A study into methods used to prevent a native midge species Chironomus zealandicus becoming a public nuisance at the Māngere Wastewater Treatment Plant in Auckland, New Zealand

Abstract

Aims. The three aims of this thesis were to measure the impact of two different treatment methods on Chironomus zealandicus larvae, investigate whether resistance to methoprene has developed in different populations of C. zealandicus, and investigate whether a linear relationship exists between environmental parameters and the seasonal emergence of C. zealandicus from the Final Effluent Channel (FEC). Methods. A Before After Control Impact (BACI) experiment was used to measure the efficacy of the chain dragging and methoprene treatment methods on midge emergence from the FEC at the Māngere Wastewater Treatment Plant (WWTP). A comparative toxicological experiment using larvae from different WWTPs with different methoprene usage histories was used to investigate the development of resistance. Seasonal constraints were investigated by analysing historical midge emergence, meteorological and water quality data using correlation analysis followed by linear regression models. Results. The BACI experiment showed a statistically significant impact from the chain dragging method on larvae in the sediment with a mean impact of -9,913 midges per m2. There was no detectable impact from the methoprene treatment. Evidence from the comparative toxicology experiment indicated that two populations may have developed resistance to methoprene with LC50 results of 4.8 μg/L and 4.1 μg/L compared to a reference population result of 1.6 μg/L. A correlation and linear regression analysis between environmental parameters and midge emergence from the FEC revealed high correlations between many of the parameters and statistically significant linear relationships between solar radiation and wind speed to midge emergence but with weak explanatory power. Conclusions. The chain dragging technique is effective at controlling chironomid emergence. It is currently being successfully used at the Māngere WWTP and Rosedale WWTP in Auckland and Bromley WTTP in Christchurch. Secondly, there is some evidence that C. zealandicus midges can develop resistance to methoprene with implications for future treatment efficacy. Lastly, the relationship between environmental parameters and midge emergence remains unproven. Direct causal relationships were not supported by the data and there is insufficient data to prove indirect relationships through the manipulation of environmental conditions that variations in these environmental parameters may cause.