Midges

The native midge (Chironomus zealandicus) is a small fly, similar in appearance to its close cousin the mosquito. During the warmer summer months, 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.

Increase in midge numbers - January 2025

The wet weather in late December 2024 and January 2025 has had a positive impact on the biological health of the ponds. The volume of rain means wastewater entering the plant wasn’t as strong, and as a result, algae are flourishing.

Algae helps the pond system operate well by generating oxygen within the ponds and preventing the anoxic conditions which can cause odour. The fragile health of the ponds is demonstrated through recent odour monitoring, which shows intermittent, low levels of odour are being detected at the community odour monitoring sites. This is an improvement in odour levels in late 2024, which spiked during the transition to summer mode. Maintaining the health of the ponds is critical to ensuring odour is at a minimum. 

Unfortunately, algae are also a food source for midges and the increase in algae has resulted in a surge in the number of midges on and around the oxidation ponds, with numbers much higher than the previous few summers. 

We have a midge control programme in place which includes native planting around the oxidation ponds. Native plants encourage midges to stay close to the ponds rather than spreading to local residential and commercial areas. Since 2018 we have planted approximately 75,000 native plants around the oxidation ponds.

In the past, when the ponds were healthy, we dredged the bottom of the ponds and altered the flow paths through the ponds to reduce midge numbers. We’ve considered these control options but have decided against them at this time as there is a real risk of disturbing and unsettling the health of the ponds, which could generate odour. In addition to this, odour caused by a change in pond biology takes time to address, and in a worst-case scenario, an increase in odour could last until winter.  

The weather has a huge part to play in how our oxidation ponds operate, and the strength of wastewater entering the ponds is now increasing due to a recent patch of reasonably dry weather. If this weather continues it could make the pond conditions less suited to algae, which could in turn result in a decrease in midges. 

We know the midges are frustrating and are grateful for the ongoing patience of residents living in impacted areas. When making decisions about the next steps we’re balancing odour and midge management to try and get the best result for the community. We’ve engaged expert external advice to get a second opinion on what the impact of control measures would be and to ensure that we’re doing what we can. 

Midge control before the fire

In recent years, until the 2017/2018 season, we relied on dosing 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, cattle and in-home insect sprays. It is not very toxic and has no significant impact on bird or water life.

In 2017/1208 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 2018/2019, we didn't apply any chemicals to either 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.

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

In 2019/2020, 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% drop in the number of midges between the 2017/2018 season when we mainly used chemicals, and the 2019/2020 season.

We're continuing to implement a range of methods in an attempt to reduce the midge numbers, both short-term and long-term:

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.