The activation of resting tsetse flies (Glossina) in response to visual and olfactory stimuli in the field

ABSTRACT Studies were conducted in Zimbabwe of the responses of Glossina morsitans morsitans Westwood and Glossina pallidipes Austen resting in a refuge to various host stimuli. Tsetse took off in response to 100% ox odour, 0.08% carbon dioxide or a visual stimulus consisting of a 0.75 × 0.75 m black target placed c . 5 m from the refuge moving at 4^o s^-1, but the level of response was low with only 35%, 19% and 29% responding, respectively. Tsetse did not take off in response to any one of 25% ox odour, 0.8% carbon dioxide, acetone (3 μg 1^-1) or octenol (0.03 μg 1^-1). In the absence of any host stimuli, flies emerged from the refuge later on hotter days (35–37^oC) than on cooler days (32–34.5^oC). Male G.pallidipes emerging later in the afternoon contained significantly more haematin than those emerging relatively earlier. There were no significant differences between the responses of G.m. morsitans and G.pallidipes. It is suggested that the initial activation of resting flies is primarily mediated through endogenous, rather than host, stimuli.     

Behaviour of tsetse flies (Glossina) in host odour plumes in the field

ABSTRACT. In Zimbabwe, studies were made of the responses of Glossina pallidipes Austen and G.morsitans morsitans Westwood to artificial host odour using an incomplete ring of electrocuting nets. In a plume of synthetic host odour tsetse flew generally upwind, with 50–60% flying within 35^o of due upwind. More than 80% of tsetse flew at < 50 cm above ground level. Upon losing contact with odour they executed a reverse turn within about 2 m, and upon regaining contact they turned upwind. There were no clear differences in the responses of G.m.morsitans and G.pallidipes. Using electrocuting nets lying horizontally on the ground it was found that tsetse landed in the vicinity of the odour source, the propensity to land being greater for G.pallidipes than for G.m, morsitans , greater for immature than mature flies, and greater for males than females.     

Flying mate detection and chasing by tsetse flies (Glossina)

Abstract Male tsetse flies, probably Glossina morsitans morsitans Westw., were video-recorded in the field as they took off and chased other tsetse flies. Chasers responded (took off) to a target fly at a maximum distance of c. 55 cm, when it subtended c. 1.6^o to their eye (–1 foveal ommatidial subtense). Chased targets were always within this range (mean subtense at take-off = 3.2^o) and approaching the chaser. The most significant difference between chased and non-chased targets was in the rate of approach of the target fly in terms of the increase in its image size immediately before the chaser took off ( 21^o s⁻¹), especially as its relative increase (690% s^-1 P< 0.005). No feature of the target's translational velocity, nor any relationship between that and the image size approached this level of significance. Chasers seemed to 'slipstream' their target at c. 20 cm directly behind it, perhaps suggesting target identification by speed matching. Chases were apparently abandoned when the target image shrank from covering at least two of the chaser's foveal ommatidia to covering only one. Parallax-free measurements of flight speeds indicated a preferred, stable mean groundspeed of 4.8±0.1 m s^_1 (SE), at a mean wing-beat frequency of 209±3 Hz.     

The effects of temperature, body mass and feeding on metabolic rate in the tsetse fly Glossina morsitans centralis

Abstract.  Metabolic rate variation with temperature, body mass, gender and feeding status is documented for Glossina morsitans centralis . Metabolic rate [mean ± SE; VCO₂= 19.78 ± 3.11 μL CO₂ h⁻¹ in males (mean mass = 22.72 ± 1.41 mg) and 27.34 ± 3.86 μL CO₂ h⁻¹ in females (mean mass = 29.28 ± 1.96 mg) at 24 °C in fasted individuals] is strongly influenced by temperature, body mass and feeding status, but not by gender once the effects of body mass have been accounted for. A significant interaction between gender and feeding status is seen, similar to patterns of metabolic rate variation documented in Glossina morsitans morsitans . Synthesis of metabolic rate-temperature relationships in G. m. centralis , G. m. morsitans and Glossina pallidipes indicate that biting frequency as well as mortality risks associated with foraging will probably increase with temperature as a consequence of increasing metabolic demands, although there is little evidence for variation among species at present. Furthermore, metabolic rate–body mass relationships appear to be similarly invariant among these species. These data provide important physiological information for bottom-up modelling of tsetse fly population dynamics.     

