Effects of manipulation of plant carbon nutrient balance on tall goldenrod resistance to a gallmaking herbivore.

Ramets from six Solidago altissima clones of known resistance to the stem gallmaker Eurosta solidaginis were grown with and without supplemental nutrients. In a greenhouse experiment, mated female Eurosta were allowed to oviposit in ramets that were subsequently grown through flowering and then harvested to determine biomass allocation. Supplemental nutrients increased plant biomass but did not affect resistance to this gallmaking herbivore. This result does not conform to the plant carbon/nutrient balance hypothesis which predicts that enhanced mineral nutrition will indirectly cause a reduction in carbon-based defensive chemistry. Our results indicate a strong genetic basis to ball gallmaker resistance since modification of host phenotype did not influence susceptibility. We suggest that evaluating the degree of genotypic or environmental control of plant resistance will be especially helpful in clarifying the patterns of defensive chemical responses.