Can international experience help us to predict the potential impacts of willow sawfly (Nematus oligospilus Förster) on willow populations in Australia

In riparian areas and wetlands of south eastern Australia, willows (Salix spp.) are a serious weed. The recent arrival of willow sawfly (Nematus oligospilus) has the potential to affect willow management activities as the larval stage of the insect can cause whole tree defoliation and eventually tree death. Willow sawfly is native to the Holarctic region of the Northern Hemisphere but since 1980 has become an important invasive species in South America, southern Africa and New Zealand. Willow sawfly is capable of completing up to six generations per season in its invasive range, with population levels affected by temperature, climate (possibly conditions in early spring) and natural enemies. Tree willows tend to be more highly preferred by willow sawfly than shrub willows. Severe defoliation has so far been observed on nine Salix taxa in Australia, however there have been no reports to date of willow sawfly causing the death of entire willow trees. If the activity of willow sawfly does result in high levels of tree death across much of the landscape, then the implications for willow management include the possible redirection of resources from tree willows to shrub willows, and the development of techniques to establish native riparian vegetation in areas currently dominated by willows. It is too early to determine if populations of willow sawfly will affect willow populations and their management in Australia.