Borrichia

Abstract: In recent years, ecologists have begun to develop a more fine-grained and integrative approach to examining the importance of top-down and bottom-up effects on herbivore populations by investigating how changes in abiotic heterogeneity affect the relative roles of these forces. We performed two factorial field experiments to determine how increasing salinity and nutrient supply affected the relative strengths of top-down and bottom-up forces among the gall-making midge, Asphondylia borrichiae, its host plant Borrichia frutescens, and a suite of parasitoids. Salinity was increased by the addition of salt pellets, and nutrient supply was increased by the addition of fertilizer. In both experiments, parasitism pressure was decreased by trapping hymenopteran parasitoids with yellow sticky traps. In both experiments, bottom-up manipulations had significant effects on gall density. Elevated salinity levels decreased the number of galls per 200 Borrichia stems, and fertilization increased the number of galls. In...

Abstract: . 1. Associational resistance theory suggests that the association of herbivore-susceptible plant species with herbivore-resistant plant species can reduce herbivore density on the susceptible plant species. Several casual mechanisms are possible but none has so far invoked natural enemies. Associational resistance mediated by natural enemies was tested for by examining densities of a gall fly, Asphondylia borrichiae (Diptera: Cecidomyiidae), and levels of parasitism on two closely related seaside plants, Borrichia frutescens and Iva frutescens, when alone and when co-occurring. 2. Both Borrichia and Iva grow alone or together on small offshore islands in Florida. Each host plant species has its own associated race of fly, but both races of fly are attacked by the same four species of parasitoids. Borrichia normally has a higher density of galls than Iva, and galls are larger on Borrichia than on Iva. 3. Gall size, gall abundance, parasitism levels, and parasitoid community composition were quantified on both Borrichia and Iva on islands where each species grew alone or together. Some islands were then manipulated by adding Borrichia to islands supporting only Iva, and by adding Iva to islands supporting only Borrichia. Subsequent gall densities and gall parasitism levels on the original native species were then examined. 4. On both natural and experimentally manipulated islands, gall densities on Iva were significantly lowered by the presence of Borrichia. This is because bigger parasitoid species that were common on Borrichia galls, which are bigger, spilled over and attacked the smaller Iva galls. Thus, parasitism rates on Iva were higher on islands where Borrichia co-occurred than on islands where Borrichia were absent. Most parasitoids from Iva were too small to successfully attack the large Borrichia galls and so gall density on Borrichia was unaffected by the presence of Iva.

Abstract: 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.

Abstract: . 1. The gall midge, Asphondylia borrichiae, attacks the terminals of three plants in the aster family: Borrichia frutescens, Iva frutescens, and I. imbricata. 2. In the field, Borrichia suffers the highest rate of galling and I. imbricata the lowest. Common garden experiments also revealed highly significant differences in the attack rates of the three host species by Asphondylia. However, these differences depended on the source of the attacking midge population. 3. Midges collected from Borrichia and I. frutescens attacked the host in which they developed almost exclusively, whereas those from I. imbricata attacked both Iva spp. 4. Each Asphondylia larva develops in its own chamber within a gall. Borrichia galls were 2.5–3.5-fold less crowded than those from I. imbricata and I. frutescens, respectively. Consequently, midges that developed in Borrichia were significantly larger and eclosed with 30–40% more eggs than those that developed in I. imbricata and I. frutescens, respectively. 5. Although performance of Asphondylia larvae was lowest on the two Iva spp., especially I. frutescens, these hosts may provide an escape from natural enemies as a trade-off for reduced offspring performance. 6. Differences in host-plant species phenology may reduce gene flow among host-associated populations of Asphondylia, thereby favouring the formation of races at the level of plant genus.