During the warmer summer months people living near the Oxidation Ponds at Bromley may notice an increased number of midges.
We appreciate midges can be annoying, but they are not dangerous as they cannot bite. Normal fly spray will kill them, long-acting surface sprays are also effective. They are attracted to light, so ensure windows and doors are closed in the evenings.
For nearly 40 years, the Council has run a summer control programme from November each year, treating every few weeks through to April, depending on the climatic conditions.
The ponds are currently treated with slow-release methoprene pellets that prevent the larvae from developing into adults.
Methoprene is commonly found in flea treatments for dogs, cats, and cattle and in home insect sprays. It is not very toxic (except to fleas, midges etc) and does not impact significantly on bird or water life.
Over the summer months, the ponds are treated every 2.5-3 weeks. This is more frequent than the supplier's recommendations, increasing the frequency any more than this will have little effect.
We will be adapting midge control strategies used in Auckland, and will be trialling the use of a contact insecticide within the pond boundaries this summer. We are also investigating other alternative chemicals with a local company.
The Council is not required to treat the ponds, but we do it because we want 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 will be monitoring the ponds while we are using the insecticide.
There are several possible reasons why the numbers of midges increase:
The native midge (Chironomus Zealandicus) is a small fly which, while similar in appearance to their close cousin the mosquito. During the warmer temperatures of the 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 ponds at the Christchurch Wastewater Treatment Plant have become an ideal breeding site.
The midge lays its eggs directly into shallow freshwater pools and may attach them to plants or stones at the waters 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).
The emergence of the adult midge is temperature dependant, they require water temperatures greater that 17 degrees for optimal breeding. As temperatures drop below 17 deg the hatching time of the midges is slower. During autumn and winter they become dormant.
They are a freshwater fly, and need clean fresh water with lots of nutrients for ideal breeding conditions. The treatment ponds unfortunately are ideal. The banks of the ponds were damaged in the earthquakes and as a result the ponds are shallower and therefore warmer, making them even more ideal for the midges.