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 othe r 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 contras ting habitats to see ii plant adaptations to local abiotic environments alt ered susceptibility to the specific gall-making midge Rhopalomyia californi ca. Further we tested if genetically based differences in susceptibility ar e better explained by changes in adapted traits per se, or if susceptibilit y 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. Clo nes were reciprocally transplanted into experimental gardens near the colle ction sites. Several morphological differences between populations were sta ble across environments, but the phenotypic expression of several other gen etically controlled differences, including height, was seen only in the sou thern, inland garden. The northern coastal plants tend to be shorter, which may be an adaptation to wind pruning. Infestation rates by gallmakers diff ered between the two plant populations when grown in the southern garden, w here genetic differences in plant height were most strongly expressed but n ot in the northern garden, where wind pruning kept plants from both populat ions to the same height. In a neutral greenhouse environment plants from th e two populations did not differ in attractiveness to ovipositing females o r in suitability for gall-induction. Thus, Irvine and Bodega plants are equ ally susceptible to the gallmaker in some environments, but not others. The results suggest that in some cases plant genotypes dispersing into novel h abitats can have lower susceptibility to enemies than in their native habit ats.