Evidence for re-evaluating the Spraying Unit: Active nymphal dispersal of Triatoma infestans challenges the 50-meter control threshold.

BACKGROUND Persistent reinfestation by Triatoma infestans in the Gran Chaco undermines Chagas disease control. Current spraying protocols are often limited to a 50-meter radius around infested houses, assuming limited nymphal mobility.
OBJETIVES To evaluate fine-scale genetic connectivity and real-time movement patterns between domestic and peridomestic ecotopes across specific operational thresholds (10 to 50 meters) to test the biological validity of the traditional spraying unit.
METHODS We analyzed the micro-geographic genetic structure of 373 effective T. infestans specimens (out of 377 entomologically collected) from La Brecha, Bolivia, using eight microsatellite loci. Spatial autocorrelation and frequency-based Bayesian assignment analyses were used to evaluate gene flow between structures separated by exact intervals of 10, 40, and 50 meters.
FINDINGS Genetic differentiation between the immediate intradomestic and peridomestic interface (10 m) was remarkably low (FST = 0.0230). Spatial autocorrelation analysis revealed a significant fine-scale genetic structure for late-stage fifth-instar nymphs (N5) extending to a maximum spatial range (Dmax) of 48.5 meters. Furthermore, frequency-based Bayesian assignment successfully identified first-generation N5 migrants crossing the 40 m and 50 m intervals between peridomestic sources and domestic sinks.
MAIN CONCLUSIONS These exploratory results indicate that T. infestans nymphs possess na active walking dispersal capacity that can reach the boundaries of the traditional 50-meter spraying radius. While limited by micro-geographic replication, our data suggest that current rigid spatial thresholds may leave peripheral refuge populations untreated. We recommend that vector control programs evaluate more flexible, block-based intervention radii adapted to local peridomestic structural density.