Constraints on host use by a parasitic plant.

Consumers do not always utilize all suitable hosts. Understanding why parasitic plants do not always parasitize potentially suitable hosts requires a better understanding of the constraints that limit host use by parasitic plants. In Texas salt marshes, the parasitic plant Cuscuta indecora rarely parasitizes three hosts that support vigorous growth in the greenhouse. We identified three constraints on host use by C. indecora. First, a mismatch between the phenology of C. indecora and some suitable hosts meant that these hosts were not abundant when C. indecora was growing most vigorously, and therefore were underutilized. Second, C. indecora preferentially parasitized tall plants versus short ones, causing relatively short species to be underutilized. Third, C. indecora overwinters in some perennial hosts but has to reinfect annual hosts each year, causing annuals and perennials that do not support overwintering to be underutilized. In combination, these constraints, which reflect the general lack of mobility of parasitic plants relative to herbivores, remove half of the potential host species from the actual diet of C. indecora, and therefore likely represent a major limitation on the success of this parasite. Similar constraints are likely to limit the realized host range of many parasitic plants and select for generalized diet preferences.