Do mosquito insecticides harm beneficial mosquitoes? Evidence from Toxorhynchites rutilus septentrionalis (Diptera: Culicidae).

Abstract

Chemical interventions targeting mosquito vectors may unintentionally affect non-target beneficial mosquitoes occupying similar ecological niches. This study evaluated the susceptibility of the beneficial mosquito Toxorhynchites rutilus septentrionalis (Dyar & Knab) to commonly used mosquito control products at operational field rates and compared its responses with those of Aedes aegypti L. Two adulticides, permethrin-based Aqua-Kontrol 30-30 and bifenthrin-based Talstar P Professional, and 3 larvicides, Vectobac 12AS (Bacillus thuringiensis israelensis [Bti]), Natular DT (spinosad), and Altosid Pellets WSP (methoprene), were evaluated. Permethrin ultra-low-volume space spray caused complete mortality in both adult Ae. aegypti and Tx. r. septentrionalis within 1 h post-treatment. In contrast, the bifenthrin-based barrier treatment on foliage caused 100% mortality in Ae. aegypti up to 28 d after application, whereas Tx. r. septentrionalis adults survived despite exhibiting transient signs of pyrethroid-induced neurotoxicity. Larval susceptibility varied according to developmental stage and insecticide type. Bti caused rapid mortality in Ae. aegypti larvae, particularly, early instars of Tx. r. septentrionalis showed 50 to 92% mortality and late instars showed lower mortality (4 to 66%). Spinosad caused 84 to 100% mortality in both mosquito species within 72 h post-exposure, except in early instars of Tx. r. septentrionalis at the lowest concentration (14% mortality). Methoprene inhibited adult emergence in Ae. aegypti at the highest concentration but produced minimal effects on Tx. r. septentrionalis. These findings demonstrate that mosquito control products differ substantially in their non-target impacts on beneficial mosquitoes and highlight the importance of incorporating ecological risk assessments into integrated vector management programs.