The flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticina
e) is polymorphic for its ability to use Barbarea vulgaris R. Br. (Brassica
ceae) as a host plant. The genetic factors influencing this ability show bo
th sex-linked and autosomal inheritance. Evidence was found for the presenc
e 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 ab
ility to survive on B. vulgaris exists in most populations in eastern Denma
rk, it is usually at a low frequency. Beetles collected on B. vulgaris, how
ever, usually produced larvae that survived on this plant. The inheritance
and the abundance of the ability to use B. vulgaris are discussed in the co
ntext of the evolution of the interaction between I! nemorum and its atypic
al host plant.