The coevolutionary arms race between a gall-inducing fly Eurosta solidaginis and the parasitoid Eurytoma gigantea
Testing for escalating coevolutionary arms races between predators and prey is difficult as it requires measuring reciprocal evolution over a long time span. Thompson (1996) proposed that geographic variation in the interaction of species will produce a geographic mosaic of coevolution where space can be used as a proxy for time to study coevolution. The interaction of the parasitoid Eurytoma gigantea and the fly Eurosta solidaginis which forms galls on Solidago altissima and S. gigantea is a model system for measuring phenotypic selection. Eurytoma ovipositors must reach Eurosta larvae in the gall’s central chamber, thus limiting successful attack to larvae in small galls. Eurytoma therefore exerts selection for larger gall size. Differences in selection by multiple natural enemies results in larger galls in the prairie than in the forest biome on S. altissima in Minnesota, and larger galls on S. gigantea in both biomes. We tested the prediction that selection on Eurytoma would result in longer ovipositors in Eurytoma attacking Eurosta on S. gigantea and on prairie S. altissima. We measured ovipositors lengths using population as the main effect to test for genetic differences between populations and gall size as the covariate to determine the impact of environmental factors.
Both the plant population and gall size had significant impact on Eurytoma ovipositor length. Eurytoma parasitoids individually reared from Eurosta that formed on galls on S. gigantea and S. altissima from the prairie had longer ovipositors than those from the population found on S. altissima in the forest. We conclude that because there is a genetic basis to both gall size and ovipositor length that there is evidence of a coevolutionary arms race between Eurosta and Eurytoma.