Pest munches through flood protection trees

By ROSALEEN MacBRAYNE

A voracious grub munching its way through thousands of riverbank willow trees is endangering multimillion-dollar New Zealand flood protection schemes.

Damage caused by the willow sawfly has major environmental implications and the race is on to find a way to stop the insect in its tracks.

The worst of the destruction is in the Bay of Plenty, Hawkes Bay and Poverty Bay.

Environment BOP works engineer Tony Dunlop says the sawfly is becoming a serious threat to willow tree riverbank protection.

This is the fourth year the sawfly has defoliated large areas of willow plantings in the eastern Bay of Plenty, stripping leaves two or three times during the growing season.

Many trees are now dying, leaving gaps in bank protection on major river schemes such as the Waioeka-Otara and Whakatane-Waimana.

Mr Dunlop says the spread of the sawfly means river managers are having to seriously review the use of willows.

One possible alternative is the poplar tree - although the fly has been known to sample those leaves, too. Native plants have proved unsuccessful.

Mr Dunlop has recently returned from Northern California with seeds for new - and hopefully more resistant - willow species.

The seeds are germinating at a HortResearch station in Palmerston North, but it is likely to be six to 10 years before significant trials of alternative bank protection species can be carried out.

Mr Dunlop says although willow poles will almost certainly be attacked, it is unwise to postpone planting while waiting for an alternative. If the sawfly continues to advance, it will destroy existing willows, diminish the strength of riverbanks and threaten billions of dollars of flood protection schemes.

Bank erosion will lead to loss of land and sedimentation of river channels and estuaries. Dead trees washed downstream could affect bridges, moorings and harbour structures.

Scientists are doing chemical trials, but an insecticide assault may not be environmentally or economically sound near waterways, says Dr Lindsay Fung, from HortResearch.

There is no known biological control for sawfly, which is suspected to have come from South Africa.



Meanwhile, regional councils in affected areas are seeking money for further research from the Environment Ministry's sustainable management fund.

The sawfly was discovered in Auckland in 1997 but quickly took up residence in the Bay of Plenty. It spread a year later to the Gisborne area and Hawkes Bay and has since moved as far south as Central Otago.