Pyrethroid Insecticide Evaluation on Different House Structures in a Chagas Disease Endemic Area of the Paraguayan Chaco

Insecticide effects of deltamethrin 2.5% SC (flowable solution) on different substrates and triatomine infestation rates in two indigenous villages (Estancia Salzar and Nueva Promesa) of the Paraguayan Chaco are reported. This field study was carried out to determine the extent to which variability in spray penetration may affect residual action of the insecticide. A total of 117 houses in the two villages were sprayed. Filter papers discs were placed on aluminium foil pinned to walls and roofs in selected houses and the applied insecticide concentration was determined by high pressure liquid chromatography (HPLC). The target dose rate was 25 mg a.i./m2. The mean actual applied dose in Estancia Salazar was 11.2 +/- 3.1 mg a.i./m2 in walls and 11.9 +/- 5.6 mg a.i./m2 in roofs while in Nueva Promesa, where duplicates were carried out, the mean values were 19.9 +/- 6.9 mg a.i./m2 and 34.7 +/- 10.4 mg a.i./m2 in walls and 28.8 +/- 19.2 mg a.i./m2 and 24.9 +/- 21.8 mg a.i./m2 in roofs. This shows the unevenness and variability of applied doses during spraying campaigns, and also the reduced coverage over roof surfaces. However, wall bioassays with Triatoma infestans nymphs in a 72 h exposure test showed that deposits of deltamethrin persisted in quantities sufficient to kill triatomines until three months post spraying. Knockdown by deltamethrin on both types of surfaces resulted in 100% final mortality. A lower insecticidal effect was observed on mud walls. However, three months after treatment, sprayed lime-coated mud surfaces displayed a twofold greater capacity (57.5%) to kill triatomines than mud sprayed surfaces (25%). Re-infestation was detected by manual capture only in one locality, six months after spraying.