Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants,such as crops and their wild relatives.Previous studies showed population genetic structuring in co leafhopper Dalbulus maidis in Mexico,such that the species consists of two sympatric,host plant-associated populations:an abundant and widespread “pestiferous” population on maize (Zea mays mays),and a small and localized “wild” population on perennial teosinte (Zea diploperennis),a maize wild relative with a limited distribution.This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of co leafhopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants.Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant;immigrant inviability was assessed by comparing,across two generations,the fecundity,survival,development time,sex ratio,and population growth rate among leafhopper populations and host plants.Our results showed that on maize,and compared to resident,pestiferous females,wild females were more likely to mate,and greater proportions of their offspring survived to adult stage and were daughters;consequently,the per-generation population growth rate on maize was greater for immigrant,wild leafhoppers compared to resident,pestiferous leathoppers.Our results suggested that wild leathoppers emigrating to maize have a fitness advantage over resident,pestiferous leafhoppers,while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.