Distribution and rapid range expansion of the introduced willow sawfly N ematus oligospilus Förster (Hymenoptera: Tenthredinidae) in A ustralasia

The willow sawfly, N ematus oligospilus   F örster ( H ymenoptera: T enthredinidae), has been introduced inadvertently across temperate regions in the S outhern H emisphere, including N ew Z ealand and A ustralia, where it has dispersed extremely rapidly. A host specialist herbivore, at high population densities it can defoliate and damage introduced willows ( S alix spp.), many species of which are invasive weeds in Australasia. In this study, we show that in just 10 years, the distribution of N . oligospilus has expanded across most of south‐eastern A ustralia and south‐western W estern A ustralia and, in less than 15 years across both the N orth and S outh I slands of N ew Z ealand. The relative population density of N . oligospilus varied widely within both geographic regions. The distribution of N . oligospilus was still expanding in A ustralia where high densities resulted in widespread willow defoliation. In contrast, the distribution of the willow sawfly had reached fully across both the North and South Islands in N ew Z ealand, but its density had declined from the early stage of invasion. All willow taxa surveyed were utilized by the willow sawfly, but host preferences were apparent, especially for the S . fragilis/ S . x rubens hybrid complex. Many factors, including natural enemies, willow range expansion and environmental conditions are likely to affect the long‐term population dynamics and spread of N . oligospilus in A ustralia.