Sodium channel activation underlies transfluthrin repellency in Aedes aegypti.

BACKGROUND: Volatile pyrethroid insecticides, such as transfluthrin, have received increasing attention for their potent repellent activities in recent years for controlling human disease vectors. It has been long understood that pyrethroids kill insects by promoting activation and inhibiting inactivation of voltage-gated sodium channels. However, the mechanism of pyrethroid repellency remains poorly understood and controversial. METHODOLOGY/PRINCIPAL FINDINGS: Here, we show that transfluthrin repels Aedes aegypti in a hand-in-cage assay at nonlethal concentrations as low as 1 ppm. Contrary to a previous report, transfluthrin does not elicit any electroantennogram (EAG) responses, indicating that it does not activate olfactory receptor neurons (ORNs). The 1S-cis isomer of transfluthrin, which does not activate sodium channels, does not elicit repellency. Mutations in the sodium channel gene that reduce the potency of transfluthrin on sodium channels decrease transfluthrin repellency but do not affect repellency by DEET. Furthermore, transfluthrin enhances DEET repellency. CONCLUSIONS/SIGNIFICANCE: These results provide a surprising example that sodium channel activation alone is sufficient to potently repel mosquitoes. Our findings of sodium channel activation as the principal mechanism of transfluthrin repellency and potentiation of DEET repellency have broad implications in future development of a new generation of dual-target repellent formulations to more effectively repel a variety of human disease vectors.

A dual-target molecular mechanism of pyrethrum repellency against mosquitoes.

Pyrethrum extracts from flower heads of Chrysanthemum spp. have been used worldwide in insecticides and repellents. While the molecular mechanisms of its insecticidal action are known, the molecular basis of pyrethrum repellency remains a mystery. In this study, we find that the principal components of pyrethrum, pyrethrins, and a minor component, (E)-ß-farnesene (EBF), each activate a specific type of olfactory receptor neurons in Aedes aegypti mosquitoes. We identify Ae. aegypti odorant receptor 31 (AaOr31) as a cognate Or for EBF and find that Or31-mediated repellency is significantly synergized by pyrethrin-induced activation of voltage-gated sodium channels. Thus, pyrethrum exerts spatial repellency through a novel, dual-target mechanism. Elucidation of this two-target mechanism may have potential implications in the design and development of a new generation of synthetic repellents against major mosquito vectors of infectious diseases.

First report of Ovomermis sinensis (Nematoda: Mermithidae) parasitizing fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) in China.

Spodoptera frugiperda invaded China in the end of 2018 and has caused severe damage to maize and other crops. Several S. frugiperda naturally parasitized by nematodes were observed in Hainan Province, China. The morphological characteristics based on the results of scanning electron microscopy indicated that the nematode belongs to the family Mermithidae. Additionally, coding sequences for the 18 S and 28 S rDNA were amplified from the nematode genome, and phylogenetic analysis revealed that the nematode belongs to Ovomermis sinensis, a known entomoparasitic nematode. Our finding is the first record that S. frugiperda was naturally parasitized by O. sinensis. The results of this study are of great significance for potential biological control of S. frugiperda by indigenous natural beneficial organisms, i.e. O. sinensis within an integrated pest management system.Spodoptera frugiperda invaded China in the end of 2018 and has caused severe damage to maize and other crops. Several S. frugiperda naturally parasitized by nematodes were observed in Hainan Province, China. The morphological characteristics based on the results of scanning electron microscopy indicated that the nematode belongs to the family Mermithidae. Additionally, coding sequences for the 18 S and 28 S rDNA were amplified from the nematode genome, and phylogenetic analysis revealed that the nematode belongs to Ovomermis sinensis, a known entomoparasitic nematode. Our finding is the first record that S. frugiperda was naturally parasitized by O. sinensis. The results of this study are of great significance for potential biological control of S. frugiperda by indigenous natural beneficial organisms, i.e. O. sinensis within an integrated pest management system.

Molecular characterization and functional expression of voltage-gated sodium channel variants in Apolygus lucorum (Meyer-Dür).

BACKGROUND: Apolygus lucorum (Meyer-Dür) is a serious worldwide agricultural pest, especially for Bt cotton in China. Pyrethroids, neonicotinoids and organophosphates are the most effective insecticides to control piercing and sucking insects, including A. lucorum. The voltage-gated sodium channel (Nav ) is major target site of pyrethroids. Extensive alternative splicing and RNA editing, two major post-transcriptional mechanisms, contribute to generate different functional sodium channel variants. In our research, we characterized the sodium channel variants of A. lucorum. RESULTS: In this study, we isolated numerous sodium channel variants that cover the entire coding region of the VGSC gene from A. lucorum. All clones could be grouped into 47 splice types based on the presence of nine alternative exons (exons j, n, o, a, p, b, s, q and t). Exons j, b and t were located independently, while exons n, o, a and p were located adjacently, as were exons s and q. We also found 35 nucleotide changes in different positions in individual variants, of which 18 nucleotide changes were A-to-I RNA editing, 11 nucleotide changes were likely due to U-to-C or C-to-U editing, and the others were likely natural sequence polymorphisms in the population. Furthermore, we expressed all of the variants in Xenopus oocytes. Eighteen of them were expressed in oocytes and sensitive to tetrodotoxin. CONCLUSION: Our results provide a functional basis for understanding how A. lucorum sodium channel variants work in regulating channel expression, pharmacology and gating properties for agricultural insects. Apolygus lucorum is widely distributed in cotton production. Our results suggest how AlNav (the sodium channel of A.lucorum) variants work in regulating channel expression, pharmacology and sodium channel gating for agricultural insects in the future. © 2020 Society of Chemical Industry.

Comparison of deltamethrin as indoor residual spray or on insecticide treated nets for mosquito control in Lake Chilwa.

We conducted a study on the control of mosquitos on Chisi Island in Lake Chilwa from August to November, 2006. The aim was to compare the cost and efficacy of deltamethrin, a pyrethroid based insecticide, when used in insecticide treated nets (ITN) and when used in indoor residual spray (IRS). Thirty village huts were enrolled in the study. Fifteen were systematically selected in a stratified manner and sprayed with deltamethrin following manufacturers' standard application procedures of 0.02g/m2. The remaining fifteen were provided with ITNs. In both groups deltamethrin KO tabs were used. Pyrethroid knockdown (PKD) spray was used for indoor rest captures in the houses monthly for three months. Houses treated with IRS had significantly reduced number of mosquitoes resting indoors than houses provided with nets (p<0.05). Based on the prevailing market price of MK550 each, the cost of five nets was calculated as MK2750.00 per house hold of five compared to five 20 g knock-out tablets costing MK300.00 using IRS. The cost of IRS is 10 times cheaper than ITN. These results suggest that it is cheaper and more effective to use deltamethrin in IRS than in ITN.