From bitter to better: evidence of shifting host preference by Euura proxima (Hymenoptera: Tenthredinidae) in New Zealand willows

The red gall sawfly Euura proxima (Serville) (Hymenoptera: Tenthredinidae) is an introduced insect in New Zealand that causes bean-shaped red galls on the leaves of species of willow (Salix spp.). The incidence and level of galling of the leaves varies, but there have been no studies regarding the host preferences and growth of this insect herbivore in New Zealand. Twelve willow clones of 4 species and a hybrid that are grown in New Zealand were investigated to correlate the development and growth of E. proxima with phenolic and nutrient content of the leaves. The Salix purpurea clones were resistant to E. proxima, with no galls observed. The S. matsudana clones had small, malformed galls without viable larvae. The S. matsudana × alba, S. alba and S. fragilis clones developed galls with viable larvae. In New Zealand, E. proxima appeared to prefer willow clones with a lower phenolic content, with these having higher levels of galling and larvae with larger head widths. This is in contrast to studies of E. proxima in its native range, which showed a preference for higher levels of phenolics. The shift in preference of E. proxima from willows with a high content of phenolic glycosides in its native range to willows with a low content of phenolic glycosides in New Zealand may be due to absence of predators in the introduced range, but further studies are needed to address this hypothesis.