Application of DNA markers to identify the individual-specific hosts of tsetse feeding on cattle

Primer sets for five different ungulate loci were used to obtain individual microsatellite DNA profiles for 29 Mashona cattle from a herd in Zimbabwe. There were 3-13 alleles for each locus and, using the entire suite of five loci, each animal within the herd, including closely related individuals, could be unequivocally distinguished. Wild-caught Glossina pallidipes Austen (Diptera: Glossinidae) were fed on specific cattle and the bloodmeal was profiled 0.5-72 h after feeding. The individual specific sources of the bloodmeals, including mixe meals produced by allowing tsetse to feed on two different cattle, were reliabl identified up to 24 h after feeding. The technique was used in field studies of hos selection by G. pallidipes and G. morsitans morsitans Westwood (Diptera Glossinidae) attracted to pairs of cattle. When the pair comprised an adult and a calf, 100% of meals were from the adult. For some pairs of adult cattle, tsetse were biased significantly towards feeding on one animal, whereas for other pairs there was no such bias. In general, feeding was greater on the animal known to have lower rate of host defensive behaviour. Results suggest that relatively slight differences in the inherent defensive behaviour of cattle produce large difference in host-specific feeding rates when the hosts are adjacent. For flies attracted to pair of cattle, < 2% contained blood from both hosts. The DNA profiling technique will be useful in studying the epidemiology of vector-borne diseases of livestock.     

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.     

Feeding behaviour of Glossina pallidipes and g. morsitans centralis on Boran cattle infected with trypanosoma congolense or T. vivax under laboratory conditions

In field studies, tsetse flies (Diptera: Glossinidae) feed more successfully on cattle infected with Trypanosoma congolense Broden (Kinetoplastida: Trypanosomatidae) than on cattle infected with T. vivax Ziemann or uninfected cattle. Here we describe the first laboratory investigation of this phenomenon. In the first experiment, caged Glossina pallidipes Austen were fed for 1 and 5 min on a Boran steer infected with T. congolense clone IL 1180 and on an uninfected steer. Feeding success was recorded in this way five times over several weeks. The same protocol was subsequently used in three additional experiments with the following combinations: G. pallidipes and a steer infected with T. vivax stock IL 3913, G. morsitans centralis Machado and a steer infected with T. congolense, and G. morsitans centralis and a steer infected with T. vivax. The four experiments were replicated once, making eight experiments in total. In three experiments there was increased tsetse feeding success, measured at 1 min, after a steer became infected (T. congolense, two experiments and T. vivax, one experiment). Analysis of all data combined found no significant differences in tsetse feeding success on the different groups of cattle prior to infection, but after infection tsetse feeding success was significantly greater on the infected cattle (P< 0.001). Trypanosoma congolense infection led to a greater increase in tsetse feeding success than T. vivax infection. The increase in feeding success was not related to changes in the level of anaemia, skin surface temperature or parasitaemia. A possible explanation is the effects of trypanosome infection on cutaneous vasodilation and/or blood clotting in infected cattle. When allowed to feed for 5 min, nearly all tsetse engorged successfully and effects of cattle infection on feeding success were not found.     

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.     

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.     

Analysis of the mating activity of male tsetse flies Glossina m. morsitans and G. pallidipes in the laboratory

ABSTRACT. The responses of male Glossina morsitans morsitans West-wood and Glossina pallidipes Austen to freeze-killed females were examined in the laboratory. Analyses were performed using a specially designed, automated, computer-based, recording system. G. pallidipes were more active than G. m. morsitans , interacting with the female decoys twice as often. Interactions with the decoys divided broadly into short-stay (<60 s) and long-stay, full copulatory attempts. For G. m. morsitans 90% of interactions resulted in full copulatory attempts, the mean duration of which was >1 h. For G. pallidipes only 40% of interactions resulted in full copulatory attempts, the mean duration of which was 35 min. The initiation of interactions showed a clear V-shaped activity pattern in G. m. morsitans but in G. pallidipes only a morning peak was observed. In neither species was there a tendency for full copulatory responses to be initiated in any specific period of the diurnal activity pattern. The results indicate that the two species have very different mating systems, and represent an initial step in the quantification of these differences.     

