The interactions of plant clone and abiotic factors on a gall-making midge

Within and around Tampa Bay, Florida, monoclonal populations of the sea daisy, Borrichia frutescens, can be found on small, isolated islands growing within the intertidal zone. Stem tips of Borrichia are attacked by the gall-making cecidomyiid, Asphondyliaborrichiae. We used reciprocal transplants of Borrichia clones between islands to assess the importance of plant genotype and local environmental conditions (shade and host-plant nitrogen) on gall abundance. In another experiment, we controlled for host genotype effects by inducing differences in local environmental conditions through the addition of NH₄NO₃ fertilizer and/or shade to field plots at the only monoclonal site with a large enough population of Borrichia to facilitate the experiment. We also examined the effect of these variables on attack levels of Asphondylia by parasitoids. In the reciprocal transplant, while some Borrichia clones always supported more galls than others, regardless of environmental conditions, all four clones developed more galls when they were placed in the shade, compared to those in the sun, at all four sites. In addition, some islands always supported more galls than others and we found a significant clone × site interaction. In the single-clone experiment, Borrichia in fertilized- and shaded-only plots developed more Asphondylia galls than those from nonmanipulated control plots, and plants that received both shading and fertilizer developed the most galls. Although shade and fertilization produced an additive increase in plant nitrogen content, their effects resulted in a synergistic decrease in C:N ratio. Neither shading nor host plant nitrogen content had a significant effect on levels of parasitism between experimental and control plants. Our results suggest that genetic differences in Borrichia's susceptibility to Asphondylia attack are important in shaping the distribution of galls, but environmental factors such as soil nitrogen and degree of shading are at least as important as genetic differences between host plants.