Comparative evaluation of pyrethroid insecticide formulations against Triatoma infestans (Klug): residual efficacy on four substrates

RESUMO

We investigated the residual efficacy of four insecticide formulations used in Chagas disease vector control campaigns cyfluthrin 12.5 percent suspension concentrace (SC), lambda-cyhalothrin 10 percent wettable powder (WP), deltamethrin 2.5 percent SC, and 2.5 percent WP on four types of circular blocks of wood, straw with mud, straw with mud painted with lime, and mud containing 5 percent of cement. Three concentrations of these insecticides were tested the LC90 (previously determined on filter paper), the double of the LC90, and the recommended operational dose. For each bioassay test, 15 third-stage nymphs of Triatoma infestans (Klug) (Hemiptera Reduviidae) were exposed for 120 h to each treatment at 24 h, 30, 60, 90, and 180 days post-spraying. Mortality rates, moulting history and behaviour were recorded at 24, 48, 72, and 120 h of exposure. Mortality rates were highest during the first 30 days post-spraying. Highest mortality rates (above 50 percent) were observed for deltamethrin 2.5 percent SC and lambda-cyhalothrin 10 percent WP on wood blocks up to three months post-spraying. Mud was the substrate on which treatments showed lowest persistence, with the other two substrates showing intermediate residual efficacy of all treatments. During the first 30 days WP formulations were not as effective as SC flowable formulations but, overall in the longer term, WP gave grater mortality rates of T. infestans nymphs exposed at up to six months post-spraying. Porous surfaces, especially mud, showed most variability presumably due to absorption of the insecticide. In contrast the less porous surfaces (i.e. wood and lime-coated mud) kept mortality rates high for longer post-treatment, irrespective of the insecticide concentration used.