P> Caste determination in eusocial insects is the process by which individuals differentiate into reproducer or helper phenotypes. Environmental caste determination (ECD) is predicted to be more efficient than genetic caste determination (GCD), yet GCD occurs in several populations of Pogonomyrmex harvester ants. We tested whether GCD reduces efficiency by comparing colony growth rates of two GCD lineage pairs (H and J) with two closely related ECD congeners, P. rugosus and P. barbatus, under laboratory conditions over a range of temperatures. In addition, we directly compared metabolic rates of GCD and ECD larvae using flow-through respirometry. Unexpectedly, colonies from GCD lineages grew faster than colonies of P. rugosus across all temperatures, and grew at the same rate as colonies of P. barbatus. Slower colony growth rates of P. rugosus were caused by lower queen fecundity and slower larval development. Variation in developmental rate was not due to differences in larval metabolic rates, which did not differ among taxa. These results suggest that GCD in Pogonomyrmex does not impose significant productivity costs during colony growth. Instead, efficiency costs are compensated by other physiological mechanisms which may or may not be directly related to the mode of caste determination. Persistence of GCD populations in contact with ECD competitors likely stems from a life-history trade-off favouring different taxa across the geographic range of the complex: the slow growing but starvation-resistant P. rugosus dominates in resource-poor regions, while faster growing and competitive GCD populations predominate in more mesic habitats. Z8 0 Z9 1