Bay of Plenty Regional Council engineer Tony Dunlop said he and his staff are grappling with how to stabilise riverbanks normally held firm by willows, the stalwart of flood protection schemes.
But one of the biggest threats to the system’s integrity is coming from the smallest of causes, the giant willow aphid.
Staff are working through remediating the major damage done by last April’s flood in the wake of Cyclone Debbie.
It left the region’s ratepayers with an estimated $50 million bill to restore flood protection systems.
“What we have noticed in the past two years is that the presence of this aphid is putting the willow trees under stress and some of the areas where banks of the rivers have given way have been due to willows being weakened by the aphid infestation.”
Willows have typically been used in riverbank remediation and hillside stabilisation with greater variety now available to suit different situations and demands in the area now known as bio-engineering.
With 40 years of experience managing rivers in the Eastern Bay of Plenty, Dunlop said the impact of the aphid on willow health had become more noticeable in the last five years.
“We have one area we needed to restore the riverbank and we found the root system of willows on the bank had been compromised by the aphids, requiring an infill of rock to stabilise them.”
Cyclone Debbie washed more than 100ha of high-value river plain farmland out to sea and in some cases created entire new river paths that engineers are having to work with.
The giant willow aphid was first sighted in Auckland in 2013 but has spread rapidly across the country.
Its presence is identified by a black sooty substance found on the leaves of affected willow trees.
But help might be in the wings for beleaguered councils with intensive work under way to determine which willows are most resistant to the impact of the aphid.
NZ Poplar and Willow Research Trust chairman Bruce Wills said the organisation is working at Massey University on willow germplasm and running field trials to provide planting options.
He had experienced problems on his own property with the aphid and had removed the most vulnerable willows to replace them with a more robust species.
Trust member Barry Foster said biological control of the aphid might become a possibility in coming years should an imported Californian aphid wasp prove effective here.
The wasp was imported late last year.
“It is currently in quarantine at Scion and will have to undergo a number of tests to ensure it does not predate on our own native aphid.”
The trials will take at least two years before the wasp can be released and its numbers will have to be bred before release.
“All willows are susceptible to a degree so the sooner we get a biocontrol in place the better,” Foster said.
