Midge Nuisance – A Swarm in a Pond

Swarms of midges can cause a seasonal nuisance near wastewater treatment ponds in New Zealand and around the world. New Zealand midges (Chironomus Zealandicus) lay eggs in layers of sediment along pond edges, whereupon larvae hatch and undergo four stages of development, prior to emergence as adults two to seven weeks later (depending on water temperature). They are not identified as a disease vector and, thus, do not pose health risk. However, midges seek out cool, shaded areas such as the underside of leaves and sheltered buildings, they are attracted to outdoor lighting at night, and can form large mating swarms. As a result midges can cause significant nuisance to nearby residents and recreationalists.

This paper summarises the outcome of a comprehensive literature review into control of midges at wastewater treatment ponds. The purpose of this review was to better understand the range of factors which can affect the intensity of midge swarms around a wastewater treatment pond. Four key factors were identified as impacting on midge population and resulting nuisance; these are pond sludge thickness, pond BOD loading, total pond bank area, and nearby vegetation planting. Persistent midge nuisance can occur for prolonged periods, over consecutive years. In these instances, a programme for pest management has been reported to successfully control the nuisance. Such programmes target the eggs, larvae, and adults at different stages separately and use different mechanisms/pathways.

Within the New Zealand context, midges do not create a nuisance at all wastewater treatment ponds. Ponds operated in Motueka, Nelson and Blenheim are not known to generate midge related complaints from nearby residents. Conversely, residents in close proximity to the Christchurch and Mangere wastewater treatment plants regularly report complaints during the spring and summer months every year.

This paper establishes a history of midge control in wastewater treatment ponds both globally and within the New Zealand context. A summary of the current knowledge of midge control around the world is presented. A qualitative assessment is made on the benefits and shortcomings of a range of midge control methods and methodology for the development of a midge management programme is proposed.