Host specialization in Cuscuta costaricensis: an assessment of host use relative to host availability

The parasitic plant Cuscuta is considered a host generalist because of the many host species on which it can grow, but C. costaricensis in Costa Rica does not use all hosts equally. Proportion similarity (PS) between resource use and availability is 0.66 on a scale of 0-1, reflecting that 88% of parasite cover is on only 2 of the 10 potential host types, those 2 comprising 54% of the total cover. Host availability does not predict parasite use: similarity distributed hosts were not equally infested. However, C. costaricensis grows significantly more vigorously on the 2 most commonly used host types in 3 different measures of vigor. Thus, relative host use by the parasite is determined by differential parasite growth and/or mortality. However, C. subinclusa in southern California, has similar patterns of host use and possesses active mechanisms for choosing among host species as well as the above passive mechanisms. -from Authors