Vaporous essential oils and isolates restore pyrethroid-treated netting efficacy to Aedes aegypti (Diptera: Culicidae)

Decreasing opportunities for mosquitoes to bite is critical in the reduction of mosquito-borne pathogen transmission, such as Plasmodium spp. or dengue, yellow fever, and West Nile viruses. Field-evolved resistance to a large selection of synthetic insecticides is crippling efforts to reduce mosquito populations and new strategies are necessary to sustain the efficacy of commercially available tools. Aedes aegypti (L.), among other species, have evolved pyrethroid resistance in the field and the pyrethroid-resistant Puerto Rico (PR) strain is a valuable tool for understanding resistance mechanisms. A previous study showed that pyrethroid efficacy can be enhanced by pre-exposing the PR strain to essential oil vapors before topical application of deltamethrin. Insecticide-treated nets (ITNs) and long-lasting insecticidal nets (LLINs) are current products commercialized for mosquito bite protection, but nets using pyrethroids are losing efficacy in the field due to field-evolved pyrethroid resistance. This study tested essential oils previously identified to affect pyrethroid efficacy, as well as their main isolates, to assess if they can restore the efficacy of pyrethroid-treated LLIN against the PR strain. We show that although amyris (Amyris balsamifera) expectedly decreases net efficacy, increased mortality can be obtained after exposure to tagetes (Tagetes bipinnata) and cajeput (Melaleuca cajuputi) oils, but also after exposure to their isolates, such as dihydro tagetone and ocimene, from 1 h after exposure to the net. This study provides a selection of promising synergists used as vaporous emanations to restore pyrethroid efficacy and counteract field-evolved resistance in mosquitoes.