A survey of natural ecotopes of Triatoma infestans dark morph and other triatomine sylvatic species was performed in an uninhabited area of the Bolivian Chaco. Among the 321 triatomines collected by light trapping, only 4 T. infestans dark morph specimens were identified. Predominant flying species were T. guasayana and T. sordida group 2 (51.7% and 37.1% of capture, respectively). The same species prevailed in terrestrial and epiphytic bromeliads where scarce T. infestans dark morph nymphal instars were also detected. In parrot nests T. delpontei prevailed broadly over other species (90.2% of the capture) and only 4 T. infestans dark morph adults were collected. In contrast, T. infestans dark morph was the predominant species captured in hollow trees (46.0% of the total collected). The abundance of immature forms (88.2% of the collection) shows that hollow trees constitute a favourable ecotope for this species. Of the 421 trees investigated, 33.7% were positive for triatomines. T. infestans dark morph, found inside 15.0% of them, also had higher apparent density than other species (average number of T. infestans in positive trees, 2.0 +/- 1.6 vs 1.3 +/- 0.6 for other species). Light trapping seems to be an efficient method to sample the T. sordida-T. guasayana complex in that it shows a similar distribution to that observed in natural ecotopes; however, this method is ineffective for the assessment of the local abundance of T. infestans dark morph.

Eight natural Bolivian populations of two closely related species of Triatominae, Triatoma sordida and T. guasayana, were analysed by multilocus enzyme electrophoresis at 20 loci. Both species were readily separated and no natural hybrid was observed. Among the silvatic sample of T. sordida, strong departure from panmixia within and between loci was detected in two sites of the Chaco, suggesting two reproductively separate populations easily recognized at Idh2 and Mdh2 loci. Genetic distance between them was in agreement with the hypothesis of distinct species. However, the detection of 3% of putative hybrids suggested a recent evolutionary divergence.

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The role of Triatoma sordida in the domestic transmission of Trypanosoma cruzi was assessed in 7 rural localities in Velasco Province, Department of Santa Cruz, Bolivia. Tri. sordida, the only triatomine species identified in these localities, was found inside 58.0% of houses but not in large numbers (3.1 bugs per infested house on average). A total of 220 faecal samples from domiciliary bugs was examined microscopically and by the polymerase chain reaction for the presence of trypanosomes: 21.4% were infected. Analysis of blood meals of domiciliary Tri. sordida showed that humans were the commonest host (70.4%), followed by chickens and dogs. Four of 418 persons tested were seropositive for Tryp. cruzi. Only 2 of a second group of 62 persons living in dwellings infested by Tri. sordida were seropositive. Tryp. cruzi infection was demonstrated in dogs and domestic rats. Three other species of small mammals were found to be infected with trypanosomes. In our study area, domestic Tri. sordida are mainly incriminated in the transmission of Tryp. cruzi to synanthropic animals, whereas transmission to humans is very rare. The presence in houses of small populations of Tri. sordida infected with Tryp. cruzi is therefore currently insufficient for this insect to constitute a major epidemiological risk factor.