Polyphagous stages of insect lineages are believed to facilitate host shifts onto novel hosts, which may then lead to specialization and diversification on these new hosts. Recent studies have indicated that transitions between specialist and generalist forms within a lineage may be common. Therefore host shifts during polyphagous stages are likely to be an important component of diversification and speciation in plant feeding insects. The polyphagous swallowtail butterfly sibling species Papilio glaucus and Papilio canadensis represent an ideal study group for such host shifts. They exhibit many ecological similarities, but differ significantly in their host use abilities. In addition, recent increased movement of Papilio glaucus into P. canadensis territory due to climate warming has lead to the formation of an allochronically separated hybrid swarm population. This recombination of the P. glaucus and P. canadensis genomes is likely to produce novel combinations of detoxification enzymes and ovipositional preferences, which may lead to novel host associations. Here we examine the ovipositional preference and larval host use abilities of this hybrid swarm population. Our results indicate that the ovipositional preference of this hybrid swarm is identical to that of P. glaucus, although the preferred host of P. glaucus is absent where this hybrid swarm occurs. In contrast, the larval host use abilities represent a mixture of P. glaucus and P. canadensis host use abilities and are highly variable even within families. Here we discuss the potential future host associations of this hybrid swarm as a case study for host use divergence within an already divergent polyphagous sibling species pair.