Larval mosquito control is one of the primary means of managing mosquito populations. A frequent control method for larval mosquitoes is the use of Bacillus thuringiensis var. israelensis (Bti), an insecticidal bacterium whose toxic activity is linked to the alkaline environment of the mosquito midgut. However, Bti applications have been known to fail to induce desired mortality in some situations. To determine if environmental pH may play a role in treatment efficacy, we conducted a series of experiments examining environmental pH and its effect on the toxicity of Bti to the mosquito Culex quinquefasciatus. First, we conducted dose-response testing of Bti on Cx. quinquefasciatus larvae to determine LC50 under a neutral pH. Next, we exposed Cx. quinquefasciatus to a putative LC50 of Bti in water of varying pH. Finally, we surveilled natural waterways in St. Tammany Parish, LA to assess what pHs may be present under natural conditions. We found that larval mortality was positively correlated with environmental pH, and that label-rate applications of Bti could fail to induce complete mortality in more acidic environments. Additionally, we found that, although rare, these pHs could be found in naturally occurring environments in Louisiana.
McNamara et al. (2026) studied this question.