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Active dispersion

Here is maybe one aspect of the biology of Triatominae, together with population density, where we can find clear-cut differences between domestic and silvatic species. Active dispersion is performed not only by flying,33,34 but also, and perhaps more frequently in domestic species, by walking.

Domestic adult bugs when discovered in their hiding place do not fly, they try to escape by walking. In silvatic conditions, the same bug discovered under a stone might decide to walk away, or might remain absolutely immobile, simulating a dead body. This behavior was observed by one of us (JPD) for silvatic T. infestans in the Cochabamba (Bolivia) foci, probably because in silvatic situation and not in domestic places the bug is surrounded by predators and cannot beat them at running. Some silvatic species are able however to immediately take flight if disturbed, like Parabelminus yurupucu and Microtriatoma trinidadensis.35

The active dispersal of T. infestans and other Triatoma species was measured recently by Abrahan et al.36 using light traps (for flying insects) and sticky dispersal barriers (for walking insects) within rural courtyards. Monthly catches were made on 30 nights in the warm season. Despite continuous and strong capture efforts a total of 8 flying adults, 6 walking nymphs, and 10 walking adults of T. infestans were captured, together with specimens of T. guasayana, T. eratyrusiformis, and T. platensis. The study demonstrated that adult T. infestans can disperse by walking, suggesting for females an adaptive strategy allowing them to move with eggs and/ or with blood reserves, which is unlikely when flying.

The distance of flight of T. infestans is around 1—2 km, however there are exceptions and some observations give T. infestans a much wider range.37 The flight activity generally needs a physiological preparation.38 It requires a previous heating during which the bug shakes its wings for a few minutes, and it is more frequent in starved specimens.39 The flight of T. infestans seems to occur more often during the hot season, and at night.40

The factors inducing flight are many, among which are the nutritional status of the bug, external temperature and relative humidity, population density, etc. and their respective role is not easy to define.35 Nutritional41,42 and reproductive status,36,43 as well as population density,44 are known factors that modulate dispersal behavior in Triatominae. Environmental conditions as external temperature, relative humidity, and wind speed could be key factors for flight initiation.33,41

Flight orientation is apparently random, but it seems that during its flight the bug can be attracted by lights, as proved by the many observations of bugs either caught inside light traps set up outside villages to catch sandflies (F. Lepont, IRD, personal communication) or, in a village, observed landing on the brighten window sills during night (La Fuente, CENETROP, personal communication). Laboratory experiments have confirmed that a true attraction by white light rather than arrival by chance does exist.45 Walking of nymphs toward a light source was reported during light trapping. It is more mysterious to understand how a silvatic habitat is left for another one. Possible but still not confirmed factors helping orientation in silvatic conditions could be a specific odor attractant46,47 or the warmth (e.g., infra-red) emitted by animal bodies, all signals probably collected by the receptors located on antennae.48 For some triatomine species that use flight as an important type of locomotion, such as T. dimidiata, light is a physical cue that might attract insects into houses, and streetlights have been associated with increased domestic infestation.49 The removal of domestic animals in infested areas may increase vector dispersal, possibly toward nearby human sleeping spaces. Castillo-Neyra et al.50 used a semifield system to characterize the dispersal of T. infestans, and compared the behavior of vector populations in the constant presence of hosts and after the removal of the hosts. The emigration rate of net insect population decline in original refuges following host removal was on average 19.7% of insects per 10 days compared to 10.2% in constant host populations. Activity of insects was significantly increased when hosts were removed.

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