Gall size in Eurosta solidaginis is influenced by plant-insect-natural enemy coevolution

Eurosta solidaginis (Diptera:Tephritidae) induces galls on tall goldenrod, Solidago altissima.  Gall size and shape determines the susceptibility of the fly larva to natural enemies, and gall size and shape are influenced by an interaction of the fly genotype, plant genotype and the environment. Diversifying selection by natural enemies has resulted in larger, more spherical galls in the prairie biome than in the forest.  We conducted a double reciprocal transplant experiment among prairie and forest sites where galls from all combinations of flies and plants were produced in each biome. We found that prairie E. solidaginis induced larger galls than those from the forest, but that prairie plants produced smaller galls that forest plants. We hypothesize that conflicting coevolutionary pressures drive this interaction.  Prairie flies are under selection to maximize gall size to protect them from natural enemies, but prairie plants are under selection to limit gall size because large galls reduce plant fitness.  Forest flies are under selection to produce smaller galls, and thus there is reduced selection for forest plants to inhibit the formation of large galls.  These data provide evidence of a plant-insect coevolutionary arms race.