Recovery of Triatoma infestans populations after insecticide application: an experimental field study

The capacity of populations of Triatoma infestans (Klug) (Hemiptera: Reduviidae) to survive and recover was assessed after application of insecticide (gamma-HCH at a rate of 0.5 g a.i./m2) at different seasons. T. infestans populations were maintained in experimental chicken houses under natural climatic conditions in a region of Argentina endemic for Chagas disease transmitted by these bugs. Based on previous studies of T. infestans populations in these habitats, each experimental group was set up with a total of 626 T. infestans, comprising 390 eggs, 204 nymphs of particular stages, fourteen male and eighteen female adults. The chicken houses were dismantled and rebuilt at monthly intervals to study the vector population changes over a period of 33 months. When the insecticide was applied during winter, spring or summer, populations of T. infestans recovered to untreated or precontrol levels during the next reproductive season (i.e. during the hot season, October-March). In contrast, populations treated during autumn (March) remained at very low densities for 2 years and then increased rapidly to surpass the untreated populations. All populations of the bugs fell to very low numbers (sometimes less than twenty individuals) after gamma-HCH applications, but none was driven to extinction. In all cases, the density of surviving populations was independent of their density before treatment. The fact that all treated populations recovered within 1-3 years, to at least the density of untreated populations, shows the high reproductive potential of T. infestans to recover from very low population densities. Moreover, the additive effect of climatic-induced mortality and insecticide-induced mortality is only apparent when insecticides are applied just before the onset of the cold winter months during which reproductive rates are at their lowest.