The maintenance of sex is difficult to explain in the face of the demographic advantages of asexuality, especially when sexual and asexual lineages co-occur and compete. Here, we test if niche divergence and host plant specialization can contribute to the maintenance of sympatric populations of two closely related, sexual and asexual Aptinothrips grass thrips species. In mesocosm experiments, ecological niche and host plant specialization were inferred from thrips performance on different grass species used as hosts in natural populations. Sexual and asexual thrips performed best on different grass species, indicating niche differentiation. The asexual species was also characterized by a broader ecological niche than the sexual one. However, niche differentiation is unlikely to explain the maintenance of the two species in sympatry, because the reproductive rate of asexual females generally exceeded that of sexual ones. Surprisingly, the asexual but not sexual species showed geographic variation in the ecological niche. This geographic variation likely stems from variation in clonal assemblages among locations because different asexual genotypes have different ecological niches. Across natural populations, the performance of asexual females on a specific grass species was furthermore positively correlated with the abundance of that grass species, consistent with adaptation to locally abundant grasses. Altogether, our results suggest that niche differentiation contributes little to the co-occurrence of sexual and asexual grass thrips and that asexuality facilitates adaptation to a diversity of co-occurring host plants.