Plant genetic variation determines the strength of intraguild predation in a gall-parasitoid food web
Theory predicts that genetic variation can govern the organization of food webs; however, there have been few experimental tests of this. Here, we address this knowledge gap by examining how genetic variation within a dominant plant species (coastal willow, Salix hookeriana) influences the strength of intraguild predation in a gall-parasitoid food web. In this food web, the resource is a leaf galling midge (Iteomyia salicisverruca), the intraguild prey is an egg, endoparasitoid (IG-prey = Platygaster sp.) and the intraguild predators are a suite of three larval, ectoparasitoids (IG-predators = Mesopolobus sp, Torymussp., and Eulophid sp. A). To isolate the effects of genetic variation, we conducted an experiment with 16 replicates of 8 unique willow genotypes (128 willows total) growing in a common garden. On half of these replicates, we bagged 20-30 galls to exclude IG-predators, which enabled us to quantify how the strength of intraguild predation varied among willow genotypes. We then collected galls on bagged and unbagged trees and brought them back to the lab to rear them for their associated parasitoids. In addition, we measured the size of individual galls as this has been shown to be a key trait determining parasitoid attack rates.
In concordance with previous work in this system, we found that the size of galls varied more than 2-fold among willow genotypes. Moreover, we found that the strength of intraguild predation varied by 20% among willow genotypes and was determined by variation in gall size. Specifically, we found that intraguild predation occurred less frequently on willows that hosted larger galls. This gall size refuge mechanism is supported by theory, which predicts that refuge for IG-prey from IG-predators is a key mechanism that mediates the persistence of intraguild predation. Taken together, our results indicate that genetic variation can play a key role in the organization and persistence of food webs. Furthermore, our results highlight the potential for microevolutionary processes to shape both the structure and dynamics of food webs.