The destructive willow sawfly is damaging willows planted for soil conservation and river control throughout the Waikato Region.

This week’s Environment Waikato Operations Committee meeting heard the pest has progressively spread south across the country, and has now been reported as far south as Christchurch. The insect’s larvae which look like a small green caterpillar are able to severely weaken or even destroy established willow trees, threatening soil stabilisation and river bank erosion control programmes.

Land management officer Rien van de Weteringh said the Waikato had over 1000 hectares of willows planted for conservation purposes and another 3000 hectares of crack and pussy willows in rivers and wetlands. The sawfly’s destructive habit could increase soil erosion in gullies, damaging water quality and loss of productive land. Other effects included increased sedimentation in lower catchments, increased congestion of waterways, increased flooding and changes to wetland habitat.

This summer the damage has been relatively light although the Region had a number of “hot spots” – Mercer-Huntly, an area south of Te Aroha and now also along the upper Waipa river near Otorohanga. In some areas older crack willows had died after two years of defoliation. Control options included insecticides which were expensive and not practical as they could pollute waterways, planting alternative species which was being investigated or using engineering structures which were also expensive.

Meanwhile, Environment Waikato is continuing to support research at HortResearch and stepping up monitoring. The Lower Waikato Enhancement Project is using native vegetation in the lower reaches, where river bank stability had been achieved and willows were not essential anymore.

He said the threat of the pest now had to be considered in designing work programmes and alternative tree species used or species identified as having some resistance.

More information was needed before more accurate assessments of the long term effects could be made and an appropriate strategy put in place.

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