The phototoxicity of certain dyes against the West Nile vector, Culex pipiens larvae (Diptera: Culicidae)

Mosquitoes play a crucial role in the transmission of some dangerous infectious diseases. Many chemical insecticides used to control mosquitoes are toxic and ineffective because of insects’ resistance. Light activation of dyes produces singlet oxygen and reactive oxygen species, which are insecticidal, in photodynamic insecticides (PDI), and scientists have long recognized this efficient alternative. Three dyes (alizarin red, bromothymol blue, and methylene blue) were evaluated as photosensitizers against the 3rd instar larvae of Culex pipiens. Data show that bromothymol blue has the highest photodynamic toxicity with an LC50 of 0.0026 mg/L, while alizarin red and methylene blue showed lower efficacy with LC50 values of 0.0505 mg/L and 0.5134 mg/L after 6 h post-treatment, compared to chlorpyrifos (0.6211 mg/L). Bromothymol blue was 238.88 times more potent than chlorpyrifos, and its efficacy increased with extended sunlight exposure, with LC50 values decreasing from 0.0204 mg/L (8:00–10:00 am) to 0.0025 mg/L (12:00 pm–14:00 pm). Absorption wavelengths of the three tested photosensitizers were recorded in the range of 200–800 nm. The high-energy molecular optical gap calculations, of the least concentration used, revealed that bromothymol blue exhibits the lowest energy (4.11 eV), compared to alizarin red (4.75 eV) and methylene blue (5.2 eV), suggesting higher reactivity and potentially increasing phototoxicity of bromothymol blue. Bromothymol blue showed the highest effect against C. pipiens larvae, supported by the lowest molecular bandgap energy. Therefore, it could contribute to integrated management programs and serve as an alternative to conventional insecticides.