Most disturbed habitats in the tropics and subtropics harbor numerous species of invasive ants,and occasionally the same species has been introduced repeatedly from multiple geographical sources.We examined how experimental crossbreeding between sexuals from different populations affects the fitness of queens of the tramp ant Cardiocondyla itsukii,which is widely distributed in Asia and the Pacific Islands.Eggs laid by queens that mated with nestmate males had a higher hatching rate than eggs laid by queens mated to males from neighboring (Hawaii x Kauai) or distant introduced populations (Hawaii/Kauai x Okinawa).Furthermore,inbreeding queens had a longer lifespan and produced a less female-biased offspring sex ratio than queens from allopatric mating.This suggests that the genetic divergence between different source populations may already be so large that in case of multiple invasions eventual crossbreeding might negatively affect the fitness of tramp ants.