Pyrethrins elicit olfactory response and spatial repellency in Aedes albopictus

A critical review of current laboratory methods used to evaluate mosquito repellents

Pathogens transmitted by mosquitoes threaten human health around the globe. The use of effective mosquito repellents can protect individuals from contracting mosquito-borne diseases. Collecting evidence to confirm and quantify the effectiveness of a mosquito repellent is crucial and requires thorough standardized testing. There are multitudes of methods to test repellents that each have their own strengths and weaknesses. Determining which type of test to conduct can be challenging and the collection of currently used and standardized methods has changed over time. Some of these methods can be powerful to rapidly screen numerous putative repellent treatments. Other methods can test mosquito responses to specific treatments and measure either spatial or contact repellency. A subset of these methods uses live animals or human volunteers to test the repellency of treatments. Assays can greatly vary in their affordability and accessibility for researchers and/or may require additional methods to confirm results. Here I present a critical review that covers some of the most frequently used laboratory assays from the last two decades. I discuss the experimental designs and highlight some of the strengths and weaknesses of each type of method covered.

Transfluthrin enhances odorant receptor-mediated spatial repellency in Aedes aegypti

Mosquito-borne diseases are an increasing global health challenge and vector-based interventions remain the most important methods for reducing the public health burden of most mosquito-borne diseases. Transfluthrin, a volatile pyrethroid insecticide, is used widely and effectively as an insecticide and as a repellent. In a recent study, we showed that at very low concentrations transfluthrin repels Aedes aegypti mosquitoes in a hand-in-cage assay without detectable stimulation of antennal olfactory responses. Furthermore, activation of sodium channels by transfluthrin enhances repellency by DEET, which has been reported to repel mosquitoes via odorant receptor (Or)-dependent and Or-independent mechanisms. However, whether activation of sodium channels by transfluthrin can serve as a general mechanism for synergizing the activities of other repellents remain unknown. In this study, we found that, in hand-in-cage assay, transfluthrin enhanced repellency by geranyl acetate and (E)-β-farnesene, which activate AaOr31. Such enhancement was not observed in AaOr31-knockout mosquitoes and dampened in a pyrethroid-resistant strain carrying two sodium channel mutations, which reduce the action of transfluthrin on sodium channels. In addition, transfluthrin also enhanced repellency by (-)-borneol, (±)-citronellal, camphor, and eucalyptol, which activate Or-mediated repellency. Our study has uncovered the ability of transfluthrin to enhance the repellency to a variety of mosquito repellents, beyond DEET, and provided experimental support for the emerging paradigm of synergistic interactions between repellency mediated by sodium channel activation and Or activation. These findings have potential implications in the development of more effective mosquito repellent mixtures.

Natural and Synthetic Pyrethrins Act as Feeding Deterrents against the Black Blowfly, Phormia regina (Meigen)

Pyrethrum is a botanical insecticide derived from pyrethrum flowers. Feeding deterrence caused by pyrethrum has been reported in several sucking insects; however, there is no account of the cause of deterrence—whether from a single component or the combination of six active ingredients, called pyrethrins. We determined the feeding deterrence of natural pyrethrins, their two main components (pyrethrins I and II), and pyrethroid insecticides on the blowfly, Phormia regina. In a dual-choice feeding assay that minimized tarsal contact with food sources but allowed feeding through proboscises, natural pyrethrins, synthetic pyrethrins I/II, and allethrin were observed to induce deterrence at a concentration 16 times lower than the lowest concentration at which the knockdown rate increased. Feeding bouts were interrupted by intensive grooming of the proboscis at the deterring concentration, but no such grooming was observed to occur while feeding on the unpalatable tastants—NaCl, quinine, and tartaric acid. The underlying mode of action for the feeding deterrence of pyrethrins at sub-lethal concentrations probably occurs on the fly oral gustatory system, while differing from that of unpalatable tastants. The potent feeding deterrence of pyrethrins may provide effective protection for pyrethrum plants by rapidly deterring insects from feeding, before insecticidal activities occur.

Bioallethrin activates specific olfactory sensory neurons and elicits spatial repellency in Aedes aegypti

BACKGROUND

Use of pyrethroid insecticides is a pivotal strategy for mosquito control globally. Commonly known for their insecticidal activity by acting on voltage-gated sodium channels, pyrethroids, such as bioallethrin and transfluthrin, are used in mosquito coils, emanators and other vaporizers to repel mosquitoes and other biting arthropods. However, whether specific olfactory receptor neurons are activated by pyrethroids to trigger spatial repellency remains unknown.

RESULTS

We used behavioral and electrophysiological approaches to elucidate the mechanism of bioallethrin repellency in Aedes aegypti, a major vector of dengue, yellow fever, Zika and chikungunya viruses. We found that bioallethrin elicits spatial (i.e. non-contact) repellency and activates a specific type of olfactory receptor neuron in mosquito antennae. Furthermore, bioallethrin repellency is significantly reduced in a mosquito mutant of Orco, an obligate olfactory co-receptor that is essential for the function of odorant receptors (Ors). These results indicate that activation of specific Or(s) by bioallethrin contributes to bioallethrin repellency. In addition, bioallethrin repellency was reduced in a pyrethroid-resistant strain that carries two mutations in the sodium channel gene that are responsible for knockdown resistance (kdr) to pyrethroids, indicating a role of activation of sodium channels in bioallethrin repellency.

CONCLUSION

Results from this study show that bioallethrin repellency is likely to be the result of co-activation of Or(s) and sodium channels. These findings not only contribute to our understanding of the modes of action of volatile pyrethroids in spatial repellency, but also provide a framework for developing new repellents based on the dual-target mechanism revealed. © 2021 Society of Chemical Industry.

Molecular and functional characterization of a conserved odorant receptor from Aedes albopictus

Background

The Asian tiger mosquito Aedes albopictus is a competent vector of several viral arboviruses including yellow fever, dengue fever, and chikungunya. Several vital mosquito behaviors (e.g., feeding, host-seeking, mating, and oviposition) are primarily dependent on the olfactory system for semiochemicals detection and discrimination. However, the limited number of studies hampers our understanding of the relationships between the Ae. albopictus olfactory system and the complex chemical world.

Methods

We performed RT-qPCR assay on antennae of Ae. albopictus mosquitoes of different sexes, ages and physiological states, and found odorant receptor 11 (AalbOr11) enriched in non-blood-fed female mosquitoes. Then, we examined the odorant preference with a panel of physiologically and behaviorally relevant odorants in Xenopus oocytes.

Results

The results indicated that AalbOr11 could be activated by ten aromatics, seven terpenes, six heterocyclics, and three alcohols. Furthermore, using post-RNA interference (RNAi) hand-in-cage assay, we found that reducing the transcript level of AalbOr11 affected the repellency activity mediated by (+)-fenchone at a lower concentration (0.01% v/v).

Conclusions

Using in vitro functional characterization, we found that AalbOr11 was a broadly tuned receptor. Moreover, we found that AalbOr11 shared a conserved odorant reception profile with homologous Anopheles gambiae Or11. In addition, RNAi and bioassay suggested that AablOr11 might be one of the receptors mediating (+)-fenchone repellency activity. Our study attempted to link odor-induced behaviors to odorant reception and may lay the foundation for identifying active semiochemicals for monitoring or controlling mosquito populations.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05158-1.