Dark-Winged Chagas Bug Clusters in Rural Hotspots, Landscape Study Finds

In the scattered villages and farmsteads of Avia Terai, in Argentina’s Chaco Province, the insect that transmits Chagas disease is quietly telling researchers something unexpected. Triatoma infestans, the main vector of the parasite Trypanosoma cruzi across South America, comes in two color forms: the familiar reddish-brown bugs and a darker, melanic variant. A new study published in Parasites & Vectors reveals that these dark individuals are not randomly scattered across the landscape. Instead, they appear almost exclusively in rural settings, cluster together in spatial hotspots, and are associated with more connected landscapes, findings that could reshape how scientists understand and predict the reinfestation of houses after insecticide spraying.

The research, led by Julieta Nattero of the Universidad de Buenos Aires and CONICET together with colleagues including Delfina Trezza-Neumayer, Federico G. Fiad, Ricardo E. Gürtler, and María S. Gaspe, set out to answer four linked questions. Does the frequency of the melanic form change along a gradient running from city to countryside? Do melanic bugs cluster in space? Do the two color forms differ in traits linked to flight and dispersal? And which features of the surrounding landscape best predict where melanic individuals turn up in and around human dwellings? The answers carry practical weight, because understanding what drives house reinfestation is critical for the final push against a disease that still burdens millions of people in Latin America.

To do this, the team assembled an unusually detailed dataset. They collected 901 adult Triatoma infestans from 67 infested houses spanning urban, peri-urban, and rural environments in Avia Terai. Rather than relying on subjective visual judgments of color, the researchers quantified dorsal coloration using red, green, and blue pixel metrics taken from two body regions: the forewing and the connexivum, the flattened lateral margin of the abdomen. They then used K-means clustering, an unsupervised machine learning method, to sort individuals into chromatic groups objectively. This quantitative approach matters because color variation in insects can be subtle, and consistent classification is the foundation for every downstream analysis.

The first striking result is rarity combined with strict environmental restriction. Melanic individuals made up just 16.98 percent of the bugs analyzed, and they were found only in rural environments, never in peri-urban or urban houses. That pattern suggests the dark morphotype is tied to particular ecological conditions that exist in the countryside but not in more built-up settings. Whether those conditions involve temperature, humidity, background substrates, or something else remains an open question, but the urban-to-rural gradient clearly acts as a filter on this phenotypic variation.

Color, however, is only one part of the story. The team also examined whether melanic and non-melanic bugs differ in traits related to dispersal, a key question because flying adults are thought to be important agents of reinfestation after insecticide campaigns. Using geometric morphometrics on the forewings, a technique that captures size and shape variation independent of overall body size, the researchers found that wing size and shape were broadly similar between the two morphotypes. The single exception was male wing shape, which differed between color forms. Flight muscle development, assessed directly, did not differ between morphotypes or between sexes, indicating that darker bugs are not inherently better or worse fliers by this measure.

One dispersal-related trait did stand out. The researchers calculated a dispersal index based on the ratio between the membranous and stiff portions of the forewing, a morphological feature that in other insects correlates with flight capacity. Non-melanic individuals had higher values of this index than melanic ones. Taken together with the similar flight muscle development, the result paints a nuanced picture: the color morphotypes are not simply fast and slow dispersers, but subtle differences in wing architecture hint that the two forms may move through the landscape in different ways, with consequences for how quickly they can colonize newly sprayed houses.

The spatial analysis delivered perhaps the most consequential finding. Using Moran’s I, a standard statistic for spatial autocorrelation, and Local Indicators of Spatial Association, or LISA, which pinpoint where clusters of similar values occur, the team detected significant positive spatial autocorrelation in melanic frequency at distances of 2,500 meters, a pattern that persisted out to 5,000 meters. In practical terms, houses with melanic bugs are not distributed at random across the map. Instead, they form localized hotspots where the dark morphotype occurs at unusually high frequency, and these hotspots are large enough to span several kilometers. Such spatial structure implies that the processes producing melanic bugs, whether local population persistence, restricted gene flow, or shared environmental conditions, operate at landscape scales rather than house by house.

What landscape features explain those hotspots? To find out, the researchers calculated landscape metrics within eight concentric buffers around each dwelling, ranging from 200 to 3,200 meters in radius. This multi-scale design allowed them to identify the scale of effect, the spatial extent at which the surrounding environment most strongly influences the phenomenon of interest. They then modeled melanic frequency using negative binomial generalized linear models combined with multi-model inference based on Akaike’s Information Criterion corrected for small samples, a rigorous framework for comparing competing explanations while accounting for overdispersed count data.

The models converged on a clear answer. Landscape metrics measured at buffers of 800 to 1,000 meters best predicted the frequency of melanic individuals, and higher values of those metrics, associated with increased landscape connectivity, corresponded to more melanic bugs. In other words, dark-colored Triatoma infestans are most frequent around houses embedded in well-connected rural landscapes, where patches of suitable habitat are linked in ways that may facilitate bug movement and population persistence. The identification of this specific scale of effect gives vector control programs a concrete spatial radius to consider when assessing reinfestation risk around treated houses.

The broader significance of the study lies in its integration of phenotypic, spatial, and landscape perspectives. Chromatic dimorphism in Triatoma infestans was first reported in sylvatic populations, bugs living in wild habitats away from houses, and later documented in domestic and peridomestic settings, but its ecological meaning has remained murky. By showing that the melanic morphotype is spatially aggregated, environmentally restricted, and landscape-dependent, the new work suggests that color variation is not mere noise. It appears to reflect genuine ecological differentiation tied to the settings where bug populations persist and from which they reinvade treated houses. If melanic bugs mark populations with particular spatial dynamics, then their presence could serve as a phenotypic signal of reinfestation risk, helping surveillance teams prioritize villages and households for follow-up inspection and spraying.

The study also underscores a methodological lesson for disease ecology more generally. Phenotypic variation in disease vectors is often studied at the level of individual insects or single houses, yet the patterns documented in Avia Terai emerge only when the surrounding landscape and spatial structure are taken into account. As the authors conclude, understanding phenotypic variation in Triatoma infestans requires integrating landscape context and spatial structure, because the occurrence of the melanic morphotype is associated with particular ecological settings and spatial dynamics that influence persistence and reinfestation. For a vector that continues to infest houses despite decades of control efforts, every clue about where and why certain bug populations thrive is a clue about how to stop them, and in the dark wings of these rural hotspots, researchers have found a marker written in color that points toward the landscapes where the battle against Chagas disease is hardest to win.

Subject of Research: Spatial distribution and landscape associations of melanic Triatoma infestans, the Chagas disease vector, along an urban-to-rural gradient in Argentina

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

Article References: Nattero, J., Trezza-Neumayer, D., Fiad, F. G., Gürtler, R. E., & Gaspe, M. S. (2026). Spatial aggregation and landscape associations of melanic Triatoma infestans in an urban-to-rural gradient. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07712-7

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07712-7

Keywords: Triatoma infestans, Chagas disease, melanism, chromatic dimorphism, spatial autocorrelation, landscape connectivity, geometric morphometrics, vector control, Argentina, Parasites & Vectors, reinfestation, flight traits