A MIXED DIET IMPROVES PERFORMANCE AND HERBIVORE RESISTANCE OF A PARASITIC PLANT

I examined how the performance of Castilleja wightii (Scrophulariaceae ), a generalist root parasite, is affected by the availability of diff erent combinations of host species. In this greenhouse study, I focuse d on pairs of hosts consisting of either two leguminous host individua ls (Lupinus arboreus; Fabaceae), two non-nitrogen-fixing hosts (Erioph yllum stachaedifolium; Asteraceae), or one individual of each of these species. Castilleja growth and reproductive performance were greatly improved by the simultaneous attack of two distinct host species, even though Castilleja grown with two Lupinus hosts had significantly high er nitrogen content. Different combinations of host species also stron gly affected the growth of aphid colonies feeding on the Castilleja us ed in this experiment. Across all treatments, the growth of aphid colo nies was positively correlated with the nitrogen content of the parasi tic plants, which, in turn depended on the combination of hosts attack ed. Aphid colonies feeding on parasites attacking a mixture of host sp ecies grew more slowly than those on parasites attacking two Lupinus i ndividuals. Therefore, simultaneous attack of a mixture of host specie s may lead to improved parasite performance in two ways-via a direct b enefit on parasite growth and flowering as well as a possible indirect benefit because of the relatively poor performance of herbivores feed ing on these parasites.