LOCAL SPECIALIZATION OF PHORETIC MITES ON SYMPATRIC CARRION BEETLE HOSTS

Wide-ranging but ecologically variable species and their close relatives provide the best opportunity to analyze the role of trade-offs in specialization. Poecilochirus carabi Can. is a holarctically distributed mite that rides between patches of carrion on the bodies of carrion beetles (Silphidae, Nicrophorus). Using behavioral tests, we demonstrated differences in local host specialization of P. carabi at two sites in Michigan, the Kellogg Biological Station (KBS) and the University of Michigan Biological Station (UMBS). At KBS, Nicrophorus orbicollis and N. tomentosus each supported a strict host specialist. These KBS "orbicollis- and tomentosus-specialists" differed significantly in both body size and a measure of dorsal setal lengths. Laboratory crosses between specialists were not successful, suggesting that the specialists are distinct species. At UMBS, in contrast, such strong specialists were not found. Mites preferred two beetle species, N. tomentosus or N. defodiens (a species not found at KBS), over all others, but moved between these two species readily. UMBS mites all resembled the KBS tomentosus-specialist in morphology. No orbicollis-specialist mites were found at UMBS, despite the high relative abundance of N. orbicollis in the beetle community. We raised mites from each site on all available hosts to test whether preference for a host species was associated with higher fitness in association with that host species. Although P. carabi were able to reproduce in association with any Nicrophorus species, mites at UMBS had the highest reproductive success on the two species that they preferred. Each KBS specialist mite was raised in association with N. tomentosus and N. orbicollis. While the tomentosus-specialist had significantly higher reproductive success in association with its preferred host species, the orbicollis-specialist showed no difference in reproductive success between the two host species. However, when the pattern of dispersal on beetles was considered, a difference in fitness between the two hosts was apparent. This difference in dispersal pattern was due to an interaction between the parental behavior of the two host species and mite generation time. We discuss the evidence for trade-offs in this system and the use of interspecific comparisons in the analysis of specialization.