Feeding frequency in relation to reproduction in Glossina morsitans morsitans and G.pallidipes

Abstract Measurements of residual haematin in males of Glossina morsitans morsitans Westwood reared in the laboratory at 25^oC suggest that blood meal digestion is completed in 4 or 5 days after feeding. However, a high proportion of haematin is present as faecal matter 2 days after feeding and it is concluded that digestion is completed sooner than indicated by the regression of logio haematin on time. Therefore, low levels of residual haematin in field-caught tsetse provide no indication of the frequency with which they feed. For this reason the effects of feeding frequency upon various reproductive parameters in the laboratory have been examined. It is concluded that the best performance is achieved by G.m. morsitans females which ingest four blood meals per inter-larval period and that for a similar performance in G.pallidipes five blood meals are required. The extent to which such feeding frequencies are a reflection of feeding activity in the field are discussed in terms of the biochemical requirements to maintain a reproductive adult female tsetse in positive energy balance.     

Genetic Variation at Structural Loci in the Glossina morsitans Species Group

Gene diversity was investigated in four taxa of tsetse flies (Diptera: Glossinidae) including Glossina morsitans morsitans, G. m. centralis, G. swynnertoni, and G. pallidipes. Histochemical tests were performed for 35–46 isozymes. Polymorphic loci were 20% in G. morsitans morsitans, 32% in G. m. centralis, 17.6% in G. swynnertoni, and 26% in G. pallidipes. Mean heterozygosities among all loci were 6.6% in G. morsitans morsitans, 6.0% in G. m. centralis, 7.1% in G. swynnertoni, and 6.8% in G. pallidipes. Allozyme gene diversities were considerably less than those reported for many Diptera. The low gene diversities are probably related to small effective population sizes.     

Flight behaviour of tsetse flies in host odour plumes: the initial response to leaving or entering odour

ABSTRACT. Free-flying, wild male and female Glossina pallidipes Aust. and G. m. morsitans Westw. were video-recorded in the field in Zimbabwe as they entered or left the side of a host-odour plume in cross-wind flight, or as they overshot a source of host odour in upwind flight (camera 2.5 m up looking down at a 3 times 2.5 m field of view at ground level). 80% of cross-wind odour leavers turned sharply ( turns 95^o), but without regard to wind direction (overshooters behaved essentially the same except that nearly 100% turned). Many fewer flies entering a plume cross wind turned ( c . 60%), and when they did they made much smaller turns ( 58^o); these turns were, however, significantly biassed upwind ( c . 70%). All three classes of fly had similar groundspeeds ( 5.5–6.5 m s^_1) and angular velocities ( 350–400^o s^-1). Clear evidence was obtained of in-flight sensitivity to wind direction: significantly more flies entering odour turned upwind than downwind, and odour losers turning upwind made significantly larger turns than average. The main basis for the different sizes of turn was the different durations of the turning flight, rather than changes in angular velocity or speed. No evidence was found of flies landing after losing contact with odour.     

