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
Alternatively, call 03 941 8999 (or 0800 800 169 for Banks Peninsula residents) for all incidents within Council-maintained areas.
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.
Preparing for the season ahead – September 2026
As part of our preparations for the forecast hot and dry summer, we've started our seasonal midge monitoring programme earlier than usual. Recent pond maintenance identified bloodworms, a natural stage in the midge lifecycle, and algae levels that can provide a food source for midges.
With significant changes made to the pond system and a hot, dry summer forecast, we're taking a proactive approach to better understand and manage any potential midge activity this season.
Part of our ongoing midge control programme 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 100,000 native plants around the oxidation ponds.
Midge control options
We implement a range of methods in an attempt to reduce the midge numbers, both short-term and long-term:
| Vegetation screening planting |
We’ve got a native planting programme underway in the paddocks surrounding the Wastewater Treatment Plant, which involves compost spreading to improve the soil conditions to give the native plants the best chance of surviving and thriving. Native vegetation barriers are a proven method of controlling midges from venturing to nearby residential areas. When this compost is applied, there can be a short-term localised odour as the compost is initially unloaded and spread in the paddocks. The duration of this odour is very brief and doesn’t impact residential areas. So far, more than 100,000 native plants have been planted, with thousands more planned over the coming years. View a PowerPoint presentation [PPTX, 14 MB] that Council staff made to elected members about the midge control and native planting programme in February 2023. |
|---|---|
| Mechanical disturbance |
We will mechanically disturb the sediment in the base of the ponds every 10 to 14 days throughout the midge season. We begin this in mid-October and 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 of 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.
