Examination of Sex Ratios and Mating Habits of Ips avulsus and I. calligraphus (Coleoptera: Scolytidae)1

Abstract

The sex ratio of brood bark beetles is usually 1:1 (males:females), whereas the sex ratio of attacking beetles often favors the female. This situation occurs for many Ips species which are polygamous. I. calligraphus (Germar) was found to closely follow this pattern. The proportion of males to total beetles which emerged and reemerged was 0.465 and 0.284, respectively. These proportions correspond to a 1:1 sex ratio for brood adults and a 1:3 sex ratio for attacking adults. Thirty-nine nuptial chambers were examined and had an average of 3.05 egg galleries per nuptial chamber. The proportion of I. avulsus (Eichhoff) males to total beetles which emerged and reemerged was 0.468 and 0.522, respectively. Both proportions correspond to a 1:1 sex ratio. However, the average number of egg galleries per nuptial chamber was 2.91. I. avulsus males and females frequently paired off, leaving the nuptial chamber open for use by other beetles. Therefore, I. avulsus males may be able to adjust their mating behavior to accommodate the appropriate number of females.