Contrasting local adaptation patterns in two sister taxa of a generalist herbivore

Insect herbivores represent the most diverse group of multicellular life on Earth and most species are dietary specialists, whereas generalists represent a minority. This incredible diversity of herbivorous insects may be due to local adaptation, which could generate local specialization and maintain generalization at the species scale. However, we lack studies that directly compare the prevalence of local adaptation in related insect herbivores that vary in diet breadth. Here, we performed a large‐scale reciprocal transplant experiment to compare sister taxa that differ in diet breadth; we tested if Hyphantria cunea, a highly polyphagous moth with two genetically distinct biotypes, shows local adaptation to multiple host plants across its native range. We found that the red‐head biotype, which has a narrower diet, showed local adaptation on two sets of hosts. In contrast, one population of the black‐head biotype, which has a broader diet, showed a pattern of general vigor, outperforming the other black‐head population on two sets of hosts. We observe that local adaptation can be maintained across hosts and may not always lead to specialization. Our results confirm that dietary generalists can be locally adapted, but the evolutionary patterns emerging from a geographically widespread generalist are diverse, with two classical patterns (local adaptation and general vigor) emerging in two closely related taxa.