Effects of herbivory and mistletoe infection by Psittacanthus calyculatus on nutritional quality and chemical defense of Quercus deserticola along Mexican forest fragments

Mistletoes are parasitic plants that show effects that can parallel or contrast with those caused by herbivores to their host plants, particularly on aspects such as host biomass, resource allocation patterns, and interspecific interactions at the community level. In this study, we evaluated the potential synergistic effects of herbivory and infection by the mistletoe Psittacanthus calyculatus on nutritional quality and chemical defense of the white oak Quercus deserticola along forest fragments in Mexico. For this, we sampled leaves of parasitized oaks, unparasitized oaks and mistletoes at four forest fragments in the Cuitzeo basin, Michoacán state, Mexico, and measured herbivory levels and foliar water content, total nonstructural carbohydrates, phenols, flavonoids, and hydrolyzable tannins in each sample. Higher levels of infection by P. calyculatus were found in the smaller forest fragments, while foliar damage by herbivores was higher in larger forest fragments. At all sites, levels of herbivory were lower in the mistletoe than in both parasitized and unparasitized oaks. However, there was a positive relationship between herbivory levels in parasitized oaks and their mistletoes. Also, foliar water content and total phenol concentration were positively correlated between the oaks and the mistletoes. The results suggest that herbivory levels in parasitized hosts and mistletoes depend on the close physiological interaction between the nutritional quality and the chemical defense of the two plants involved. This is one of the few studies analyzing the chemical ecology of the interaction between plant hosts and plant parasites.