systems, but it may also generate parallel specialization in parasitoid associates in a cascading effect of "sequential divergence." However, sequential divergence has never been successfully identified in parasitoids of externally feeding insects. To address this gap, we consider the Pinus-Neodiprion-Perilampus neodiprioni system, where plant-associated, externally feeding Neodiprion demonstrate ecological divergence, as supported by body size as a magic trait in sympatric species. It seems possible that this body size differentiation may cascade to Neodiprion's parasitoids via sequential divergence. Therefore, we asked, "Does P. neodiprioni exhibit host-specific divergence in body size that parallels patterns in Neodiprion hosts?" Further, "Does P. neodiprioni exhibit host-related variation in the abdominal region or sexually dimorphic features, supporting reproductive barriers?" Using by-catch samples, we measured body size of P. neodiprioni, reared from Neodiprion lecontei and Neodiprion pinetum hosts. We analyzed body size variation as predicted by host species, parasitoid sex, host sex, host cocoon size, latitude, and their interactions. In addition, we used mixed models to analyze the effects of parasitoid sex and Neodiprion host on three size-based facial features. We found that P. neodiprioni demonstrates host-associated body size variation that mirrors size differences in their Neodiprion hosts. In addition, host-associated size variation occurs in the abdominal region and two sexually dimorphic characters of P. neodiprioni, supporting possible reproductive isolation. Together, these patterns suggest that host magic traits may drive not only within-species ecological divergence, but they can facilitate parasitoids' subsequent morphological differentiation and possible sequential divergence.