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

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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.

In recent years, until the 2017/18 season, we 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. In 2017/18 Methoprene was dosed into the ponds every two weeks and the amount applied was a big increase over the 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.

In 2018/19, we didn't apply any chemical to either to the ponds or the vegetation. Instead we undertook a trial of mechanical disturbance. 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.

Comparison of the data collected from 30 midge monitoring traps, showed a 35% drop in the number of midges between the 2017/18 season (when chemicals were the primary form of control) and the 2018/19 season (when mechanical disturbance was primary form of control).

In 2019/20, we continued to use mechanical disturbance, and didn’t apply any chemicals to the oxidation ponds. We also saw benefits from the native vegetation we planted in the area, which is now well established, and by altering the flow paths through our ponds. The result has been a 73 per cent drop in the number of midges between the 2017/18 season, when we mainly used chemicals, and the 2019/20 season.

This year, we will continue to implement a range of methods in an attempt to reduce the midge numbers, both short-term and long-term:

Vegetation screening planting We will plant around 10,000 trees and shrubs in the paddocks between the oxidation ponds and Cuthberts Road this year, in addition to the 24,000 we planted over the last two years. 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.
Mechanical disturbance We will mechanically disturb the sediment in the base of the ponds every 10–14 days throughout the midge season. We will begin this in mid-October and will continue until the number of midges reduces at the end of the season.
Changing flow paths We have six oxidation ponds in total, and some recent infrastructure improvements at the plant mean we can now reverse the flow of water through the first two ponds, and alternate the water quality. Midges are very sensitive to changes in water quality at the early stages in their life-cycle, and this should help reduce numbers. This won't affect the treatment and water quality of the wastewater effluent at the end of the pond system.

We are 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.