Biology subjects

Lees, R. S.

Publications and source records attributed to Lees, R. S..

6 recordsLinked to original sources

bioRxiv2026

Multi-site phenotypic characterisation of Kisumu colonies for comparison of insecticide susceptibility between testing centres

Testing of insecticides and insecticide-based vector control tools relies on the use of bioassays whereby insects are exposed and mortality, or another relevant entomological endpoint, is measured. To be able to compare results between products and across time and space standard laboratory strains are used. Insecticide-susceptible strains are often used as comparators in studies involving insecticide-resistant populations. Kisumu is a long-established, widely-used insecticide-susceptible strain of Anopheles gambiae. In this study, 16 facilities across different countries exposed their Kisumu colonies aged 3-5 days to four insecticides (permethrin, alpha-cypermethrin, DDT and pirimiphos-methyl) at a range of concentrations in WHO tube assays to assess susceptibility. Insecticide-treated filter papers were made at the Liverpool School of Tropical Medicine and sent to testing centres, along with a standard protocol and testing conditions requirements, to standardise testing as much as possible. A first observation was that DDT results were affected by the use of Risella oil, the WHO-recommended carrier oil but immiscible in acetone, leading to uneven coating of papers and increased variability in bioassay results (40x variance between colonies). Bioassay data generated by the testing facilities suggest a wide range of susceptibility to different insecticides between colonies of Kisumu: LC50 values between colonies differed, though not significantly, by up to 4x for permethrin, 6x for pirimiphos-methyl, and 15x for alpha-cypermethrin. Some colonies are classified as insecticide resistant based on the WHO definition of <98% mortality on exposure to a discriminating concentration. The assumption that Kisumu is a standard susceptible strain, consistent between facilities is challenged, which has implications when comparing results between studies. We propose genotypic analysis of the colonies to identify signs of resistance markers indicative of contamination from resistant strains or wild mosquitoes, and generation of guidance to support the maintenance and monitoring of susceptible mosquito strains.

zoology↗

bioRxiv2026

The Two Tube Volatile Assay: a non-contact benchtop bioassay for monitoring susceptibility to transfluthrin

IntroductionProgress against malaria has stalled since 2015, with insecticide resistance a key driver. Spatial emanators release volatile insecticides into the air, exposing mosquitoes through a route distinct from the tarsal contact used by treated nets, indoor residual spraying, and standard bioassays. Transfluthrin is currently monitored using the WHO bottle bioassay, which combines contact and vapour exposure and cannot isolate airborne effects. A scalable, vapour-only method is needed to characterise susceptibility to volatile pyrethroids. MethodsWe adapted the WHO tube bioassay to deliver non-contact transfluthrin vapour exposure without contacting treated papers. Non-blood-fed female Anopheles were exposed to acetone control or transfluthrin papers at 0.005, 0.1 and 2 mg/paper. Knockdown was recorded over 60 minutes and mortality at 24 hours. Testing spanned three laboratories (LSTM, AIRID and KEMRI) using susceptible reference and resistant lab colonies. Dose response models estimated EC{square}{square} for knockdown (EC{square}{square}KD) and mortality (EC{square}{square}Mort), with resistance ratios (RR) against the local susceptible reference. ResultsThe assay generated clear concentration- and time-dependent responses. At LSTM, Kisumu gave an EC{square}{square}KD of 0.052 and EC{square}{square}Mort of 0.054 mg/paper; Siaya was comparable (RR{square}{square}KD 1.17, RR{square}{square}Mort 0.89), whereas Tiassale 13 and KDR showed reduced susceptibility (RR{square}{square}KD 2.42 and 2.85; RR{square}{square}Mort 5.56 and 6.65). At AIRID, the resistant Cove strain did not reach 50% knockdown or mortality. At KEMRI, Siaya showed reduced susceptibility (RR{square}{square}KD 3.36/RR{square}{square}Mort 5.65). Kisumu at AIRID and KEMRI was comparable to LSTM (EC{square}{square}KD 0.007/EC{square}{square}Mort 0.030; EC{square}{square}KD 0.044/EC{square}{square}Mort 0.023). Despite inter-laboratory variation, susceptible strains remained distinguishable from resistant ones. Lower RRs than for contact pyrethroids suggest contact-based phenotypes may not predict vapour susceptibility. ConclusionThe assay provides a practical benchtop method using standard WHO hardware, distinguishing susceptible from resistant strains across three laboratories. Twenty-four-hour mortality is the recommended endpoint, 60-minute knockdown secondary. Not yet validated, it provides a foundation for further validation and adaptation to other actives.

zoology↗

bioRxiv2026

Multi-centre laboratory study to determine discriminating concentrations for broflanilide and isocycloseram resistance monitoring in mosquitoes

1In 2024-2025 a multi-centre study involving seven international laboratories was conducted with the support of the World Health Organization (WHO). The aim of the study was to establish and validate discriminating concentrations (DCs) in WHO bottle bioassays for monitoring susceptibility to broflanilide and isocycloseram in Anopheles gambiae s.s., An. funestus, An. stephensi and Aedes aegypti. The following values are recommended for adoption as DCs for broflanilide: 10 {micro}g/bottle for An. gambiae and Ae. aegypti, 15 {micro}g/bottle for An. funestus, and 25 {micro}g/bottle for An. stephensi. The recommended DCs for isocycloseram are 15 {micro}g/bottle for Ae. aegypti, 30 {micro}g/bottle for An. gambiae, 50 {micro}g/bottle for An. stephensi, and 60 {micro}g/bottle for An. funestus. Based on the experiences of conducting this study, which represents the application of a generic protocol for establishing discriminating concentrations produced by the WHO, technical recommendations are made on the generation and analysis of DC data for insecticides in future.

