Technical Abstract: From both theoretical and experimental work, it is well-known that genetic variation is necessary for adaptive evolution. Less understood is to what extent the lack of genetic variation may serve as an important constraint on adaptive evolution in natural populations. Among plant feeding insects exposed to a novel or non-traditional plant species, patterns of genetic variation in both acceptance and performance on the novel plant determine whether or not the plant will be incorporated into the diet. A lack of genetic variation in either acceptance or performance can constrain the insect from evolving to feed on the plant regardless of any selective advantage. This study investigated patterns of genetic variation in host-associated characters in the leaf-mining fly Amauromyza flavifrons. This insect readily oviposits on Beta vulgaris, a non-traditional plant upon which the larvae suffer high mortality. Experiments found significant tfamily-level variation in oviposition response to B. vulgaris but not in either larval survival or growth on this plant species. In regions with high densities of B. vulgaris, it is predicted that the lack of genetic variation in larval survival and growth on this plant will constrain the adoption of this plant as a "true host plant" and, instead, females will evolve to have lower acceptance of this plant for oviposition.