Co-Toxicity Factor Analysis Reveals Numerous Plant Essential Oils Are Synergists of Natural Pyrethrins against Aedes aegypti Mosquitoes

Abstract

With insecticide-resistant mosquito populations becoming an ever-growing concern, new vector control technologies are needed. With the lack of new chemical classes of insecticides to control mosquito populations, the development of novel synergists may improve the performance of available insecticides. We screened a set of 20 plant essential oils alone and in combination with natural pyrethrins against Aedes aegypti (Orlando) female adult mosquitoes to assess their ability to synergize this natural insecticide. A co-toxicity factor analysis was used to identify whether plant oils modulated the toxicity of natural pyrethrins antagonistically, additively, or synergistically. Both knockdown at 1 h and mortality at 24 h were monitored. A majority of oils increased the toxicity of natural pyrethrins, either via an additive or synergistic profile. Many oils produced synergism at 2 µg/insect, whereas others were synergistic only at the higher dose of 10 µg/insect. Amyris, cardamom, cedarwood, and nutmeg East Indies (E.I.) oils were the most active oils for increasing the mortality of natural pyrethrins at 24 h with co-toxicity factors greater than 50 at either or both doses. A number of oils also synergized the 1 h knockdown of natural pyrethrins. Of these, fir needle oil and cypress oils were the most successful at improving the speed-of-action of natural pyrethrins at both doses, with co-toxicity factors of 130 and 62, respectively. To further assess the co-toxicity factor method, we applied selected plant essential oils with variable doses of natural pyrethrins to calculate synergism ratios. Only the oils that produced synergistic co-toxicity factors produced statistically significant synergism ratios. This analysis demonstrated that the degree of co-toxicity factor correlated well with the degree of synergism ratio observed (Pearson correlation coefficient r = 0.94 at 2 µg/insect; r = 0.64 at 10 µg/insect) and that the co-toxicity factor is a useful tool in screening for synergistic activity.

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

“…For Ae. aegypti , synergism assays were performed similar to the topical applications described previously, with the following modifications: 2 doses (2 µg/mosquito and 10 µg/mosquito) of each monoterpenoid were applied as synergists, similar to previous studies exploring the synergistic potential of natural products in combination with an intermediate dose-level of insecticide alone [ 51 ]. The concentrations of permethrin (0.6 ng/mosquito) and methomyl (6 ng/mosquito) used were chosen as they produced an average mortality among replicates between 10 and 75% at 24 h, with a 24-h mortality of 36 ± 6% and 57.5 ± 25.3%, respectively.…”

Efficacy of metal nanoparticles as a control tool against adult mosquito vectors: A review

“…Future research efforts should seek to address these areas. Plants possess bioactive compounds including alkaloids, ketones, tannins and essential oils which have demonstrated insecticidal effects, but many of these same compounds can also act as synergists with existing insecticides (66,67). By synthesizing metal nanoparticles in conjunction with these bioactive compounds, researchers aim to confer beneficial properties to enhance insecticide formulations, thus providing potential new tool in managing resistance in mosquito populations.…”

Insecticidal and Synergistic Potential of Three Monoterpenoids Against the Yellow Fever Mosquito, Aedes aegypti (Diptera: Cu-Licidae), and the House Fly, Musca domestica

“…aegypti, synergism assays were performed similar to the topical applications described previously, with the following modifications. Two doses (2 µg/mosquito and 10 µg/mosquito) of each monoterpenoid were applied as synergists similar to previous studies exploring the synergistic potential of natural products in combination with an intermediate dose-level of insecticide alone [50]. The concentration of permethrin (0.6 ng/mosquito) and methomyl (6 ng/mosquito) used were chosen as they produced an average mortality among replicates between 10-75% at 24 hr, with a 24-hr mortality of 36 ± 6% and 57.5 ± 25.3%, respectively.…”