Ovarian development, ovariole number, and relationship to body size in Psithyrus spp. (Hymenoptera: Apidae) in southern Alberta

Oocyte development, potential fecundity and relationship to body size were evaluated in Psithyrus species, social bee parasites of the ancestral Bombus. Oocyte size in female Psithyrus from southern Alberta increased linearly with time from emergence since hibernation in the spring and was not synchronous with the finding of a suitable host nest. None of the Psithyrus females collected from host nests in the autumn had any ovariole development. Significant differences between years in body size occurred for P. insularis (Smith) and P. suckleyi (Greene), but not for P. fernaldae (Franklin), and the number of ovarioles and oocytes differed yearly for P. insularis, but not P. fernaldae and P. suckleyi. The number of ovarioles per ovary varied within females and species and among species. The number of ovarioles in P. fernaldae (12.3 ± 2.1), P. insularis (15.7 ± 2.2), and P. suckleyi (21.3 ± 3.0) was more than in Bombus species (8). As body size of Psithyrus females increased, there was a significant increase in the number of ovarioles when all specimens over years and species were considered. Significant differences in body size and potential fecundity within and between species suggest considerable plasticity in physiology, morphology, and behaviour and differences in reproductive strategies within the genus Psithyrus.