zoology↗

bioRxiv2026

Widely circulating pyrethroid resistance mechanisms reduce the efficacy of transfluthrin and pose a risk for mosquito-borne disease control with spatial emanators

Spatial emanators (SE) are a promising complement to existing tools for preventing mosquito transmitted diseases. In 2025, the WHO updated the WHO Guidelines for Malaria to include a conditional recommendation for the indoor use of prequalified SE products in malaria control. Both prequalified, and many other SE products contain the volatile pyrethroid transfluthrin, which shares the same target site as other (contact/solid phase) pyrethroids. Therefore, an assessment of cross resistance is critical to predict effectiveness against mosquitoes with existing pyrethroid resistance. Our results show that resistance to solid phase pyrethroids is correlated with resistance to transfluthrin in Anopheles and Aedes species. Moreover, commonly-selected resistance mechanisms including target site mutations and over-expression of P450 detoxification enzymes can confer resistance to transfluthrin. Furthermore, we show that resistant mosquitoes are less impacted by transfluthrin in terms of flight activation (irritancy) and reduced blood feeding inhibition, with the response correlating with resistance strength. Transfluthrin did not elicit an electroantennography response in Anopheles gambiae and surgically ablating mosquitoes antennae did not result in differences in flight activation upon transfluthrin exposure, suggesting the antennae are not required for transfluthrin to elicit behavioral responses. These results provide new insight regarding the mode of action of transfluthrin and the risk of resistance reducing transfluthrins efficacy in vector control interventions.

pharmacology and toxicology↗

bioRxiv2025

The beta-triketone, nitisinone, kills insecticide-resistant mosquitoes through cuticular uptake

The efficacy of numerous vector control initiatives is compromised by growing insecticide resistance among disease-transmitting arthropods of agricultural, veterinary, and public health significance. Previous investigations on hematophagous (blood-feeding) arthropod vectors, including mosquitoes, have indicated that ingesting blood containing inhibitors of the second enzyme in the tyrosine metabolism pathway, 4-hydroxyphenylpyruvate dioxygenase (HPPD), results in high insect mortality. Building upon this foundation, we evaluated the insecticidal efficacy of the HPPD inhibitor, nitisinone, against susceptible and pyrethroid-resistant strains of three mosquito species: Anopheles gambiae, Aedes aegypti and Culex quinquefasciatus. These mosquitoes are vectors of historical diseases such as malaria, emerged diseases such as Dengue and Zika and emerging viral diseases such as the Oropouche and Usutu viruses. We demonstrate, by employing standard screening assays designed to assess the cuticular uptake of mosquitocidal agents, that nitisinone has mosquitocidal activity when blood-fed mosquitoes contact a nitisinone-coated surface. Notably, there is no discernible disparity in susceptibility to nitisinone between an insecticide-susceptible strain of Anopheles gambiae and two strains carrying multiple insecticide-resistance mechanisms. We conclude that the mosquitocidal mode of action of nitisinone differs from any of the current 37 classes of insecticides as none have a mode of action that specifically interferes with blood digestion. By highlighting the efficacy of nitisinone as a contact-based insecticide, our findings support the potential expansion of vector control strategies where nitisinone is incorporated into classic interventions like treated bednets and indoor residual spraying.

zoology↗

bioRxiv2020

Demonstration of resistance to satyrization behavior in Aedes aegypti (Linnaeus) from La Reunion island

Aedes aegypti and Aedes albopictus are competent vectors of arboviruses such as dengue and chikungunya viruses which co-exist in some areas including La Reunion island. A kind of reproductive interference called satyrization has been described in sympatric species where a dominant species tends to control the spread of the other species. Here, we investigated satyrization in laboratory experiments to provide insights on the potential impact on Ae. aegypti of a control campaign including a sterile insect technique component against Ae. albopictus. Different mating crosses were used to test sympatric, conspecific-interspecific and allopatric effects of irradiated and non-irradiated male Ae. albopictus on female Ae. aegypti, including in a situation of skewed male ratio. Our results suggest that there was only a low level of satyrization between sympatric populations of Ae. aegypti and Ae. albopictus colonised from La Reunion island. A male Ae. albopictus to female Ae. aegypti ratio of 3:1 did not increase the level of satyrization. Female Ae. aegypti previously mated to male Ae. albopictus were not prevented from being inseminated by conspecific male Ae. aegypti. A satyrization effect was not seen between allopatric Ae. albopictus and Ae. aegypti strains from La Reunion Island either. The tested Ae. aegypti strain from La Reunion island has therefore developed full resistance to satyrization and so releasing sterile male Ae. albopictus may not suppress Ae. aegypti populations if an overflooding of irradiated male Ae. albopictus leads to similar results. The management strategy of two competent species in a sympatric area is discussed.

ecology↗