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

Report a problem with pests, odour or insects

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 dosing of the chemical Methoprene to try and control the midges. 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 is not very toxic and has no significant impact on bird or water life. Over the last midge season (2017/18), the Methoprene was dosed into the ponds every two weeks and the amount applied was a big increase over previous year. This was alongside the application of contact insecticide onto the vegetation surrounding the ponds. This approach had mixed results – it appeared to work early on, but midge numbers returned to their historic high numbers later in the season.

This year, the Council is implementing a range of methods in an attempt to reduce the midge numbers, both short-term and long-term:

Vegetation screening planting Vegetation is being planted in the paddocks surrounding the oxidation ponds this year. In the long-term, this should discourage midges from moving from the ponds to the nearby residential area. The vegetation should also help block light from the residential area, which attracts midges.
Sun bakes Improvement work will continue on the ponds’ infrastructure. This is a long-term measure that will enable more of the ponds to be emptied regularly throughout the year, temporarily drying out the sediment and killing the midge larvae.
Mechanical disturbance In the short-term, a method we call mechanical disturbance will be tried for the first time this coming 2018/19 season. Midge larvae are at their most sensitive when buried in the sediment at the bottom of the ponds to grow and hatch, and this method involves physically disturbing the sediment by dragging chains or something similar across the bottom. In other areas around the country where midges are a problem, this approach has resulted in a high rate of mortality of the larvae.

The Council is not required to treat the midges in 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. 

Timeframes for mechanical disturbance

  Last date Next planned date
Mechanical disturbance 13–14 December TBC