Assessment of the susceptibility status of Aedes aegypti (Diptera: Culicidae) populations to pyriproxyfen and malathion in a national wide monitoring of insecticide resistance in Brazil, 2017-2018.

Abstract Background: Chemical control using malathion has been applied in Brazil, since 1985. To obtain chemical control effectiveness, it is essential to monitor vector susceptibility to insecticides. This study aimed to describe bioassay standardizations and determine the susceptibility profile of Ae. aegypti populations to malathion and pyriproxyfen, used on a national scale in Brazil between 2017 and 2018, and discuss the observed result impacts in arbovirus control.Methods: The diagnostic-doses (DD) of pyriproxyfen and malathion were determined as the double of adult emergence inhibition (EI) and lethal doses for 99% of the Rockefeller reference strain, respectively. For the monitoring of natural populations, samplings were performed in 132 Brazilian cities through egg traps. Colonies were raised in the laboratory for one or two generations (F1 or F2) and submitted to susceptibility tests with larvae exposed to the pyriproxyfen DD (0.03 µg/L) and adults to the malathion DD obtained herein (20 µg), in addition to the indicated World Health Organization (WHO) DD (50 µg) in a bottle assay. Dose-response (DR) bioassays with pyriproxyfen were performed with populations which did not achieve 98% EI in the DD assays. Results: Alteration of susceptibility to pyriproxyfen was recorded in 6 (4.5%) Ae. aegypti populations located in the states of Bahia and Ceará, with Resistance Ratios (RR95) ranging from 1.51 to 3.58. For malathion, 73 (55.3%) populations distributed throughout the country were resistant when exposed to the local DD 20µg/bottle. On the other hand, no population was resistant, and only 10 (7.6%) populations from 8 states were considered as exhibiting decreased susceptibility (mortality ratios between 90 and 98%) when exposed to the WHO DD (50 µg/bottle). Conclusions: The feasibility of conducting an insecticide resistance monitoring action at a national wide scale was confirmed herein, employing standardized and strongly coordinated sampling methods and laboratory bioassays. Brazilian Ae. aegypti populations with decreased susceptibility to pyriproxyfen were identified. The local DD for malathion was more sensitive than the WHO DD for the early detection of decreased in susceptibility.