Pest munches $6m hole in flood control plans

The willow sawfly has damaged flood protection systems on Hawkes Bay plains to the extent the local Regional Council needs to spend more than $6.1 million to fix the problem.

The sawfly (Nematus oligospilus) entered New Zealand in 1997 and has thrived in the Heretaunga Plains. It has a demonstrated ability to defoliate and even kill willow trees.

The council monitored the sawfly's spread to see whether there was a balance between it, its food sources and predators. There was a balance in Bay of Plenty and Waikato - but not in Hawkes Bay.

A re-evaluation of the asset value of the Heretaunga Plains willows' last year saw their value drop from $5.6 million to $2.25 million.

The willows are important for flood protection. Defoliation by the sawfly causes the willow roots to die back, reducing the trees' ability to prevent erosion.

In some areas, the willows' effectiveness has been 90 per cent compromised, the council has discovered.

A report by council staff, to be discussed by councillors at a meeting on tomorrow, says there is now "an increased risk of a stopbank breach placing large areas of the Heretaunga Plains at risk of flooding".

The council previously joined with other regional councils to fund a HortResearch study on sawfly control, but the Government's refusal to continue funding the research means the programme is likely to collapse by June.

Staff recommend the council perseveres with willows, in the hope that a form of sawfly control is found, while also turning to other species of trees.

They suggest planting 279,835 trees to replace the dead and dying willows over the next five years. They also recommend significant structural works in certain areas.

The work will take five years at a forecast cost of $6,147,636.

While funding will be discussed tomorrow, staff recommend various rates rises coupled with a loan.

The council will continue lobbying the Government for funding of the work.

- NZPA