Escape from natural enemies may favor the incorporation of a novel host plant into the diet of an herbivorous insect. This scenario has been suggested for the recent host-plant shift by the goldenrod stem galler, Eurosta solidaginis, from the ancestral host Solidago altissima to the derived host S. gigantea. In this study, we examined the effects of predation from downy woodpeckers and black-capped chickadees on these two host races at the western edge of their zone of sympatry. Based on a field census, bird predation was concentrated near the cover of trees where S. gigantea tends to occur; very few attacks occurred in the open where S. altissima is prevalent. We conducted a field experiment to evaluate the preference of these avian predators for galls of the two host races when differences in the micro-geographic distribution, size and height of galls were controlled. In allopatric sites where only S. gigantea occurs, attacks by birds were 58% more frequent on S. gigantea than S. altissima galls. Similar results were found for sympatric sites, although the difference in attack was only 26% and not significant. We could find no difference in the toughness of galls or the nutritional value of a larva within the gall (in terms of biomass) to explain avian preference for the S. gigantea host race. However, we did find that from 1999-2000, the S. gigantea race offered a 27% to 107% higher reward rate, i.e. the probability that a gall harbored a Eurosta larva, than the S. altissimarace. While our studies indicated that birds could not assess a gall's content prior to pecking it open, it is possible that birds have either learned through experience, or evolved through natural selection, to choose the more profitable S. gigantea galls. Finally, our results suggest that avian predators act against the maintenance of two distinct host races in the Great Plains region.
Key words. Long-distance dispersal; Dispersal curve; Dispersal distance; Spatial Scale; Realized dispersal; Dispersal Data; Biogeography; Tracking individuals; Population Redistribution; Genetic analysis; Models; Human-Mediated Dispersal