Effects of the Mistletoe Tristerix aphyllus (Loranthaceae) on the Reproduction of Its Cactus Host Echinopsis chilensis

Abstract

We investigated the effect of parasitism by the mistletoe Tristerix aphyllus (Loranthaceae) on the reproduction of its host the columnar cactus Echinopsis chilensis. The frequency of parasitized individuals increased with cactus size. Reproduction, measured as both the frequency of individuals reproducing and the number of flowers and fruits produced per individual also increased with cactus size. Parasitism had a significant effect on the reproduction of E. chilensis. The fraction of individuals producing fruit was significantly lower among parasitized than non-parasitized cacti. For cacti of similar size the production of buds, flowers, and fruit decreased with parasitism intensity - defined as the fraction of cactus branches showing evidence of parasitism. Tristerix aphyllus is a peculiar mistletoe because it is a phloem-parasite with greatly reduced photosynthesis, and hence it depends more on its host's resources than xylem-tapping hemiparasitic mistletoes. We hypothesize that its detrimental effect on the fitness of its hosts is larger than that exhibited by hemiparasitic mistletoes.