The aim of the present work was to study the deltamethrin susceptibility of eggs from Triatoma infestans populations and the contribution of pyrethroid esterases to deltamethrin degradation. Insects were collected from sylvatic areas, including Veinte de Octubre and Kirus-Mayu (Bolivia) and from domiciliary areas, including El Palmar (Bolivia) and La Pista (Argentina). Deltamethrin susceptibility was determined by dose-response bioassays. Serial dilutions of deltamethrin (0.0005-1 mg/mL) were topically applied to 12-day-old eggs. Samples from El Palmar had the highest lethal dose ratio (LDR) value (44.90) compared to the susceptible reference strain (NFS), whereas the Veinte de Octubre samples had the lowest value (0.50). Pyrethroid esterases were evaluated using 7-coumaryl permethrate (7-CP) on individually homogenised eggs from each population and from NFS. The El Palmar and La Pista samples contained 40.11 and 36.64 pmol/min/mg protein, respectively, and these values were statistically similar to NFS (34.92 pmol/min/mg protein) and different from Kirus-Mayu and Veinte de Octubre (27.49 and 22.69 pmol/min/mg protein, respectively). The toxicological data indicate that the domestic populations were resistant to deltamethrin, but no statistical contribution of 7-CP esterases was observed. The sylvatic populations had similar LDR values to NFS, but lower 7-CP esterase activities. Moreover, this is the first study of the pyrethroid esterases on T. infestans eggs employing a specific substrate (7-CP).

To increase our knowledge of the natural susceptibility of Triatoma infestans to an organophosphate insecticide, we performed toxicological and biochemical studies on three sylvatic populations from Bolivia and two populations from domestic dwellings from Bolivia and Argentina. Fifty-per-cent lethal doses (LD50) were determined based on the topical application of fenitrothion on first instar nymphs and mortality was assessed at 24 h. Both type of populations exhibited LD50ratios significantly higher than 1 with a range of the values (1.42-2.47); the maximum value were found in a sylvatic (-S) population, Veinte de Octubre-S. Samples were biochemically analysed using a glutathione S-transferase activity assay. The highest significant activity was obtained for Veinte de Octubre-S and the lowest activity was obtained for the reference population (102.69 and 54.23 pmol per minute per mg of protein respectively). Two out of the three sylvatic populations (Veinte de Octubre-S and Kirus Mayu-S) exhibited significantly higher glutathione S-transferase activity than that of the reference population. Based on this analysis of the natural susceptibility of this organism to organophosphate insecticides, continental and focal surveys of organophosphate susceptibility should be conducted to evaluate the evolution and distribution of this phenomenon.

The objective of this work was the study of the relative contribution of general esterases: á-napthyl acetate (á-NA) and p-nitro phenyl acetate (p-NPA) in pyrethroid-resistant T. infestans from these locations. We used deltamethrin-resistant populations previously collected in two areas in Argentina (Salta and La Rioja) and one area in Bolivia (Tarija). In this study, frequency histograms of á-NA and p-NPA esterase activities, exhibited similar patterns between Bolivian population and the susceptible counterpart. In contrast, Argentinean field populations showed higher proportion of increased enzyme activity individuals. These results clearly demonstrated that resistances in Argentinean and Bolivian field populations are based on different mechanisms, suggesting that enzyme-based pyrethroid resistance in this species has multiple origins.

El objetivo de este trabajo fue el estudio de la contribución relativa de esterasas generales: á-naftil acetato (á-NA) y p-nitro fenil acetato (p-NPA) a la resistencia a piretroides de T. infestans provenientes de esas localidades. Se evaluaron poblaciones resistentes a deltametrina previamente recolectadas en dos áreas de Argentina (Salta y La Rioja) y un área en Bolivia (Tarija). En este estudio, los histogramas de frecuencias de actividades de á-NA y p-NPA esterasas mostraron perfiles similares entre la población boliviana y la población susceptible. Por el contrario, en las poblaciones argentinas de campo se observó una elevada proporción de individuos con actividad enzimática incrementada. Estos resultados demuestran claramente que la resistencia en poblaciones argentinas y bolivianas de campo se origina a partir de distintos mecanismos, sugiriendo que la resistencia a piretroides basada en la acción de enzimas presentan múltiples orígenes en estas especies.