Variation in Herbivore Population Sizes: The Effects of Host Quality, Host Genotype, Natural Enemies and the Formation of Demes

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9309298 Stiling The gall fly, Asphondylia borrichiae, develops internally in galls which form at the apical meristems of sea oxeye daisy, Borrichia frutescens. Larvae feed on an obligate fungus which they initiate in the gall cavity. Along the west central coast of Florida, local populations of B. frutescens are found only on small barrier islands and small "spoil" islands and adjacent to the intracoastal waterway. Gall densities between islands vary by as much as 2 3 orders of magnitude. This could be because: (1) some patches of sea oxeye are unsuitable for galls (i.e of poor quality or genetically resistant), (2) some sites fail to be discovered by the files, (3) some sites encourage high levels of attack by natural enemies or (4) on some islands, gall files have become locally adapted to existing host patches (i.e. deme formation). Three sets of experiments will assess the importance of plant clone, attack levels of natural enemies and gall fly dispersal and local adaptation on gall midge population dynamics: (1) reciprocal transplants of gall free plants between sites, (2) reciprocal transplant of galled plants between sites and (3) reciprocal transplant of plants that have been maintained in a common garden for one year to remove maternal effects. %%% This system will allow research that has rarely been possible in the past understand the factors controlling a four trophic level system. On a more practical level, this project may identify the reasons why some patches or genotypes of sea oxeye are resistant to damaging herbivorous insects while others are not. This is important because this plant and several other coastal and dune plants suffer high rates of attack by galling insects. Understanding the reasons behind their different rates of attack may greatly aid in selecting the most insect resistant species and genotypes for marsh and beach restoration projects.