Potential Selective Pressures by Parasitoids on a Plant‐Herbivore Interaction

The potential role of the third trophic level in the evolution of plant—herbivore relationships was examined in the case of the goldenrod Solidago altissima, and the fly Eurosta solidaginis, which forms a round stem gall. Previous observation had shown that galls attacked by parasites are significantly smaller than those in which the gall maker survives. Two different mechanisms could cause such as pattern: parasite attack could occur before galls reach full size and the attacks could cause early cessation of growth, or attack could occur after galls reach their mature size, but with inherently small galls being more prone to attack. In the first instance, parasite attack would diminish the cost of the gall to the plant, and thus favor plant genotypes that facilitate parasites. In the second instance, parasites would exert selection pressure on the gall marker to induce larger galls. Monitoring of marked plants in the field, and field experiments in which parasites were excluded from gall—bearing plants except during controlled periods, showed that parasite attack does not stop gall growth. The parasitoid wasp Eurytoma gigantea is limited to attacking small galls because of the limited reach of its ovipositor. This created a selection intensity of 0.50, favoring Eurosta that induce larger galls. Evolutionary response to selection could be realized directly through change in the gall marker's stimulus ability, or less directly through phenological changes. Plant activity to the gall marker declined with plant age, so that late—starting galls were more vulnerable to parasite attack.