Mate Discrimination in a Pseudogamous Bark Beetle (Coleoptera: Scolytidae): Male Ips acuminatus Prefer Sexual to Clonal Females

Abstract

Pseudogamous females reproduce clonally but require sperm; pseudogamous and sexual females of the bark beetle lps acuminatus coexist in apparently stable proportions in southwestern Norwegian populations. In lps bark beetles, females must court males to gain access to breeding sites. We studied courtship success of sexual and clonal females in simple discrimination experiments. Sexual females were more successful in courtship than clonal females, and clones from different localities differed in courtship success, one foreign clone was rejected by all males, while a second foreign clone was rejected only slightly more often than were sexual females. Electrophoretic analysis verified that the parthenogenetic lineages used were different clones. Clonal mating success was also dependent upon density (of females), both in a laboratory experiment and in field data. In the laboratory, more clonal females were inseminated at a high female density than at a low one. Relative success of clonal and sexual females was also affected by male experience. Contrary to the expectations of models for the maintenance of sperm-dependent parthenogenesis, our experiments and observations suggest that the relative success of sexual females might be positively, rather than negatively, frequency dependent. If this proves generally true for mixed lps acuminatus populations, then previous models may not apply to this species.