In the fall armyworm, Spodoptera frugiperda (Lepidoptera, Noctuidae), two sympatric strains have been recognized that have been termed corn strain (C) and rice strain (R), referring to their most common host plants. Both strains are reproductively isolated via a distinct prezygotic barrier as well as via an intriguing postzygotic phenomenon: when R females have mated with C males, the resulting RC hybrid females exhibit dramatically reduced fertility independent of their mating partner. Here we demonstrate that the reduced fertility is caused by the fact that these females refrain from mating, i.e. females are behaviorally sterile. We identified a Z-chromosomally linked sterility locus that is most likely incompatible with yet-to-be-identified autosomal (or cytoplasmic) factors leading to the observed sexual abstinence. Within-chromosome mapping revealed the sterility locus to be located in an area of strongly reduced inter-strain recombination.