Optimized simulation of the control of tsetse flies Glossina pallidipes and G. m. morsitans (Diptera: Glossinidae) using odour-baited targets in Zimbabwe

In 1984-1985 insecticide-treated targets were deployed in the 600-km2 Rifa Triangle, Zambezi Valley, Zimbabwe. Trap catches of Glossina pallidipes Austen were modelled using a function combining logistic growth with diffusive movement. A simulation routine was linked to a non-linear least-squares optimization programme and fits optimized with respect to population carrying capacities, rates of growth and movement, and to levels of imposed mortality. In March-September 1984, the overall additional mortality was 2% per day of adult female G. pallidipes, increasing thereafter to 8% per day, due to the deployment of more targets, the onset of the hot, dry season and the ground-spraying of the adjoining Zambezi escarpment with DDT. For G. m. morsitans Westwood the corresponding estimates were 1 and 2% per day. For both species, the deployment of four targets km(-2) in a closed population will ensure eradication. For G. m. morsitans a halving of target efficacy would reduce the killing rate to the point where eradication would be unlikely. Estimated daily displacements were c. 200 m for G. m. morsitans and 660 m for G. pallidipes. The lower rate for G. m. morsitans means that, while targets kill this species less effectively, re-invasion of cleared areas is slower. Targets do not markedly affect robust populations outside the deployment area. The Zambian tsetse population adjacent to the Rifa Triangle declined markedly during the experiment, however, suggesting that it is largely maintained by immigration. The methods developed here will be applied to data from other campaigns with the aim of improving the efficiency of tsetse control programmes.     

A comparison of African buffalo, N''Dama and Boran cattle as reservoirs of Trypanosoma congolense for different Glossina species

Teneral Glossina morsitans centralis Machado were fed on the flanks of the African buffalo (Syncerus caffer Sparrman), N'Dama (Bos taurus L.) or Boran (Bos indicus L.) cattle infected with Trypanosoma congolense Broden. The infected tsetse were maintained on rabbits and on day 30 after the infected feed, the surviving tsetse were dissected to determine the infection rates. The mean infection rates (% +/- SE) in the midgut of tsetse fed on buffalo, N'Damas and Borans were 23.5 +/- 3.3, 31.6 +/- 2.7 and 33.7 +/- 4.6, respectively. The differences were not significant. However, the mean mature infection rate in tsetse fed on the buffalo (13.2 +/- 2.1%) was significantly lower compared to the rates in tsetse fed on the N'Dama (20.4 +/- 1.4) or the Boran cattle (21.4 +/- 1.1). When groups of teneral G.m.centralis, G.pallidipes Austen, G.p.gambiensis Vanderplank, G.f.fuscipes Newstead, G.brevipalpis Newstead and G.longipennis Corti were fed simultaneously on either an infected buffalo, an N'Dama or a Boran steer, the mature infection rates ranged from 0 to 16.1%. Irrespective of the host species used, the T.congolense infection rate was highest in G.m.centralis, lowest in the palpalis and fusca group tsetse, with G.pallidipes being intermediate. Nevertheless, the trypanoresistant African buffalo and N'Dama may serve as reservoirs of T.congolense as can trypanosusceptible Boran cattle.     

The influence of sex and fly species on the development of trypanosomes in tsetse flies

Unlike other dipteran disease vectors, tsetse flies of both sexes feed on blood and transmit pathogenic African trypanosomes. During transmission, Trypanosoma brucei undergoes a complex cycle of proliferation and development inside the tsetse vector, culminating in production of infective forms in the saliva. The insect manifests robust immune defences throughout the alimentary tract, which eliminate many trypanosome infections. Previous work has shown that fly sex influences susceptibility to trypanosome infection as males show higher rates of salivary gland (SG) infection with T. brucei than females. To investigate sex-linked differences in the progression of infection, we compared midgut (MG), proventriculus, foregut and SG infections in male and female Glossina morsitans morsitans. Initially, infections developed in the same way in both sexes: no difference was observed in numbers of MG or proventriculus infections, or in the number and type of developmental forms produced. Female flies tended to produce foregut migratory forms later than males, but this had no detectable impact on the number of SG infections. The sex difference was not apparent until the final stage of SG invasion and colonisation, showing that the SG environment differs between male and female flies. Comparison of G. m. morsitans with G. pallidipes showed a similar, though less pronounced, sex difference in susceptibility, but additionally revealed very different levels of trypanosome resistance in the MG and SG. While G. pallidipes was more refractory to MG infection, a very high proportion of MG infections led to SG infection in both sexes. It appears that the two fly species use different strategies to block trypanosome infection: G. pallidipes heavily defends against initial establishment in the MG, while G. m. morsitans has additional measures to prevent trypanosomes colonising the SG, particularly in female flies. We conclude that the tsetse-trypanosome interface works differently in G. m. morsitans and G. pallidipes.     

