During the warmer summer months people living near the oxidation ponds at Bromley may notice an increased number of midges.

What are midges?

The native midge (Chironomus Zealandicus) is a small fly, similar in appearance to its close cousin the mosquito. During the warmer summer months the adult flies can form large swarms, which can reach nuisance proportions.

The native midge has always lived in the still, fresh waters near the Avon-Heathcote Estuary. Lake Ellesmere, Lake Forsyth, and around Horseshoe Lake are also common breeding spots of the midge. The oxidation ponds at the Christchurch Wastewater Treatment Plant have become an ideal breeding site, with their nature as a wastewater treatment process creating an imbalanced ecosystem that favours one species over another – in this case midges.

The midge lays its eggs directly into shallow freshwater pools and may attach them to plants or stones at the water's edge. They have a 20 to 40 day lifecycle (depending on water temperature) and only spend two to three days as adults, during which time they mate and breed.

Adult midges require water temperatures greater than 17 degrees for optimal breeding. During autumn and winter they become dormant.

What we are doing to control midges

Traditionally the Council relied on chemical dosing alone to try and control the midges. This has had mixed results, and an integrated approach using various control techniques is required. That's why this year, the Council has produced an Integrated Midge Management Plan (IMMP).

For the 2017/18 season, Methoprene is being dosed into the ponds from the first week of October. Methoprene prevents midge larvae from developing into adults, and is commonly found in flea treatments for dogs, cats and cattle and in home insect sprays. It has a low toxicity and no significant impact on bird or water life. It is being dosed into the ponds every two-to-four weeks in a slow-release pellet form.

A contact insecticide is also being applied to the vegetation around the ponds. This will act like the domestic long-lasting surface fly sprays. As with Methoprene, the chemical has a low toxicity and precautions are being taken to avoid spray drift.

As part of the IMMP, a robust monitoring programme has been established to track the number of midges as they emerge from the ponds and the number surrounding the ponds. This programme will allow the Council to evaluate how the control techniques are working.

What else we are doing:

  • vegetation screening planting (to block the direct light from the nearby residential areas that attracts midges)
  • UV traps (a UV light attracts midges that are then trapped in a bag)
  • light attraction and insecticide spray (midges are attracted by light to an area that is regularly ‘fogged’ with spray)
  • pond optimisation (assessing whether we have enough capacity at the treatment plant to be able to reduce the number of ponds, which would mean fewer areas for midges to spawn)
  • sun bakes (regularly draining water out of the ponds to let them dry, killing the midge larvae).

The Council is not required to treat the ponds, but we do it to be a good neighbour to the residents of Bromley and Aranui.

The Council continues to meet all Hazardous Substances and New Organisms requirements under the Act, including recording the amounts of organophosphate in our ponds. Additionally, an ornithologist monitors the ponds while we use the insecticide. 

Pond dosing and contact insecticide spraying

  Last date Next planned date
Pond dosing 6 to 7 November 2017 20 to 21 November 2017
Insecticide spraying 7 to 8 November 2017 21 to 22 November 2017