Complex Effects of Genotype and Environment on Insect Herbivores and Their Enemies

We performed reciprocal transplants of different Borrichia clones between offshore islands in Pinellas County, Florida, to assess the relative strengths of environment and genotype on the success of the gall midge Asphondylia borrichiae. We measured gall abundance, gall size, gall abortion rate, parasitism rate, stem—tip diameter, nitrogen level, and parasitoid community composition from both experimental and native plants. We found a significant effect of plant clone and environment on all these variables with the exception of gall abortion rate. The effect of environment was usually stronger than that of genotype. This is an unusual result. At sites with higher densities of galls, plants were higher in nitrogen, had bigger stem tips, and produced larger galls. Bigger galls suffered lower parasitism because the smallest species of the parasitoid guild, which is also the most common species, was largely excluded from large galls. The interaction between plant clone and environment was significant for all variables, including abortion rate, which indicates that plant responses, herbivore density, and herbivore mortality are phenotypically plastic. While some authors have found significant interactions between plant genotype and site on herbivore densities, the demonstration of similar phenomena on natural—enemy attack rates and abortion rates of galls is novel, at least for a natural system. In this system, "bottom—up" factors indirectly determine the strength of "top—down" control from parasitoids, by regulating the size of galls.