Does Herbivory Drive Local Adaptation in Common Milkweed? A Test Using a Common Garden Experiment.

Recent declines in Monarch (Danaus plexippus) populations may be caused by declining populations of Common Milkweed (Asclepias syriaca), their larval host plant. Efforts to restore milkweed populations may be hindered by local adaptation, a phenomenon in which local environmental conditions favor specific fitness-related traits that are potentially unfavorable elsewhere. I conducted a common garden experiment to test the hypothesis that Common Milkweed populations are locally adapted in their response to insect herbivory. Seeds representing 95 genotypes collected from 19 populations across the species’ range were germinated and transplanted into a common garden in central Virginia. Two individuals of each genotype were transplanted. To manipulate herbivory, one plant in each pair was sprayed weekly for 5 weeks with the insecticide spinosad, and the other plant was sprayed with water as a control. I quantified plant damage, plant growth, and two defensive traits. The difference in plant damage between control and experimental plants varied significantly with both latitude and longitude of the source populations, suggesting local adaptation. There was no evidence for a geographic cline associated with differences in plant defense traits, but longitude significantly affected the difference between control and experimental plants for one plant growth trait, number of leaves