The southern house mosquito doesn’t distinguish one source of sewage-polluted habitat from another. Wherever organically enriched water remains for more than seven days it could be producing vector mosquitoes. Sewage treatment facilities including those that treat large municipalities can be sources of southern house mosquito production. Though up to two-thirds of St. Tammany Parish relies on onsite wastewater treatment at each individual household or business, each of the eight municipalities treat sewage at centralized facilities. Additionally, many subdivisions treat sewage in medium-sized facilities (called package plants). Many of these package facilities are owned and managed by the Tammany Utilities public agency, but others are privately owned and operated by contracted utility providers.
Beginning in 2016, STPMAD started to surveil package treatment facilities for the production of southern house mosquitoes. Though most package plants kept water circulating inhibiting oviposition and development of mosquitoes, several became local sources of vector production. At these sites we initiated larval treatments to manage these populations until facilities managers could resolve the physical problems with the plants.
A collaboration began amongst STPMAD, Tammany Utilities, and other sewage treatment operators to educate their personnel to prevent mosquito production. Over the subsequent years we have given many presentations to educate sewage and water operators across Louisiana about the threat of sewage to produce mosquitoes.
In addition, Laboratory Manager/Entomologist Nick DeLisi performed a series of experiments to evaluate the efficacy of larvicides that can be used in package plants to manage both the treatment of mosquitoes and wastewater. Plant operators were concerned that insecticide application would cause their plants to fail an inspection.
However, none of the three treated plants were negatively impacted by our treatments. All insecticides were briquet formulations, and were suspended on the water surface by pool tablet dispensers. Bti killed approximately 85% of larvae after 24 hours, but efficacy dwindled after one week. Spinosad killed approximately 80% of larvae
after 24 hours, but also declined rapidly after the first week. Methoprene failed to inhibit eclosion at any point. Considering efficacy, non-target concerns, and cost, we
decided to use Bti for spot treatments when larvae are found in package plants.
Though any sewage-polluted water source can produce mosquitoes, the total area of ditches polluted by effluent from OWTS dwarfs the production potential from municipal or package treatment facilities in St. Tammany.