The flight and landing of tsetse (Glossina) in response to components of host odour in the field

ABSTRACT. Studies were conducted in Zimbabwe of the responses of Glossina morsitans morsitans Westwood and Glossina pallidipes Austen to various host odours using either arrangements of electrocuting nets or visual observations. Tsetse flying upwind in a plume of carbon dioxide, acetone and octenol turned downwind upon flying into a plume of acetone or octenol, but did not turn upon flying into a plume of carbon dioxide. They also turned in response to a transient decline in odour concentration. Tsetse landed on the ground in the vicinity of a source of natural odour or artificial odour containing carbon dioxide but not at sources of acetone or octenol only. The proportion of female G.pallidipes caught at a source of natural odour (37%) was significantly different from that caught at a source of synthetic odour (17%). Resting tsetse stimulated by natural odour took off sooner than non-stimulated flies and had a strong upwind bias in the direction of take off. Tsetse stimulated with artificial odour did not take off sooner than non-stimulated flies. It is suggested that there is an unidentified components) of ox odour that activates resting tsetse.     

'Anemotactic' flight paths of tsetse flies in relation to host odour: a preliminary video study in nature of the response to loss of odour

ABSTRACT. Free-flying, wild Glossina pallidipes Aust. and G. morsitans Westw. were video-recorded in the field in Zimbabwe as they flew out of air permeated with host odour (camera 2.5 m up, looking down at the ground). Analysis of the flight tracks supports the proposal of Bursell (1984) that tsetse flies attracted to an invisible source of host odour respond weakly if at all to wind direction while in flight: on losing contact with the odour the flies made a sharp turn that was uncorrelated with wind direction. The size of the turn varied considerably, with a marked discontinuity in the log-survivorship curve at 120° (a fly which had turned through at least 120° was 5 times as likely to stop the turn as a fly which had turned <120°). Over half the flies made turns of >90° (and <2 m diameter) within the 2×2.5 m field of view of the camera. It is suggested that these turns initially served to arrest the upwind progress of the fly, with the size of the turn determining the degree to which the fly backtracked towards where it last detected odour or continues cross-wind. Mean flight speed was c. 5 ms^-1 (min. 2.5, max. probably 7ms^-1).     

Flight responses of tsetse flies (Glossina) to octenol and acetone vapour in a wind-tunnel

Abstract. Female Glossina morsitans morsitans Westwood were video-recorded in a wind-tunnel as they entered, in crosswind flight, a broad plume of either octenol or acetone (two components of ox odour). Both odours produced upwind turning responses (in-flight anemotaxis) to a range of concentrations, with thresholds at around 10^-8mg1^-l for octenol and 10^-6mg1^-1 for acetone. Kinetic responses were unaffected by octenol at low concentrations, but flight speed was significantly reduced and sinuosity (^om^-1) and angular velocity (^os^-1) significantly increased by concentrations at or above those in ox breath; for acetone, these effects were apparent but inconsistently related to concentration. It is concluded that octenol and acetone vapour are used by tsetse flies to locate hosts by upwind anemotaxis, probably combined with kinetic responses. The behavioural basis for the 'repellency' of high octenol concentrations in the field is discussed in the context of the virtual loss of upwind anemotaxis to octenol at the highest concentration tested in the tunnel (30 × ox breath).     

Univariate analysis of tsetse habitat in the common fly belt of Southern Africa using climate and remotely sensed vegetation data

Abstract Tsetse are vectors of trypanosomes that cause diseases both in humans and livestock. Traditional tsetse surveys, using sampling methods such as Epsilon traps and black screen fly rounds, are often logistically difficult, costly and time-consuming. The distribution of tsetse, as revealed by such survey methods, is strongly influenced by environmental conditions, such as climate and vegetation cover, which may be readily mapped using satellite data. These data may be used to make predictions of the probable distribution of tsetse in unsurveyed areas by determining the environmental characteristics of areas of tsetse presence and absence in surveyed areas. The same methods may also be used to characterize differences between tsetse species and subspecies. In this paper we analyse the distribution of Glossina morsitans centralis, Glossina morsitans morsitans and Glossina pallidipes in southern Africa with respect to single environmental variables. For G.m.centralis the best predictions were made using the average NDVI (75% correct predictions; range > 0.37) and the average of the maximum temperature (70% correct predictions; 27.0–29.2°C). For G.m.morsitans the best prediction was given by the maximum of the minimum temperature (84% correct predictions; range > 18.8°C), and for G.pallidipes , also by the maximum of the minimum temperature (86% correct predictions; range > 19.6 °C). The following paper compares a range of multivariate techniques for making predictions about the distribution of these species in the same region.     

