We investigated the residual efficacy of four
insecticide formulations used in
Chagas disease vector control campaigns:
cyfluthrin 12.5%
suspension concentrace (SC),
lambda-cyhalothrin 10% wettable
powder (WP),
deltamethrin 2.5% SC, and 2.5% WP on four types of circular blocks of wood, straw with mud, straw with mud painted with
lime, and mud containing 5% 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%) were observed for
deltamethrin 2.5% SC and
lambda-cyhalothrin 10% 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.