Reproductive rates of tsetse flies in the field in Zimbabwe

Abstract. Tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen were marked and released within 12 h of emergence at Rekomitjie Research Station, Zambezi Valley, Zimbabwe, and on Redcliff Island, Lake Kariba. Ovarian dissections were performed on recaptured flies and on wild collected samples. At Rekomitjie >90% of female G. m. morsitans were inseminated by age 4 days and G. pallidipes by 7 days. For both species at both sites the length of the largest oocyte, for flies in ovarian category zero, increased approximately linearly for about the first 6 days and was ovulated at c. 6–8 days. The largest oocyte grew significantly more slowly in later cycles. For G. m. morsitans , but not for G. pallidipes , the rate increased with temperature; the rates were always higher than observed in the laboratory. At Rekomitjie, for both species and at a mean screen temperature of 22C, the first larva was produced at c. 18 days and subsequent larvae at 11–day intervals; the intervals decreased with temperature by c. 0.5 days/C. On Redcliff Island the intervals for both species were 2 days shorter than at Rekomitjie at any given screen temperature and were sometimes as short as 7 days. The length of the larva in utero increased exponentially during pregnancy.     

Less is more: restricted application of insecticide to cattle to improve the cost and efficacy of tsetse control

Studies were carried out in Zimbabwe of the responses of tsetse to cattle treated with deltamethrin applied to the parts of the body where most tsetse were shown to land. Large proportions of Glossina pallidipes Austen (Diptera: Glossinidae) landed on the belly ( approximately 25%) and legs ( approximately 70%), particularly the front legs ( approximately 50%). Substantial proportions of Glossina morsitans morsitans Westwood landed on the legs ( approximately 50%) and belly (25%), with the remainder landing on the torso, particularly the flanks ( approximately 15%). Studies were made of the knockdown rate of wild, female G. pallidipes exposed to cattle treated with a 1% pour-on or 0.005% suspension concentrate of deltamethrin applied to the (a) whole body, (b) belly and legs, (c) legs, (d) front legs, (e) middle and lower front legs, or (f) lower front legs. The restricted treatments used 20%, 10%, 5%, 2% or 1% of the active ingredient applied in the whole-body treatments. There was a marked seasonal effect on the performance of all treatments. With the whole-body treatment, the persistence period (knockdown > 50%) ranged from approximately 10 days during the hot, wet season (mean daily temperature > 30 degrees C) to approximately 20 days during the cool, dry season (< 22 degrees C). Restricting the application of insecticide reduced the seasonal persistence periods to approximately 10-15 days if only the legs and belly were treated, approximately 5-15 days if only the legs were treated and < 5 days for the more restricted treatments. The restricted application did not affect the landing distribution of tsetse or the duration of landing bouts (mean = 30 s). The results suggest that more cost-effective control of tsetse could be achieved by applying insecticide to the belly and legs of cattle at 2-week intervals, rather than using the current practice of treating the whole body of each animal at monthly intervals. This would cut the cost of insecticide by 40%, improve efficacy by 27% and reduce the threats to non-target organisms and the enzootic stability of tick-borne diseases.     

Is there safety in numbers? The effect of cattle herding on biting risk from tsetse flies

In sub-Saharan Africa, tsetse (Glossina spp.) transmit species of Trypanosoma which threaten 45-50 million cattle with trypanosomiasis. These livestock are subject to various herding practices which may affect biting rates on individual cattle and hence the probability of infection. In Zimbabwe, studies were made of the effect of herd size and composition on individual biting rates by capturing tsetse as they approached and departed from groups of one to 12 cattle. Flies were captured using a ring of electrocuting nets and bloodmeals were analysed using DNA markers to identify which individual cattle were bitten. Increasing the size of a herd from one to 12 adults increased the mean number of tsetse visiting the herd four-fold and the mean feeding probability from 54% to 71%; the increased probability with larger herds was probably a result of fewer flies per host, which, in turn, reduced the hosts' defensive behaviour. For adults and juveniles in groups of four to eight cattle, > 89% of bloodmeals were from the adults, even when these comprised just 13% of the herd. For groups comprising two oxen, four cows/heifers and two calves, a grouping that reflects the typical composition of communal herds in Zimbabwe, approximately 80% of bloodmeals were from the oxen. Simple models of entomological inoculation rates suggest that cattle herding practices may reduce individual trypanosomiasis risk by up to 90%. These results have several epidemiological and practical implications. First, the gregarious nature of hosts needs to be considered in estimating entomological inoculation rates. Secondly, heterogeneities in biting rates on different cattle may help to explain why disease prevalence is frequently lower in younger/smaller cattle. Thirdly, the cost and effectiveness of tsetse control using insecticide-treated cattle may be improved by treating older/larger hosts within a herd. In general, the patterns observed with tsetse appear to apply to other genera of cattle-feeding Diptera (Stomoxys, Anopheles, Tabanidae) and thus may be important for the development of strategies for controlling other diseases affecting livestock.     