Responses of tsetse flies, Glossina morsitans morsitans and Glossina pallidipes, to baits of various size

Recent studies of Palpalis group tsetse [Glossina fuscipes fuscipes (Diptera: Glossinidae) in Kenya] suggest that small (0.25 × 0.25 m) insecticide-treated targets will be more cost-effective than the larger (≥1.0 × 1.0 m) designs currently used to control tsetse. Studies were undertaken in Zimbabwe to assess whether small targets are also more cost-effective for the Morsitans group tsetse, Glossina morsitans morsitans and Glossina pallidipes. Numbers of tsetse contacting targets of 0.25 × 0.25 m or 1.0 × 1.0 m, respectively, were estimated using arrangements of electrocuting grids which killed or stunned tsetse as they contacted the target. Catches of G. pallidipes and G. m. morsitans at small (0.25 × 0.25 m) targets were, respectively, ～1% and ～6% of catches at large (1.0 × 1.0 m) targets. Hence, the tsetse killed per unit area of target was greater for the larger than the smaller target, suggesting that small targets are not cost-effective for use against Morsitans group species. The results suggest that there is a fundamental difference in the host-orientated behaviour of Morsitans and Palpalis group tsetse and that the former are more responsive to host odours, whereas the latter seem highly responsive to visual stimuli.     

Comparative study on the susceptibility of different laboratory strains of Glossina species to Trypanosoma simiae

Abstract. Teneral tsetse of four Glossina species from laboratory-reared colonies were fed on four Large White pigs infected with three different stocks of Trypanosoma simiae isolated in Coast Province, Kenya. Thereafter the tsetse were maintained on goats and dissected on day 28 to determine the trypanosome infection rates. Glossina brevipalpis was as susceptible as G.pallidipes whilst G.palpalis gambiensis was not susceptible to T.simiae CP 11 a stock causing acute infection, which was isolated from a wild G.austeni. Glossina brevipalpis was as susceptible as G.pallidipes to another stock causing acute infection, T.simiae CP 813 isolated from a wild G.pallidipes. Glossina morsitans centralis was also as susceptible as G.brevipalpis and G.pallidipes whilst G.p.gambiensis was not susceptible to this T.simiae stock. Glossina m.centralis showed very low susceptibility to a stock causing chronic infection, T.simiae CP 1896 isolated from a bushpig, whilst G.brevipalpis, G.p.gambiensis and G.pallidipes could not be infected by this T.simiae stock. Male Glossina were generally more susceptible than females to the three T.simiae stocks.     

The effect of wind speed on the flight responses of tsetse flies to CO2: a wind-tunnel study

Abstract. Female Glossina morsitans morsitans Westwood were video-recorded in a wind-tunnel as they entered, in cross-wind flight, a broad plume of CO₂ (a component of host odour). At a wind speed that corresponds with peak catches in the field (c. 0.6 ms-¹) odour produced both significant upwind turning responses (in-flight anemotaxis) and kinetic responses (reduced flight speed and increased sinuosity (m-¹). At a wind speed of c. 0.2 ms-¹ flies displayed anemotactic, but not kinetic, responses to odour. At very low wind speeds (0.1ms-¹) neither upwind turning responses nor kinetic responses to odour were detected. The results are discussed with regard to current theory of host-location by tsetse.     

