Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient

Triatoma infestans , the main Chagas disease vector in South America, continues to infest houses. Understanding ecological and phenotypic factors driving house reinfestation is therefore critical. Intraspecific chromatic dimorphism—particularly melanism—has been reported in sylvatic and later in domestic and peridomestic habitats, but its ecological significance and links to environment, landscape features, dispersal traits, and reinfestation remain unclear. Here, we assess whether: (1) the frequency of chromatic morphotypes varies across rural, peri-urban, and urban environments; (2) melanic morphotype shows spatial aggregation; (3) dispersal-related forewing traits differ between morphotypes, and (4) landscape attributes are associated with the presence of melanic individuals in domestic and peridomestic settings. We analysed the frequency, spatial distribution, and ecological correlates of melanic and non-melanic morphotypes along an urban-to-rural gradient in Avia Terai (Chaco Province, Argentina). Dorsal colouration was quantified from the forewing and connexivum using red, green and blue metrics in 901 adult T. infestans collected from 67 houses. Chromatic groups were identified using K-means clustering. Spatial patterns of the melanic morphotype were assessed with Moran’s I and Local Indicators of Spatial Association. Dispersal-related traits were evaluated using forewing geometric morphometrics, a membranous-to-stiff portion ratio, and flight muscle development. Landscape metrics were calculated within eight buffers (200–3,200 m) around each dwelling, and their effects on melanic frequency were tested using negative binomial GLMs with AICc-based multi-model inference. Melanic individuals were rare (16.98% of the individuals analysed) and restricted to rural environments. Flight muscle presence did not differ between morphotypes or sexes. Forewing size and shape were similar between morphotypes, except for male wing shape. The dispersal index was higher in non-melanic individuals. Significant positive spatial autocorrelation was detected at 2,500 m and persisted at 5,000 m, forming localized hotspots of high melanic frequency. Landscape metrics at 800–1,000 m best predicted melanic frequency, with higher values associated with increased landscape connectivity. This study underscores the need to integrate landscape context and spatial structure to understand phenotypic variation in T. infestans . The occurrence of the melanic morphotype appears to be associated with particular ecological settings and spatial dynamics that influence persistence and reinfestation.