A number of empirical studies have concluded that reproductive interference, RI, contributes to parapatric species distributions or sexual exclusion. However, the possibility that divergent host-plant use in phytophagous insects is due to sexual exclusion has seldom been considered. Here we present evidence that RI is responsible for different host-plant use by two Pierid butterfly species, Pieris napi and P. melete . When a novel host species was introduced about 50 years ago, two Pierid butterfly species at first used both the ancestral host species and the novel one. Subsequently, P. napi shifted to use only the novel host, while P. melete shifted to specialize on the ancestral host. To explain these patterns, we investigated whether the two host species differ in suitability for larval growth and survival. Additionally, we tested whether RI occurred between the two species using large outdoor field cages. Courtship of females by conspecific and heterospecific males reduces the number of eggs laid by approximately half. However, RI is asymmetric and would generate selection on P. melete females to evolve to avoid the more suitable host species preferred by P. napi . Thus, our study suggests that sexual exclusion can explain the shift in host use by these two butterfly species.