Preliminary data on the pyrethroid resistance status of Aedes caspius populations sympatric with resistant Aedes albopictus and Culex pipiens

INTRODUCTION. Pyrethroid insecticides - targeting the voltage-gated-sodium-channel (VGSC) in the insect nervous system - are the main pesticides allowed in Europe to control adult mosquitoes outdoors. While for mosquito control, they are only recommended in case of ongoing arbovirus transmission, they are also widely used to reduce biting nuisance (as well as for insect agricultural pest control). This has caused development and spreading of pyrethroid resistance (PR) in both the most widespread and public health relevant mosquito species, Aedes albopictus and Culex pipiens (Moyes, et al., 2017. PLoS Negl. Trop. Dis., 11(7): e0009084). Herein, we focus on Aedes caspius, a floodwater species whose marked anthropophily and aggressive biting behavior strongly affects outdoor activities and represents a major target of mosquito control programs in Italian coastal touristic areas. (Veronesi et al., 2012. Jour. Vect. of Entoml. 37(1): 49-61.). Despite this no data are available concerning its PR status in Italy. MATERIALS AND METHODS. 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 (Fan, et al., 2020. PLoS Negl. Trop. Dis., vol. 14: 1–22.; Kasai et al., 2011. Jpn. J. Infect. Dis., 64: 217-221.) of the vgsc was carried out to investigate the presence of mutations with known impact on PR in mosquitoes. Phenotypic PR was evaluated by exposing specimens from one coastal site for one hour to permethrin 0,75% according to WHO guidelines (WHO. 2016.). RESULTS AND CONCLUSIONS. DNA was extracted from 121 specimens and sequencing was successful for 43, 101 and 61 specimens for domains I, II and III respectively. No known kdr mutations were found in the examined sequences. Exposure of Ae.caspius specimens to permethrin 0.75% resulted in a 99% mortality, suggesting complete susceptibility. 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), (Pichler et al., 2018. Pest Manag. Sci., 74: 1319–1327; Pichler et al., 2022. Med. Vet. Entomol., 36: 390–395.) The present study did not detect any signs of genotypic or phenotypic PR. The absence of diagnostic dosages for this species limits interpretation of bioassay results and further studies will be needed to confirm the susceptibility status of Ae. caspius and to understand the possible differential selective pressure of pyrethroid usage on the different mosquito species.