Rapid Immobilization of Adult Aedes aegypti Caused By Plant Essential Oils At Sublethal Concentrations

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

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.

Mosquitocidal activity of p,p'-difluoro-diphenyl-trichloroethane (DFDT)

New insecticides are urgently needed for the control of arthropod vectors of public health diseases. As resistance to many insecticides used for the control of public health pests is ubiquitous, all available chemistries should be evaluated for their potential to effectively control both insecticide-susceptible and insecticide-resistant strains of mosquitoes. This study aimed to evaluate p-p'-difluoro-diphenyl-trichloroethane (DFDT) as a mosquito control technology and relate its activity to that of DDT. We found that topical DFDT was significantly less toxic than DDT to both pyrethroid-susceptible and pyrethroid-resistant strains of Anopheles gambiae and Aedes aegypti. Direct nervous system recording from Drosophila melanogaster CNS demonstrated that DFDT is approximately 10-times less potent than DDT at blocking nerve firing, which may explain its relatively lower toxicity. DFDT was shown to be at least 4500 times more vapor-active than DDT, with an LC50 in a vapor toxicity screening assay of 2.2 μg/cm2. Resistance to DFDT was assessed in two mosquito strains that possess target-site mutations in the voltage-gated sodium channel and upregulated metabolic activity. Resistance ratios for Akdr (An. gambiae) and Puerto Rico (Ae. aegypti) strains were 9.2 and 12.2, respectively. Overall, this study demonstrates that DFDT is unlikely to be a viable public health vector control insecticide.