The willow bud galler Euura mucronata Hartig (Hymenoptera: Tenthredinidae): one polyphage or many monophages?

The nematine sawfly Euura mucronata Hartig (Hymenoptera: Tenthredinidae) induces galls in the buds of over 30 willow species across the Holarctic region. This extensive host range is surprising, since the other Euura gallers are mostly monophagous; thus, the feeding habit of E. mucronata would represent a switch from monophagy to extreme polyphagy. Previous morphological studies have divided E. mucronata into separate species, but the feeding ranges of these species are unknown, and it is even doubtful whether multiple species really exist. To study whether or not E. mucronata consists of cryptic host-associated sibling species, an allozyme study was conducted using gallers collected from six willow species occurring in northern Fennoscandia. Electrophoretic data from seven variable enzyme loci show that: (1) "E. mucronata" probably comprises at least three species with restricted host ranges, but the species may not be completely reproductively isolated from each other; (2) the pattern of host use is not explained by the phylogeny of willows; (3) the pattern of host use is not concordant with the overall chemical similarity of the hosts; and (4) simple allopatric speciation does not appear to explain the host associations. Consequently, it is possible that reasons such as differences in host phenology, habitat, or morphology, are responsible for the limits in host use in the group.