Activation of three species of tsetse (Glossina spp.) in response to host derived stimuli

Recordings were made of the activation of hungry Glossina morsitans morsitans Westwood, G. pallidipes Austen, and G. austeni Newstead in response to odours from ox breath and ox urine, and a moving visual stimulus, in a wind tunnel. The spontaneous activity of G.m.morsitans was very low (less than 4% of males and 2% of females active per min during control periods). That of G.austeni and G.pallidipes was in the region of 20% except for G.pallidipes females when in excess of 40% were active during control periods. Addition of ox urine odours to the airstream had no effect on activity in any of the species investigated but addition of ox breath odours to the airstream significantly increased activity of G.pallidipes and of G.m.morsitans, although for the latter only approximately 12% of flies were active. For G.austeni the addition of ox breath odours resulted in a significant increase in activity of females but not of males. The moving visual stimulus resulted in a significant increase in the activity of both sexes of G.austeni and G.m.morsitans but no change in the activity of G.pallidipes. The low level of spontaneous activity and the low response to ox breath odours in a strain of G.m.morsitans maintained in the laboratory since 1969 was compared with a new colony of this species which originated from puparia collected in Zimbabwe in 1991. No differences in either spontaneous activity or the response to ox breath odour was recorded, but females from the new colony were significantly more responsive to a moving visual stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

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.     

Reproductive abnormalities in tsetse flies in Zimbabwe

Between November 1988 and July 1995 five technicians carried out ovarian dissections on 16,013 Glossina morsitans morsitans Westwood and 123,848 G. pallidipes Austen captured at Rekomitjie Research Station, Zambezi Valley, Zimbabwe. The ovarian age and uterine content were recorded, as were the lengths (l₁, l₂ and l_u) of the largest and second largest oocyte, and of any uterine inclusion. Major abnormalities and abnormal spermathecal contents were found in <0.1% of all flies dissected. Apparent abortions rates varied significantly between dissectors and occurred at frequencies of 0.8–4.5% in G. m. morsitans and 0.3–2.8% in G. pallidipes. The lowest estimates give the best picture of the field situation. Abortion rates were higher in flies caught on electric nets than in trapped flies where the rate was only 0.15%, indicating that reproductive losses are negligible for most of the year at Rekomitjie. The rates did, however, increase to >2% when mean temperatures exceeded 27°C and flies were captured in artificial refuges. There was little effect of ovarian age on the abortion rate, but the frequency of empty uteri declined markedly with age – with a suggestion, however, that it might increase again in the oldest flies. A knowledge of the rates of reproductive loss is important for the construction of realistic models of the dynamics of tsetse populations.     

Artificial larviposition sites for field collections of the puparia of tsetse flies Glossina pallidipes and G. m. morsitans (Diptera: Glossinidae)

