BACKGROUND The mosquito, Aedes aegypti (Diptera: Culicidae), is a vector of dengue fever, zika, chikungunya, and yellow fever viruses, and in many areas possesses significant levels of resistance to pyrethroids. Behavioral performance was assessed in 15, 30, and 60 min exposures in a high throughput vapor phase spatial repellency assay to three contact repellent standards: N,N‐diethyl‐3‐methylbenzamide (DEET), ethyl 3‐[acetyl(butyl)amino] propanoate (IR3535), and 2‐undecanone, as well as pyrethrum extract, transfluthrin, and metofluthrin in susceptible (Orlando) and a pyrethroid‐resistant Puerto Rico strain of Aedes aegypti. Additionally, electroantennographic studies were used to investigate the antennal sensitivities to these compounds in both strains. RESULTS Resistance was found to all tested insect repellents in the Puerto Rico strain of Ae. aegypti. Resistance ratios at the different time points were about 2 for DEET, 3 for 2‐undecanone, and 12 for IR3535. Resistance was also observed to pyrethrum extract (∼9‐fold), transfluthrin (∼5‐fold), and metofluthrin (∼48‐fold) in repellent behavioral response. Electrophysiological analysis found decreased antennal sensitivity to all repellents tested, consistent with their behavioral effects. CONCLUSION The reduced sensitivity to these repellents may represent a fitness cost arising from the kdr mutation present in Puerto Rico Aedes aegypti. This work highlights the need for understanding collateral effects from the evolution of pesticide resistance in mosquitoes, and the importance of finding alternative strategies to control resistance development. © 2019 Society of Chemical Industry

Pyrethroids are one of the most commonly used classes of insecticides, and their acid and alcohol components are esterase degradation products, usually considered to be biologically inactive. In this study, it was found that several pyrethroid acids had a spatial repellent activity that was greater than DEET, often more active than the parent pyrethroids, and showed little cross resistance in a pyrethroid-resistant Puerto Rico strain of Aedes aegypti mosquitoes. Further investigation revealed that the acids can synergize not only contact repellent standards but also other pyrethroid components as well as the parent pyrethroids themselves. Synergism by the pyrethroid acids is expressed as both increased spatial repellency and vapor toxicity as well as human bite protection. Electrophysiological studies confirmed that pyrethroid acids (100 μM) had no effect on neuronal discharge in larval Drosophila melanogaster CNS and were detected by electroantennography, and there was little resistance to olfactory sensing of these acids in antennae from Puerto Rico strain mosquitoes carrying kdr mutations. Thus, the data suggest that the pyrethroid acids have a different mode of action than the parent pyrethroids, unrelated to the voltage-sensitive sodium channel. The results highlight the potential of pyrethroid acids to be useful in future repellent formulations.

Spatial repellents are an essential tool for personal protection against mosquitoes that bite and transmit disease pathogens to humans. Current repellent screening methods, such as olfactometers and alternative choice tests, are complex systems that require a relatively large quantity of compound (mg). The present study validates a high‐throughput spatial repellent screening method using a glass tube that has the ends covered with netting, in addition to treated filters and plastic end caps. The apparatus occupies relatively little space, is easy to decontaminate, and requires small amounts of compound (μg). In a horizontal tube orientation, DEET (N,N‐diethyl‐meta‐toluamide), citronella oil and IR3535 had 1 h half repellent concentration (EC50) values of 32, 32 and 298 μg/cm2, respectively, against the Orlando strain of Aedes aegypti (L.) (Diptera: Culicidae). Vertical tube orientation increased EC50 values by approximately two‐fold, except IR3535, which remained essentially unchanged. Transfluthrin showed concentration‐dependent spatial repellency (1 h EC50 = 0.5 μg/cm2) without any knockdown, although only in vertical tubes. Transfluthrin showed 50% knockdown in 1 h at 0.5 μg/cm2 and 50% mortality at 0.15 μg/cm2 in horizontal tubes. In conclusion, this high‐throughput screening method is useful for assessing vapour toxicity and the spatial repellency of candidate molecules prior to semi‐field and field studies.

Mosquitoes, such as Aedes aegypti and Anopheles gambiae, are important vectors of human diseases. Fluralaner, a recently introduced parasiticide, was evaluated as a mosquitocide in this study. On Ae. aegypti and An. gambiae fourth-instar larvae, fluralaner had 24-h LC50 (lethal concentration for 50% mortality) values of 1.8 ppb and 0.4 ppb, respectively. Following topical application to adult Ae. aegypti, fluralaner toxicity reached a plateau in about 3 days, with 1- and 3-day LD50 (lethal dose for 50% mortality) values of 1.3 ng/mg and 0.26 ng/mg, suggesting a slowly developing toxicity. Fipronil outperformed fluralaner by up to 100-fold in adult topical, glass contact, and feeding assays on Ae. aegypti. These data show that fluralaner does not have exceptional toxicity to mosquitoes in typical exposure paradigms. In electrophysiological recordings on Drosophila melanogaster larval central nervous system, the effectiveness of fluralaner for restoring nerve firing after gamma-aminobutyric acid (GABA) treatment, a measure of GABA antagonism, was similar in susceptible Oregon-R and cyclodiene-resistant rdl-1675 strains, with EC50 (half maximal effective concentration) values of 0.34 µM and 0.29 µM. Although this finding suggests low cross resistance in the presence of rdl, the moderate potency, low contact activity, and slow action of fluralaner argue against its use as an adult mosquitocide for vector control.

The present study focused on the toxicity of the aphid anti-feedant flonicamid and its main metabolite, 4-trifluoromethylnicotinamide (TFNA-AM) to Aedes aegypti and Anopheles gambiae mosquitoes.The compounds were toxic to both species via topical application, resulting in un-coordinated locomotion and leg splaying, with a favorable An. gambiae LD 50value of 35 ng/mg for TFNA-AM, but no significant lethality to Ae. aegypti at 10 μg/female.There was mild cross resistance in the Akron-kdr (Akdr) strain of An. gambiae. Both compounds were non-toxic to intact larvae (LC 50 >300 ppm); however, headless Ae. aegypti larvae displayed spastic paralysis, with PC 50 values of 2-4 ppm, indicating that the cuticle is a significant barrier to penetration. TFNA-AM showed low mammalian toxicity, with an LD 50 of >2000 mg/kg in mice. Electrophysiological experiments showed larval Aedes muscle depolarization and Kv2 channel blocking activity that required near mM concentrations, suggesting that this potassium channel is not the main target for flonicamid nor its metabolite.However, TFNA-AM was a potent blocker of evoked body wall sensory discharge in dipteran larvae, suggesting that some component of the chordotonal organ system may be involved in its toxicity. Finally, flonicamid and TFNA-AM showed about 2-fold synergism of permethrin toxicity against An. gambiae adult females whose mechanism should become more clear once the mode of action of these compounds is better defined.