Evaluation of insecticide-treated cattle as a barrier to re-invasion of tsetse to cleared areas in northeastern Zimbabwe

A field trial in Zimbabwe investigated the efficacy of insecticide-treated cattle as a barrier to prevent the re-invasion of tsetse, Glossina morsitans and G. pallidipes (Diptera: Glossinidae), into cleared areas. The original tsetse barrier consisted of insecticide-treated odour-baited targets, at an operational density of four to five targets per km2, supported by insecticide-treatments of cattle with either deltamethrin dip (Decatix, Coopers) at two-weekly intervals, or deltamethrin pouron (Spoton, Coopers) at monthly intervals, in a band approximately 20 km wide from the re-invasion front. Tsetse catch, and trypanosomiasis incidence in nine sentinel herds was recorded for 7-8 months, respectively, before the targets were removed, leaving only the insecticide treatment of the local cattle to stem the re-invasion of tsetse. After the removal of the target barrier, the tsetse readily invaded the trial area and the incidence of trypanosomiasis in sentinel herds increased, while their PCVs decreased. After seven months without the targets in place, trypanosomiasis prevalence in the local stock had reached alarmingly high levels; the trial was terminated prematurely and the target barrier re-deployed. Immediately after the re-deployment of the target barrier, the tsetse catch in the trial area reverted to acceptable levels along the re-invasion front, and trypanosomiasis incidence in the sentinel cattle decreased. It is concluded that, under the conditions of the field trial, the insecticidal treatment of local cattle did not in itself form an effective barrier to tsetse re-invasion. By contrast, the target barrier performed as was predicted by mathematical and experimental analysis, and readily cleared the tsetse infestation and reduced trypanosomosis incidence in the trial area.     

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.     

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.     

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 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.     

The role of cattle as hosts of Glossina longipennisat Galana Ranch, south-eastern Kenya

Abstract. Glossina longipennis were recorded visiting and engorging on cattle in an enclosure and on a single ox in a crush using transparent electrocuting nets in an incomplete ring. Of the total flies caught, 3–6% of males and 5–6% of females in the total catches were engorged (a feeding success rate of up to 16.6% and 12.6%, respectively, depending on assumptions made about the proportion which had an opportunity to feed). Direct observation of tsetse from an observation pit showed 57% landing on the front legs, 13% on the hind legs, and 11 % on the belly of the host. The largest number of bloodmeals was taken from the front legs, although only 14% of landings there terminated in feeding; a higher proportion of the flies alighting on the hind legs and flank succeeded in feeding (28% and 21% respectively). Glossina longipennis were attracted to targets baited with ox odour from an underground pit in a dose-dependent manner. Odour of humans was much less attractive to G.longipennis than that of oxen (for equivalent biomass). Analysis of bloodmeal samples from tsetse caught in two sites on die ranch showed that G.longipennis preferentially feeds on suids, bovids and hippopotamus.     

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.     

Interactions between cattle and biting flies: effects on the feeding rate of tsetse

In Zimbabwe, studies were made of the effect of host behaviour on the feeding success of Glossina pallidipes Austen and G. morsitans morsitans Westwood (Diptera: Glossinidae) attracted to cattle of different age and sex. The mean feeding rates for male and female G. pallidipes attracted to oxen were 60% and 58%, respectively, compared to 33% and 53% for male and female G. m. morsitans. The feeding rate of G. pallidipes varied between oxen and was inversely correlated with a host's rate of defensive leg movements, which, in turn, was positively correlated with the density of Stomoxys spp. (Diptera: Muscidae) caught in the vicinity of the host. Tsetse were significantly less successful in feeding from young cattle. For G. pallidipes, the feeding rate on calves (<6 months) was 11%, whereas for male and female G. m. morsitans the rates were 12% and 20%, respectively. Significantly lower feeding rates were apparent for cattle aged up to 2 years, when the feeding rate for G. pallidipes (31%) was still significantly less than that on mature oxen (68%). Feeding rates for G. pallidipes on adult female cattle were lower than those on oxen (45% vs. 61%). The lower feeding rates in young animals were attributed to higher rates of defensive movements. The results suggest that higher rates of defensive activities by young cattle reduce the risk of them contracting trypanosomiasis.     

Could insecticide‐treated cattle reduce Afrotropical malaria transmission? Effects of deltamethrin‐treated Zebu on Anopheles arabiensis behaviour and survival in Ethiopia

