Goodbye flies: Swarming midge fly problem gone from Māngere

The pesky midge fly population on the Māngere foreshore is at its lowest ever thanks to the novel innovation of a Watercare environmental scientist determined to reduce chemicals for pest control.

Chris Garton’s world-first ‘lawn mowing’ approach involves a jet ski pulling multiple chains with spikes through the 17-hectare treated wastewater channel near our Māngere Wastewater Treatment Plant.

After an extensive controlled trial, which Garton completed his master’s thesis on, and following the peak of the midge fly season, he’s confident the method is a full success.

“It’s working wonders – we’ve now had more than two years with no midge complaints,” Garton says.

“Nobody wants these flies around their homes and in their community, including Watercare, we want to be good neighbours and doing everything we can to reduce the midge population certainly helps with this.”

His innovative chain drag method kills the midge fly larvae that live in the shallow water on impact and through burying them with the sediment that is temporarily stirred up.

If left untreated the flies can create huge clouds that cover houses, washing lines and swarm neighbourhoods.

The approach has quartered costs as we no longer need to import the expensive chemical methoprene from the USA for pest control.

Methoprene is a highly-targeted and environmentally safe chemical that was applied every three weeks using slow-release pellets and cost about $300,000 a year.

By comparison, the chain dragging method is done by a jet ski operator once a week and costs about $75,000 a year.

“It’s not that methoprene is a bad technique, but it’s very expensive and I knew there must be a better option that’s chemical free,” Garton explains.

“Results from the last two seasons have been better than when we were using methoprene and at a fraction of the cost.”

The main species that live in the water channel are a native midge fly called Chironomus zealandicus.

They look like mosquitos, but they don’t bite. They are prolific breeders, with females able to lay 1000 eggs at a time. They have a short lifecycle of about three weeks when the weather gets warm in the spring and summer.

A very small number can quickly multiply into billions and get out of control if not treated properly.

We originally stopped using methoprene and replaced it with the chain dragging method in August 2017.

“I was blown away by the reduction in population numbers over the first season of 2017/2018,” says Garton.

The midges are monitored every week by an experienced entomologist and the peak numbers received in the traps in the two seasons prior to using the chain method were 6352 and 4852.

But the 2017/2018 season saw this number drop to just 1554.

However, Garton wanted more data to back up his observations and scientifically prove his method.

From August 2018 to August 2019, he ran an experiment on the channel to compare the effectiveness of methoprene and the jet ski chain methods and monitor a 10 per cent controlled area that received no treatment at all.

“This experiment provided some excellent data,” Garton says.

“But because some areas were left untreated this, unfortunately, led to higher numbers of midges (about what we experienced when we were treating them with methoprene) around the channel from October 2018 – March 2019.

“Consequently, there was a big spike in population numbers from September 2019 to December 2019.”

The following months were spent getting the flies under control, but it’s been smooth sailing since then.

“Thanks to the chain dragging method the last two seasons have been the best ever recorded at the Māngere Wastewater Treatment Plant and we’ve had no midge complaints from the community since 2019.”

With two seasons of excellent results, Garton’s happy to say the flies are under control and will be writing up an academic paper documenting his findings to date.

His method is such a hit it’s now being used at our Rosedale Wastewater Treatment Plant and was also adopted by Christchurch City Council at the Bromley Wastewater Treatment Plant.

As far as he’s aware, there are no other wastewater treatment plants around the world that are using this method – yet.

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