Members of the genus Ips are polygamous. The male excavates the entrance hole and a small nuptial chamber between the inner bark and sapwood. The females then enter and each excavates an egg gallery starting from the nuptial chamber. The number of females per gallery system ranges from 1 to 5 but the most common number is 3. This sex ratio of approximately 3 ♀ ♀: 1 ♂ has been attributed to some mortality factor more lethal to the males in some stage of development. Breeding of isolated pairs under laboratory conditions was carried out primarily to determine the variability between individuals from the same brood and between individuals from different broods. The objective was to resolve the complex of closely related species in the Ips tridens–I. engelmanni group. An unexpected result was a clarification of the sex ratio problem.