The evolutionary implications of genetic variation among Solidago altissima L. (Asteraceae) genotype in the morphology of galls induced by Eurosta solidaginis Fitch, 1855 (Diptera: Tephritidae)

To contribute to the understanding of the coevolution of the gall-inducing tephritid fly, Eurosta solidaginis Fitch, 1855 (Diptera: Tephritidae), and its host plant, tall goldenrod, Solidago altissima L. (Asteraceae), we tested hypotheses on how plant genotypic variation influenced gall size and shape. A field survey showed that stem diameter was correlated with gall diameter, indicating that plant phenotype influences gall size. To test the hypothesis that plant genotypic variation in stem growth influenced gall diameter, length, and shape, 10 goldenrod genotypes were grown in a common garden and exposed to flies in a cage in 2019 and 2020. Plant genotype significantly influenced gall diameter and length in both years and gall shape in the second year of the experiment. Our results support those of Weis & Abrahamson (1986), who found that S. altissima gall diameter results from the interaction of plant genotype, insect genotype, and the environment. Neither stem diameter nor length significantly impacted gall diameter, length, or shape in the common garden experiments, suggesting plant genotype influences these traits through other factors uncorrelated with stem growth. Because genetic variation in gall size is weakly correlated with stem growth, this removes restraints on plant response in the evolution of gall size in response to selection by Eurosta.