Tsetse flies and other hematophagous Diptera may use a variety of sensory cues to locate host animals. Olfaction is used by many species for long-range attraction, and manipulation of responses to host odors has provided new methods for control of tsetse and the potential to control other species. Although progress has been made in characterizing the chemistry of host odors, the dynamics of odor plumes and how these affect the insects’ responses are less well understood. Carbon dioxide (CO2) is an attractant for many hematophagous insects and provides a convenient model for studying odor plumes, although use of this particular compound must take account of the fact that it is already present in the atmosphere at significant levels.
Studies were undertaken in riverine and mopane tsetse habitats in Zimbabwe during the dry season to determine the range at which CO2 from a host animal can be detected against the background concentration of atmospheric CO2. Continuous background monitoring of CO2 was performed at 1 Hz through seven diurnal cycles, providing a baseline for further experimental readings. CO2 was released at 2, 4, 10, 20 and 200 L/min from a cylinder and from two young oxen in a pit at an estimated 4 L/min, and CO2 levels were measured at distances 8, 16, 32 and 64 m downwind from the release point. Subsequent CO2 mixing ratios and background levels were measured at 10 Hz for 10-20 min periods between dawn and dusk. The three spatial components of wind vector and air temperature were monitored simultaneously at the same positions downwind of the source.
A peak analysis was performed on short sections of the high resolution time series to determine what distance from the release point a plume CO2 signal could be significantly distinguished from the background signal. Due to sparse vegetation and high windspeeds at the mopane site, the background CO2 signal showed an approximately normal distribution and the threshold could be defined in terms of standard deviations. In contrast, the more dense forest canopy and lower windspeed at the riverine site led to a skewed background CO2 distribution. Thus the threshold had to be quantified by means of percentile ranges. Peaks were identified as any CO2 signal above threshold, and bursts were characterized as > 2 continuous peaks. Peaks were detected at all doses of CO2. Numbers of peaks and burst length increased significantly with increasing dose and decreased with increasing distance from the source.
Results to date on the stability of the atmospheric background and the CO2 intensity within peaks and bursts suggest tsetse may well be able to detect CO2 from large distances (> 64 m) downwind of an animal host in certain habitats. These studies of plume structure will be extended in parallel with electrophysiological investigations of the sensitivity of tsetse to CO2 and field studies of tsetse behaviour.