Resource-directed foraging of the Neotropical mistletoe Struthanthus flexicaulis (Loranthaceae).

Some parasitic plants are capable of vegetative propagation, which allows them access to new hosts and improves nutrient availability. We aimed to determine what factors positively influence this propagation in the xylem-tapping Struthanthus flexicaulis, focusing on the use of directed foraging as a means of optimising access to resources. The study site was a rupicolous plant community in southeast Brazil. We evaluated how the success of branch propagation (by contagion) of the mistletoe is influenced by the crown height and density of its main host, the legume Mimosa calodendron. Oriented foraging was tested through comparing the response of mistletoe branches toward a potential host (resources) or a plastic net (no resources). Successful contagion was related to high density and the departure height of a branch from a host crown. In a high-density area, 89% of branches reached a new host versus 21% in a low-density area. Formation of an appressorium on either the plastic nets or hosts elicited branch growth reorientation towards it, as well as development of new branches. These responses were significantly stronger towards hosts than nets, and attributed to effective xylem tapping. The foraging strategy of S. flexicaulis benefits from branch response (growth and reorientation) to the development of appressoria and their resulting haustoria. The growth of appressoria on non-resource substrates and the ensuing response optimises the mistletoe spread by allowing these substrates to act as physical support for spreading branches and to infect a host that was previously out of reach.