Assessment of the efficacy of BTI Larvicide, VectoBac 12AS® (BTI AM65-52) against “nuisance” flies at water treatment plants in North West England

In treating wastewater using percolating filters, nature employs diverse biota, which includesnematoceran larvae, to sustain filter function. This study sought to generate evidence on theefficacy of a Bacillus thuringiensis var. israelensis-derived larvicide widely used in England forthe control of adult flies that emerge. This was in part-fulfilment of the European Union’s newbiocidal products licencing requirements. Adult and larval fly counts of four species of “nuisance flies”, Limnophyes minimus, Metriocnemuseurynotus, Psychoda alternata and Sylvicola fenestralis were monitored at two wastewatertreatment works (WWTW) in North West of England. At selected points relative to peak flypresence, selected filters were treated with the larvicide at concentrations of 80mg/L (low dose),160 mg/L (medium dose) or 230 mg/L (high dose). Fly reduction and effectiveness of differentdoses were determined based on pre- and post-treatment counts. A single treatment against S. fenestralis using the medium (recommended) dose achieved 100%larval and adult fly reductions within two and fourteen days respectively. Two treatments againstP. alternata at the low (recommended) dose achieved 93% and 95% larval and adult reductionsoverall, 48 hours and 23 days after the second treatment respectively. Forty-eight hours and onemonth after second treatment, larval and adult L. minimus respectively showed 100% and 30%overall reductions at low dose, 100% and 91% at medium (recommended) dose and, 100% and90% at high dose. At similar post-treatment points, larval and adult M. eurynotus showed 100%and 94% overall reductions respectively at low dose, 97% and 99% at medium (recommended)dose and 100% across both larvae and adults at high dose. The biolarvicide showed no effect against non-target organisms but was harmful and effectiveagainst target-species. Reduced and increased treatment strengths trialled against chironomidspecies showed slightly reduced efficacy and no added benefit respectively