Tsetse flies Glossina pallidipes Austen and G. morsitans morsitans Westwood deposit their larvae in warthog burrows, in August-November, at Rekomitjie Research Station, Zambezi Valley, Zimbabwe. Artificial burrows, made from 200-l steel drums, were used to sample these flies and to collect their puparia. Sand-filled plastic trays in the burrows served as a substrate for larval deposition. The sand was covered with c. 2 cm of leaf litter after it was shown that only 3% of larvae were deposited on bare sand if both substrates were available. Other burrow modifications - artificially shading the burrow entrance, increasing the relative humidity inside the burrow, or reducing the size of the burrow entrance - significantly decreased deposition rates. The use of burrows in the hot season results in a reduction in the temperature experienced by the puparium towards an assumed optimum level of 26 degrees C. Artificial burrows maintained a mean temperature of 28.5 degrees C during October-November 1998, c. 2.5 degrees C cooler than ambient; earlier work has shown that natural burrows can be c. 5 degrees C cooler than ambient at these times. This may explain why natural burrows in full sunlight were used for larviposition, whereas artificial burrows were used only when they were in deep shade, and why significantly higher proportions of G. pallidipes were found in natural (66%) than in artificial burrows (34%). Better-insulated artificial burrows might produce more puparia with higher proportions of G. pallidipes. Burrows become waterlogged during the rains and may be too cool for optimum puparial development during the rest of the year. The percentages of G. m. morsitans in catches of females from artificial burrows, refuges and odour-baited traps were 34, 26 and < 10% respectively. Traps are biased in favour of G. pallidipes; artificial burrows may show a bias in favour of G. m. morsitans that is a function of temperature. Artificial warthog burrows provide a convenient way of studying the puparial stage in tsetse and for the first time facilitate the capture of females as they deposit their larvae.     

Use of deltamethrin 'pour-on' insecticide for the control of cattle trypanosomosis in the presence of high tsetse invasion

A deltamethrin 'pour-on' insecticide was applied monthly to over 2000 cattle exposed to a high challenge of drug-resistant trypanosomes and high tsetse re-invasion pressure in the Ghibe valley, south-west Ethiopia. Blood samples were taken monthly from an average of 760 cattle for determination of PCV and presence of trypanosomes. The area of the valley is approximately 350 km2 and the cattle grazed in roughly four locations covering about a quarter to half of the area. Two years before the trial commenced, Glossina morsitans submorsitans Newstead (Diptera: Glossinidae) began to invade the valley. Despite the use of the pour-on the mean apparent density of G. m. submorsitans continued to rise, and, during the 4 years of tsetse control, was more than three-fold higher than that recorded during the previous 18 months. Over the same period there was little change in the apparent density of Glossina pallidipes Austen (Diptera: Glossinidae). By contrast, the mean monthly prevalence of trypanosome infections in cattle over 36 months of age decreased from 38.3 to 29.0%, the incidence of new infections decreased from 26.6 to 16.0% (a reduction of 40%), and packed cell volume in cattle increased from 21.7 to 24.1%. Evidence of a change in apparent parasite transmission rate was demonstrated by regression of infection incidence in cattle on the logarithm of apparent density of G. m. submorsitans. Before the trial started the regression coefficient was 45.8 +/- 6.3 and this reduced to 9.2 +/- 2.5% incidence per log(e) (flies/trap/day) during the period of tsetse control. It was concluded that this indicated reductions in tsetse numbers in the immediate vicinities of cattle in a way that was not reflected in overall tsetse catches. Nevertheless, the comparatively high levels of trypanosome prevalence that persisted in the cattle demonstrates that, where invasion prevalence is high, treatment of small pockets of cattle will not eradicate tsetse. To achieve more significant reduction in trypanosome prevalence in cattle, integrated methods of control utilizing target barriers in the major routes of invasion will be needed.     

Factors affecting the landing and feeding responses of the tsetse fly Glossina pallidipes to a stationary ox

In Zimbabwe, studies were made of the landing and feeding responses of Glossina pallidipes on an ox. Of the tsetse approaching an ox, ≈ 70% fed. Increasing densities of tsetse increased the grooming responses of the ox but had no significant effect on the percentage of tsetse that engorged. The landing site of tsetse on the ox varied with density, with ≈ 50% landing on the legs at low densities (< 20 flies per ox), compared to ≈ 80% at densities > 40 flies per ox. For male G. pallidipes , the mean bloodmeal size was 37 mg. The probability of feeding was negatively correlated with fat content, declining from 91% for flies with < 1 mg fat to < 50% for flies with > 4 mg fat. Bloodmeal size was also negatively correlated with fat content; the regression equation relating bloodmeal size and fat content indicated that the mean wet weight declined from 42 mg for flies with 1 mg of fat to 31 mg for flies with 5 mg of fat. For females, the probability of feeding was not significantly affected by age as determined by ovarian category but there was a paucity of young (ovarian category 0) flies attracted to the ox. Pregnancy status had no significant effect on the probability of feeding, but samples of flies attracted to the ox showed a relative dearth of females approaching larviposition and a preponderance just after.