Co-toxicity factor analysis reveals numerous plant essential oils are synergists of natural pyrethrins against Aedes aegypti mosquitoes

Authors: Norris, Edmund; BLOOMQUIST, JEFFREY - University Of Florida

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2021
Publication Date: 2/11/2021
Citation: Norris, E.J., Bloomquist, J.R. 2021. Co-toxicity factor analysis reveals numerous plant essential oils are synergists of natural pyrethrins against Aedes aegypti mosquitoes. Insects. 12(2):154,1-11. https://doi.org/10.3390/insects12020154.
DOI: https://doi.org/10.3390/insects12020154

Interpretive Summary: With insecticide-resistant mosquito populations becoming an ever growing concern, new vector control technologies are needed. There is a general lack of new insecticides being developed, which may hinder public health pest control in the near future. Plant essential oils represent new insecticides and repellents, which are generally safer to mammals and non-target organisms than conventional insecticides. We screened a set of 20 plant essential oils alone and in combination with a natural insecticide, natural pyrethrins, against the yellow fever mosquito. We assessed the potential of these plant essential oils to produce immobilization (knockdown at 1 hr) and lethality when applied alone and to improve or reduce the intrinsic knockdown and mortality produced by natural pyrethrins when applied in combination. Overall, very few oils were effective when applied alone at the doses used in this study. However, we found that a number of these oils significantly synergized and antagonized the toxicity of natural pyrethrins when applied in mixture. These findings highlight select novel plant essential oils that may represent important leads for the development of future insecticide synergists. Moreover, synergism or antagonism was highly dependent on the amount of the oil applied (e.g. some oils were more effective at improving natural pyrethrins toxicity at low doses, whereas others were better at improving its toxicity at high doses). These data demonstrate that the effects of these natural product-containing mixtures are dependent on the dose of each individual constituent. This is an important consideration in the development of future insecticidal formulations that contain natural products.

Technical 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 1h and mortality at 24h was monitored. Of the oils, a majority of oils increased the toxicity of natural pyrethrins, either via an additive or synergistic profile. Of the oils, many 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 successful oils for increasing the mortality of natural pyrethrins at 24h with co-toxicity factors greater than 50 at either or both the 2 and 10 µg/insect application level. A number of oils also significantly improved the 1h knockdown of natural pyrethrins at these application levels. Of these, fir needle oil and cypress oils were the most successful at improving the speed-of-action of natural pyrethrins across both application levels, with co-toxicity factors of 130 and 62, respectively. To further assess the co-toxicity factor method, we applied select plant essential oils with variable concentrations of natural pyrethrins to calculate synergism ratios. Only the oils that produced synergistic co-toxicity factors produced 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). This study demonstrates the co-toxicity factor is nonetheless a useful tool in screening for synergistic combinations.