Role of Host Plant Variation, Avian Predators, and a Parasitoid in Populations of a Gall Fly, Eurosta solidaginis

Abstract

Tall goldenrod, Solidago altissima, is the host plant of a stem-galling fly Eurosta solidaginis. The fly larvae inhabit spherical galls that form on plants. Black-capped chickadees, downy woodpeckers, and a parasitoid Eurytoma gigantea are important causes of mortality in E. solidaginis. Galls were collected from abandoned fields near Mendota Heights and Brooklyn Park, MN. Larval mass, gall size, and rate of parasitism differed between populations. To elucidate plant characteristics that contribute to higher rates of gall fly oviposition, individual goldenrod clones were identified in the Brooklyn Park population. Among clones plant density, plant height, and clone area were not correlated with percentage galled plants. Clones of similar area, plant density, and plant height with exceptionally high proportions of galled plants were examined further. Basal stem diameter, gall height, gall diameter, larval mass, and rate of parasitism differed significantly among the clones equally susceptible to gall fly oviposition. Previous research has found that larger galls are more likely to suffer avian predation and smaller galls experience higher rates of parasitism. From indirect evidence it has been concluded that birds avoid galls containing a parasitoid. Alternatively, birds may simply preferentially attack larger galls. To remove the confounding factor of gall size, we are conducting avian feeding trials with gall fly and parasitoid larvae removed from galls.