Dynamics of a Host-Cleptoparasite Relationship: Holcopasites ruthae as a Parasite of Calliopsis pugionis (Hymenoptera: Anthophoridae, Andrenidae)

Holcopasites ruthae Cooper is a cleptoparasite of Calliopsis pugionis Cockerell. At five sites, parasitism rate of host cells varied from 0 to over 30%, with an overall parasitism rate of 6.6% when data from all sites were combined. Significant influences on parasitism rate included site, nest density, and date of excavation, and density had a significant interaction with date. Together these factors account for 97% of the variance in parasitism rate. The sex ratio of the host bee was significantly influenced by parasitism rate, host nest density, and date, which together explain 88% of the variance in host sex ratio. Possible explanations for the dynamics of this host-parasite relationship are discussed. Host nest density may influence parasitism via predator confusion or selfish herding. The influence of parasitism on sex ratio is probably not directly a result of differential parasitism reducing the number of surviving females. However, both equal vulnerability by sex and differential vulnerability resulting from differences in the time spent by females provisioning male and female cells remain possibilities: the latter could indirectly influence sex ratio through facultative behavioral changes in the host bee.