Plant Foraging: A Marginal Value Model and Coiling Response in Cuscuta Subinclusa

Stems of the parasitic plant Cuscuta subinclusa (Convolvulaceae) discriminate among host species and invest in resource acquisition (coil around the stem of a host plant) relative to host quality in a way predicted by a marginal value model of resource use. C. subinclusa can be said to forage in that: (1) stem soiling, the necessary antecedent and determinant of resource uptake, precedes exploitation of host materials; and (2) mean coiling on a host species is proportional to mean reward per unit of investment (measured as biomass accumulation per unit length of stem in coil). Preliminary evidence indicates that coiling in C. subinclusa is induced by host bark flavonoids. Larger parasites are more likely to overwinter and set seed a second season than smaller ones, and parasites that start from overwintered tissue are significantly larger at flowering than are those that have started from seed. Thus, seed set is correlated with parasite size at the end of the season, linking the foraging response and fitness of the plant.