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Council pursuing midges.

9 March 2007

Christchurch City Council has enlisted help from AgResearch and NIWA to find out why numbers of native midges in the Bromley area have escalated, annoying local residents.

The native midge Chironomus Zealandicus has always lived in the still, fresh waters near the Avon-Heathcote Estuary. The ponds at the Christchurch Wastewater Treatment Plant are also a habitat for the small harmless, non-biting insect.

Mike Bourke, the Council’s Operations and Maintenance Manager says as the midge has a 21-day lifecycle, the Council used to spray the ponds every three-to-four weeks to keep the population under control.

"This year we increased the spraying to every fortnight and then every ten days. However monitoring and resident complaints have revealed the spray programme is not keeping the midges to an acceptable level which is why we’re seeking help from AgResearch and NIWA," says Mr Bourke.

There are several theories why the numbers are increasing. On Auckland’s North Shore the improving water quality at the Lake Rosedale wastewater pond has seen an increase in the number of midges. Here in Christchurch, the water quality in the 250 hectares ponds has also improved over the last two years which may be one cause of the problem.

Other possibilities include the warmer weather triggering an increase in the population around the Avon-Heathcote Estuary or a run of nor’east winds blowing the midges from the fresh water areas near the Estuary into the suburbs surrounding the wastewater plant.

Crown research institutes NIWA and AgResearch have been contracted to immediately review the existing midge spray programme and recommend improvements based on best practice elsewhere. They will also identify the species of midges living at the ponds and assess resistance of the pond midge population to the spray. If needed, they will recommend an alternative spray.

In the longer term, the scientists will review the Council’s monitoring programme and make recommendations for the ponds and surrounding area. They will also monitor the midges throughout the year to relate environmental conditions to midge outbreaks. The scientists will also review ways to control the midges including enhancing the biological diversity of the ponds, and controlling the midges through predation and competition.

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