Interspecific transfer of bacterial endosymbionts between tsetse fly species: infection establishment and effect on host fitness

Tsetse flies (Glossina spp.) can harbor up to three distinct species of endosymbiotic bacteria that exhibit unique modes of transmission and evolutionary histories with their host. Two mutualist enterics, Wigglesworthia and Sodalis, are transmitted maternally to tsetse flies' intrauterine larvae. The third symbiont, from the genus Wolbachia, parasitizes developing oocytes. In this study, we determined that Sodalis isolates from several tsetse fly species are virtually identical based on a phylogenetic analysis of their ftsZ gene sequences. Furthermore, restriction fragment-length polymorphism analysis revealed little variation in the genomes of Sodalis isolates from tsetse fly species within different subgenera (Glossina fuscipes fuscipes and Glossina morsitans morsitans). We also examined the impact on host fitness of transinfecting G. fuscipes fuscipes and G. morsitans morsitans flies with reciprocal Sodalis strains. Tsetse flies cleared of their native Sodalis symbionts were successfully repopulated with the Sodalis species isolated from a different tsetse fly species. These transinfected flies effectively transmitted the novel symbionts to their offspring and experienced no detrimental fitness effects compared to their wild-type counterparts, as measured by longevity and fecundity. Quantitative PCR analysis revealed that transinfected flies maintained their Sodalis populations at densities comparable to those in flies harboring native symbionts. Our ability to transinfect tsetse flies is indicative of Sodalis ' recent evolutionary history with its tsetse fly host and demonstrates that this procedure may be used as a means of streamlining future paratransgenesis experiments.     

Responses of tsetse flies (Glossina spp.) to compounds on the skin surface of an ox: a laboratory study

The behaviour of male Glossina morsitans morsitans Westwood and Glossina pallidipes Austen (Diptera: Glossinidae) alighting on targets with or without ox sebum was compared. The presence of ox sebum did not increase significantly the number of flies alighting on the target in either species. However, after contact with the sebum coated target, both species showed an increase in flight activity, and G. m.morsitans showed a greater tendency to return to the target. This behaviour resulted in a number of short flights which may reflect the search for a feeding site on a host. The duration of each visit to the target was significantly reduced when sebum was present for G. m. morsitans but not for G. pallidipes. This is explained by documented differences in the resting behaviour of the two species which shows that G. m. morsitans normally rests for longer periods on the surface of an untreated black target than does G. pallidipes. Other experiments showed that the presence of sebum elicited a probing response in G. m. morsitans and G. pallidipes. The results are discussed with reference to the possible use of host sebum to improve trap catches in the field.     

Metabolic rates of tsetse flies in the field as measured by the excretion of injected caesium

ABSTRACT Teneral tsetse flies, Glossina morsitans morsitans Westw., were injected with labelled caesium (¹³⁷Cs) <18 h after emergence and released in the Zambezi Valley of Zimbabwe between May 1983 and June 1984, and again in February 1985. Radioactivity in flies recaptured time t days after injection indicated a three-stage exponential loss of caesium, identical for both sexes. For t≫4 the estimated rate constant (-0.119 per day) was significantly lower than for 4≫t≫12 (–0.252 per day). By day 15 about 97% of the isotope had been excreted; thereafter the loss rate fell by an order of magnitude. The data for t>4 days were well fitted by the sum of two exponentials but no smooth function was found to fit all three phases. The loss rate from the rapidly metabolized pool increased exponentially with temperature at the same rate as for male tsetse kept in the dark in the laboratory. However, the loss rate in the field was lower at every temperature, suggesting that these flies live at 2–6^oC lower than the average Stevenson screen temperature. Published estimates of hunger cycle and daily flight durations, made on the basis of measured rates of caesium excretion, are invalid because they use the assumption that flies are living in the field at screen temperatures. The data suggest that both sexes have the same metabolic rate up to the age of about 15 days, which implies that the females (being larger and having to nourish a larva in the latter stages of this period) must be less active and/or live at even lower temperatures than the males.     

