Lack of pyrethroid resistance alleles in Aedes caspius sympatric with resistant Ae. albopictus and Culex pipiens

Mosquitoes are not only a relevant nuisance, but also important pests causing damage to human and animal health and are therefore target of extensive pesticide treatments. This has caused development and spreading of resistance to most largely used insecticide molecules in many species. In the European Union, only pyrethroids – targeting the voltage- gated-sodium-channel (VGSC) – are allowed for the control of adult mosquitoes and for the limitation of ongoing disease transmission, but their excessive usage has led to the rise of pyrethroid resistance (PR) in both the most widespread species, Aedes albopictus and Culex pipiens. Herein, we focus on Aedes caspius, a floodwater species whose marked anthropophily and aggressive biting behaviours strongly affect outdoor activities, and which is a major target of mosquito control programmes in Italian touristic areas. Despite this no data are available concerning its PR status. Aedes caspius specimens were sampled in 2020 in three coastal sites from Ferrara province in Italy were previous studies highlighted high phenotypic PR and high frequencies of knock-down-resistance (kdr) mutations within the vgsc gene associated with PR in sympatric Ae. albopictus and Cx. pipiens. Sequencing of domains I, II and III of the vgsc was carried out to investigate the presence of mutations with known impact on PR in mosquitos. DNA was extracted from 121 specimens and sequencing was successful for 43, 101 and 61 specimens for domain I, II and III respectively. No known kdr mutations were found in the examined specimens. Despite the reported high insecticidal usage in the sampling sites and the high presence of phenotypic and genotypic PR in sympatric Cx. pipiens and Ae. albopictus (where frequencies of kdr alleles were above 90% and 25%, respectively), the present study did not detect any kdr mutations in the Ae. caspius sample analysed. This might be due to actual full susceptibility of the analysed populations to pyrethroids, which will be evaluated soon by bioassays. On the other hand, the species may rely on metabolic or cuticular resistance mechanisms as an alternative to the target-site resistance mechanisms investigated herein. Further investigations are ongoing to clarify these aspects and to understand the possible differential selective pressure of pyrethroid usage on different mosquito species.