Odour movement, wind direction, and the problem of host‐finding by tsetse flies

Abstract

ABSTRACT. Movement of host odour was modelled in natural tsetse habitats with smoke and ultra‐light 7‐cm‐long wind vanes; the speed and direction of the air movements were analysed from video recordings thereof. Wind of <1 ms‐1 did not move in straight lines, since large packets of air (>10 m across) often changed direction together. The rate of this change of direction (meander) correlated negatively with windspeed. In open woodland with a shrubby understorey (in which windspeed was reduced by a factor of >5 from that above the canopy, to ax 0.3 m s‐1), this wind meander fell by 2d̀ s‐1 change of direction for each 0.1 m s‐1 increase in windspeed (r2=0.96). Over open ground without shrub cover, the meander fell by 0.5d̀ s‐1 per 0.1 m s‐1 increase in windspeed (r2=0.85). In both situations, such meandering virtually ceased in winds of > 1 m s‐1. In woodland, the relationship between the direction of air movement near the surface of bare earth (one potential tsetse landing site) and that c. 0.5 m above ground level (flight height) was often weak (r2=0.2‐0.4), but this problem would be reduced if the fly averaged the ground‐level wind for at least 30 s. Odour (smoke) travelling from a source 15 m ‘upwind’ over open ground arrived at a notional tsetse fly for 80% of the time from a direction within 10d̀ of the true source direction. In typical tsetse woodland, however, the ‘odour’ arrived from all directions (including >90d̀ away from the source), with only a 30% bias towards the true source direction (±10d̀). Evidently, tsetse must navigate up odour plumes by means that get round these difficulties‐simple, moth‐type upwind anemotaxis alone seems unlikely to be adequate.

Olfactory responses of Glossina fuscipes fuscipes (Diptera: Glossinidae) to the monitor lizard Varanus niloticus niloticus

“…Hargrove (1980) showed that the landing responses of tsetse, especially of female G. pallidipes, increased with increase in the size of the model. Savanna tsetse (G. m. morsitans and G. pallidipes) respond to wind direction while in flight using optomotor cues generated by wind drifting them from their heading; this occurs, even at low wind speeds (<0.5 m/s) such as those encountered in typical tsetse habitats (Gibson & Brady, 1988, Colvin et al, 1989Brady et al, 1989). Bursell (1987) and Torr & Mangwiro (1996) reported that the same species took off upwind in response to carbon dioxide and whole ox odour.…”

Filament-Based Atmospheric Dispersion Model to Achieve Short Time-Scale Structure of Odor Plumes

“…Transport of odor filaments by this flow field yields a plume centerline with realistic characteristics. In particular, the meandering nature of the plume (with 'semi-independent' instantaneous wind direction and plume centerline alignment) is a realistic characteristic of natural plumes [42,43] and therefore is a significant advancement relative to the wind-tunnel type of puff models used previously for analysis of insect flight response, e.g. see [34,33].…”

Odor-modulated orientation in walking male cockroachesPeriplaneta americana, and the effects of odor plumes of different structure

“…The direction and speed of this flow are typically the most reliable and useful information indicating direction to the source location and are incorporated with information about the odor stimulus during odor-tracking behavior (Arbas et al, 1993;Belanger and Willis, 1996;Weissburg, 2000;Grasso and Basil, 2002). However, under certain conditions, in certain forest and woodland environments, the wind directions may change so continuously that at any given moment an animal experiencing odor and orienting to the wind may be aimed away from the source (Elkinton et al, 1987;Brady et al, 1989).…”