Insect herbivores are one of the most diverse groups known and host plant use is an important factor influencing their diversification. We tested how diet breadth, host plant species, and geographic distance influence population divergence on fall webworm (Hyphantria cunea, referred as FW). FW is one of the most generalist species known and they have a broad geographic range occurring in almost all US states. Also, FW has two types that have been considered as possibly two different species: the black and the red-headed larvae. The two types differ in behavior, voltinism, host plant use, development time and are also reproductively isolated in time. We sampled FW larvae from Connecticut (allopatric for black-headed type), Illinois, Missouri, Maryland, New Jersey, Ohio, Pennsylvania (sympatric populations), and Colorado (allopatric for red-headed type). We used ddRAD-seq and next generation sequencing for library preparation, Fst, PCA and Structure to define population genomics, and RDA and memgene to investigate the influence of geography and host plant.
Results/Conclusions
We found low gene flow between red and black larvae, and the two types were well separated in our principal component analysis. Also, we found 3 genetic structures: red and black form distinctive structures, whereas Colorado FW had a distinct genetic structure from the other locations. Geographic distance and host plant use influenced the genetic distance of FW, however, the pattern varied depending on the type and geographic scale we analyzed. The genetic distance of red-headed FW in a broad geographic scale was influenced by both host use and geographic distance, while the genetic distance of black-headed FW and Colorado red-heads were influenced mainly by geographic distance. Our research shows new evidence that host plant use can influence divergence of generalist herbivores, but geographic isolation is also an important factor.