Host Specialization in Cuscuta Costaricensis: An Assessment of Host Use Relative to Host Availability

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Abstract
The parasitic plant Cuscuta is considered a host generalist because of the many host species on which it can grow. Nonetheless, we found that 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 between 0 and 1, reflecting that 88% of parasite cover is on only two of the 10 potential host types, those two comprising 54% of the total cover. Host availability does not predict parasite use: similarly distributed hosts were not equally infested. However, C. costaricensis grows significantly more vigorously on the two most commonly used host types, in three different measures of vigor. Thus, relative host use by the parasite would appear to be determined by differential parasite growth and/or mortality. However, C. subinclusa in southern California, USA, has similar patterns of host use and possesses active mechanisms for choosing among host species as well as the above passive mechanisms. At this time, then, we draw no conclusions as to the causes of the observed differences among hosts of degree of infestation.