Adaptation of Coyote Brush to the Abiotic Environment and Its Effects on Susceptibility to a Gall-Making Midge

Many plant traits that affect susceptibility to insect attack may have other functions important to the plant. If so, susceptibility could evolve as a correlated response to selection imposed through these other functions. We studied two populations of coyote brush, Baccharis pilularis, from contrasting habitats to see if plant adaptations to local abiotic environments altered susceptibility to the specific gall-making midge Rhopalomyia californica. Further we tested if genetically based differences in susceptibility are better explained by changes in adapted traits per se, or if susceptibility changes due to increased general vigor of adapted plants. Plant genotypes were cloned from an inland population at Irvine in southern California and from a coastal population surrounding Bodega Bay, 800 km to the north. Clones were reciprocally transplanted into experimental gardens near the collection sites. Several morphological differences between populations were stable across environments, but the phenotypic expression of several other genetically controlled differences, including height, was seen only in the southern, inland garden. The northern coastal plants tend to be shorter, which may be an adaptation to wind pruning. Infestation rates by gallmakers differed between the two plant populations when grown in the southern garden, where genetic differences in plant height were most strongly expressed but not in the northern garden, where wind pruning kept plants from both populations to the same height. In a neutral greenhouse environment plants from the two populations did not differ in attractiveness to ovipositing females or in suitability for gall-induction. Thus, Irvine and Bodega plants are equally susceptible to the gallmaker in some environments, but not others. The results suggest that in some cases plant genotypes dispersing into novel habitats can have lower susceptibility to enemies than in their native habitats.