Insecticidal and Synergistic Potential of Three Monoterpenoids against the Yellow Fever Mosquito, (Diptera: Culicidae), and the House Fly, (Diptera: Muscidae).

As resistance to the limited number of insecticides available for medical and veterinary pests becomes more widespread, there is an urgent need for new insecticides and synergists on the market. To address this need, we conducted a study to assess the toxicity of three monoterpenoids-carvone, menthone, and fenchone-in comparison to permethrin and methomyl against adults of two common pests: the yellow fever mosquito () and the house fly (). We also examined the potential for these monoterpenoids to enhance the effectiveness of permethrin and methomyl when used together. Finally, we evaluated the ability of each monoterpenoid to inhibit acetylcholinesterase, comparing them to methomyl. While all three monoterpenoids performed relatively poorly as topical insecticides (LD > 4000 ng/mg on ; >6000 ng/mg on ), they synergized both permethrin and methomyl as well as or better than piperonyl butoxide (PBO). Carvone and menthone yielded synergistic co-toxicity factors (23 and 29, respectively), which were each higher than PBO at 24 h. Currently, the mechanism of action is unknown. During preliminary testing, symptoms of acetylcholinesterase inhibition were identified, prompting further testing. Acetylcholinesterase inhibition did not appear to explain the toxic or synergistic effects of the three monoterpenoids, with IC values greater than 1 mM for all, compared to the 2.5 and 1.7 µM for methomyl on and , respectively. This study provides valuable monoterpenoid toxicity and synergism data on two pestiferous insects and highlights the potential for these chemistries in future pest control formulations.