Midgut lectin activity and sugar specificity in teneral and fed tsetse

Abstract. . Midgut infection rates of Trypanosoma congolense in Glossina palpalis palpalis and of Trypanosoma brucei rhodesiense in Glossina pallidipes are potentiated by the addition of D+ glucosamine to the infective feed, but not to the levels of super-infection reported for G. m. morsitans. G. p. palpalis and G.pallidipes are shown to possess two trypanocidal molecules: a glucosyl lectin which can be inhibited by D+ glucosamine and a galactosyl molecule inhibited by D+ galactose. Addition of both D+ glucosamine and D+ galactose to the teneral infective feed promotes super-infection of the midguts of G.p.palpalis. The glucosyl lectin is specific for rabbit erythrocytes and is present in guts of fed G.m.morsitans and G.p.palpalis , titres of lectin activity do not increase substantially after the second bloodmeal. The galactosyl specific molecule does not show any erythrocyte specificity, although haemolytic activity is observed only in G.p.palpalis and not in G.m.morsitans. The presence of two trypanocidal molecules in some species of tsetse may account for the innate refractoriness of these flies to trypanosome infection.
As D+ glucosamine also inhibits the killing of procyclic trypanosomes taken as an infective feed, it is suggested that the midgut lectin is normally responsible for the agglutination of trypanosomes in the fly midgut by binding to the pro-cyclic surface coat, prior to establishment in the ecto-peritrophic space.     

Tsetse mating behaviour: effects of age and hunger in Glossina morsitans morsitans and G.pallidipes

ABSTRACT. The effects of age and hunger on the responses of male Glossina morsitans morsitans Westwood and G.pallidipes Austen to freeze-killed female decoys, were examined in the laboratory. In both species, activity, estimated as the total number of interactions between males and decoys, increased with both age and hunger. Interactions were divided into short-stay (<60 s) and long-stay, full copulatory responses. In both species, young, unfed males were significantly less likely to attempt to copulate with a decoy after encounter than were fed males. Among fed males the proportion of interactions that proceeded to full copulatory attempts did not change with increasing age, but decreased consistently with increasing hunger. At all ages and hunger levels, G.pallidipes were more active than G.m.morsitans. However, after encountering a decoy, G.pallidipes were less likely to attempt to copulate than G.m.morsitans. In both species the duration of copulatory attempts did not change with age, but declined with increasing hunger. Copulatory attempts by G.pallidipes were significantly shorter than those of G.m.morsitans. The results are discussed in relation to the behaviour of tsetse in response to control devices such as traps and targets.     

Rate of trypanosome killing by lectins in midguts of different species and strains of Glossina

The activity of lectins in different species of tsetse was compared in vivo by the time taken to remove all trypanosomes from the midgut following an infective feed and in vitro by agglutination tests. Teneral male Glossina pallidipes Austen, G. austeni Newstead and G. p. palpalis R-D. removed 50% of all Trypanosoma brucei rhodesiense Stephens & Fantham infections within 60 h. A 'refractory' line of G. m. morsitans Westwood took 170 h to kill 50% infections while a 'susceptible' line of the same species failed to kill 50%. Agglutination tests with midgut homogenates showed differences between fly stocks which accorded with differences in rate of trypanosome killing in vivo. Flies fed before an infective feed were able to remove trypanosomes from their midguts more quickly than flies infected as tenerals. Increasing the period of starvation before infection increased the susceptibility to trypanosome infection of non-teneral flies. Teneral flies showed little agglutinating activity in vitro, suggesting that lectin is produced in response to the bloodmeal. Feeding flies before infection also abolished the differences in rate of trypanosome killing found between teneral 'susceptible' and 'refractory' G. m. morsitans, suggesting that maternally inherited susceptibility to trypanosome infection is a phenomenon limited to teneral flies. Electron micrographs of midguts of G. m. morsitans suggest that procyclic trypanosomes are killed by cell lysis, presumably the result of membrane damage caused by lectin action.