Larvicidal activity of the photosensitive insecticides, methylene blue and rose bengal, in Aedes aegypti and Anopheles gambiae mosquitoes

BACKGROUND: Insecticides are critical for controlling mosquito populations and mitigating the spread of vector-borne disease, but their overuse has selected for resistant populations. A promising alternative to classical chemical insecticides is photosensitive molecules - here called photosensitive insecticides or PSIs - that when ingested and activated by light, generate broadly toxic reactive oxygen species. This mechanism of indiscriminate oxidative damage decreases the likelihood that target site modification-based resistance evolves. Here, we tested whether the PSIs, methylene blue (MB) and rose bengal (RB), are viable insecticides across the mosquito lineage. RESULTS: MB and RB are phototoxic to both Aedes aegypti and Anopheles gambiae at micromolar concentrations, with greatest toxicity when larvae are incubated in the dark with the PSIs for 2 h prior to photoactivation. MB is ten times more toxic than RB, and microscopy-based imaging suggests that this is because ingested MB escapes the larval gut and disperses throughout the hemocoel whereas RB remains confined to the gut. Adding food to the PSI-containing water has a bidirectional, concentrationdependent effect on PSI toxicity; toxicity increases at high concentrations but decreases at low concentrations. Finally, adding sand to the water increases the phototoxicity of RB to Ae. aegypti. CONCLUSION: MB and RB are larvicidal via a light activated mechanism, and therefore, should be further investigated as an option for mosquito control.

Automated summary

This study tested the phototoxicity of methylene blue (MB) and rose bengal (RB) as photosensitive insecticides on mosquitoes. The results showed that MB is more toxic than RB, and ingested MB can disperse throughout the mosquito's body while RB remains in the gut. The concentration of food and the presence of sand in the water also affect the toxicity of the photosensitive insecticides.