Anthony M. Rossi á P. Stiling The interactions of plant clone and abiotic factors on a gall-making midge Received: 12 June 1997 / Accepted: 6 April 1998 Abstract Within and around Tampa Bay, Florida, monoclonal populations of the sea daisy, Borrichia fru- tescens, can be found on small, isolated islands growing within the intertidal zone. Stem tips of Borrichia are attacked by the gall-making cecidomyiid, Asphondylia borrichiae. 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 eects by inducing dierences in local environmental conditions through the addition of NH 4 NO 3 fertilizer and/or shade to ®eld plots at the only monoclonal site with a large enough population of Borrichia to facilitate the experi- ment. We also examined the eect of these variables on attack levels of Asphondylia by parasitoids. In the re- ciprocal transplant, while some Borrichia clones always supported more galls than others, regardless of envi- ronmental 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 signi®cant 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 eects resulted in a synergistic decrease in C:N ratio. Neither shading nor host plant nitrogen content had a signi®cant eect on levels of parasitism between experimental and control plants. Our results suggest that genetic dier- ences 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 dierences between host plants. Key words Asphondylia borrichiae á Borrichia frutescens á Clonal and abiotic dierences Introduction Gall-forming insects, which are essentially sessile and bound to the site of their oviposition, have an intimate relationship with their host plant. For most galling insects, the majority of their life cycle is spent sheltered within the host's tissues (Gagne 1989). As a result of this intimate relationship between galler and host, Weis and Abrahamson (1986) proposed that gall phenotype is not only in¯uenced by the host's genotype, but is also an extension of the insect's genotype. Gall-makers not only aect the chemistry of their host, but are in turn greatly aected by the host plant's nutritional status and, although galls do not do not create new photosynthetic pathways, they do alter preexisting patterns (Weis et al. 1988). For instance, several studies have reported that, while total nitrogen decreases within galls, levels of soluble nitrogen such as proteins and amino acids increase (Warwick and Hildebrandt 1966; Birch 1974; Bronner 1977; Birch et al. 1992). These nutrients are shunted away from use by the plant for growth and become available to the gall-former, which in turn leads to stress of the host. For instance, Birch et al. (1992) reported that galled plants apparently require a higher level of moisture than do nongalled plants. Gall-formers may provide model systems in which to investigate the eects of host plant genotype and environmental inter- actions on phytophagous insect populations (Weis et al. 1988). Oecologia (1998) 116:170±176 Ó Springer-Verlag 1998 M. Rossi (&) 1 á P. Stiling Department of Biology, University of South Florida, Tampa, FL 33620, USA Present address: 1 Department of Natural Sciences, University of North Florida, Jacksonville, FL 32224-2645, USA