Polymorphism in a flea beetle for the ability to use an atypical host plant

The flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae) is polymorphic for its ability to use Barbarea vulgaris R. Br. (Brassicaceae) as a host plant. The genetic factors influencing this ability show both sex-linked and autosomal inheritance. Evidence was found for the presence of major genes such as those found in earlier studies, but also of genes with a smaller effect which have not previously been found. Although the ability to survive on B. vulgaris exists in most populations in eastern Denmark, it is usually at a low frequency. Beetles collected on B. vulgaris, however, usually produced larvae that survived on this plant. The inheritance and the abundance of the ability to use B. vulgaris are discussed in the context of the evolution of the interaction between I! nemorum and its atypical host plant.