Anopheles arabiensis Patton (Diptera: Culicidae) is the most widespread vector of malaria in the Afrotropical Region. Because An. arabiensis feeds readily on cattle as well as humans, the insecticide-treatment of cattle--as employed to control tsetse (Diptera: Glossinidae) and ticks (Acari: Ixodidae)--might simultaneously affect the malaria vectorial capacity of this mosquito. Therefore, we conducted field experiments in southern Ethiopia to establish whether Zebu cattle (Bos indicus L.) treated with a pour-on pyrethroid formulation of 1% deltamethrin, widely used to control ticks and tsetse, would be effective against An. arabiensis or cause the female mosquitoes to feed more frequently on humans, due to behavioural avoidance of insecticide-treated cattle. Contact bioassays (3 min exposure) showed that the insecticide remained effective for about 1 month (kill rate > 50%) against mosquitoes feeding on the flanks of treated cattle. A novel behavioural assay demonstrated that An. arabiensis readily fed on insecticide-treated cattle and were not deflected to human hosts in the presence of treated cattle. DNA-fingerprinting of bloodmeals revealed that An. arabiensis naturally feeds most frequently on older animals, consistent with the established practice of applying insecticide only to older cattle, while allowing younger untreated animals to gain immunity against infections transmitted by ticks. These encouraging results were tempered by finding that > 90% of An. arabiensis, An. pharoensis and An. tenebrosus females feed on the legs of cattle, farthest from the site of pour-on application along the animal's back and where the treatment may be least residual due to weathering. Observations of mosquitoes feeding naturally on insecticide-treated cattle showed that the majority of wild female anophelines alighted on the host animal for less than 1 min to feed, with significantly shorter mean duration of feeding bouts on insecticide-treated animals, and the effective life of the insecticide was only 1 week. Thus the monthly application of deltamethrin to cattle, typically used to control tsetse and ticks, is unlikely to be effective against An. arabiensis populations or their vectorial capacity. Even so, it seems likely that far greater impact on anopheline mosquitoes could be achieved by applying insecticide selectively to the legs of cattle.     

The efficacy of various pyrethroid insecticides for use on odour-baited targets to control tsetse

The efficacy of various pyrethroid insecticides for use on odour-baited targets to control tsetse was compared in Zimbabwe. Formulations were applied to cotton cloth and polyester net and, at various intervals, the materials were bioassayed by exposing fed female Glossina pallidipes (Austen) (Diptera: Glossinidae) to cloth for 45 s or by inducing them to collide briefly with net. Trial formulations were compared with deltamethrin suspension concentrate (s.c.), the insecticide currently used in tsetse control operations in Zimbabwe. Applying 0.8% suspension of alphacypermethrin to cloth or net produced high mortalities for 9 months which was similar in performance to 0.4% suspension of deltamethrin s.c. Deltamethrin s.c. and beta-cyfluthrin s.c. applied to cloth as 0.1% suspensions were equally effective, producing high mortalities for 2 months during the wet season, and 0.8% suspension of beta-cyfluthrin was effective for 12 months. Suspensions of 0.1% lambdacyhalothrin capsule suspension or 0.1% lambdacyhalothrin wettable powder were significantly less effective than 0.1% deltamethrin s.c. Chemical analyses showed that increasing the concentration of insecticide applied to material increased the initial amount of insecticide on the material and decreased the subsequent rate of loss; 0.1% suspension of beta-cyfluthrin s.c. applied to cloth produced an initial concentration of approximately 280 mg/m2 which declined by 94% in 12 months whereas 0.8% suspension showed no significant decrease in concentration (mean= 1304 mg/m2) over the same period. For controlling tsetse by means of pyrethroid-treated targets, it is suggested that beta-cyfluthrin s.c. is as effective as deltamethrin s.c. but that alphacypermethrin s.c. should be used at twice the concentration of deltamethrin s.c. to obtain the same performance.     

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.     

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 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.     

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.     

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.     

Seasonal distribution and abundance of tsetse flies (Glossina spp.) in the Faro and Deo Division of the Adamaoua Plateau in Cameroon

Ten years after the large-scale tsetse control campaigns in the important cattle rearing areas of the Faro and Deo Division of the Adamaoua Plateau in Cameroon, the seasonal distribution and abundance of tsetse flies (Glossina spp.) were determined. During a period of 12 consecutive months (January-December 2005), the tsetse population was monitored along four trap transects consisting of a total of 32 traps and two flyround transects traversing the study area, which comprised the tsetse-infested valley, a buffer zone and the supposedly tsetse-free plateau. Throughout the study period, a total of 2195 Glossina morsitans submorsitans and 23 Glossina tachinoides were captured in the traps and 1007 G. m. submorsitans (78.8% male flies) were captured along the flyround transects. All G. tachinoides and almost all G. m. submorsitans were captured in the valley. Five G. m. submorsitans were captured in traps located in the buffer zone, whereas no flies were captured in traps located on the plateau. The index of apparent abundance (IAA) of G. m. submorsitans was substantially higher in the areas close to game reserves. In the remaining part of the valley, where wildlife is scarce and cattle are present during transhumance (dry season), the IAA of tsetse was substantially lower. In this part of the valley, the abundance of tsetse seemed to be associated with the presence of cattle, with the highest IAA during transhumance when cattle are present and the lowest apparent abundance during the rainy season when cattle have moved to the plateau. It is concluded that the distribution of tsetse in a large part of the valley undergoes substantial seasonal changes depending on the presence or absence of cattle. The repercussions of those findings for the control of tsetse in the valley and the probability of reinvasion of the plateau are discussed.     

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.     

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.     

Field studies on the effect of cattle skin secretion on the behaviour of tsetse

Abstract. The effect of ox skin secretions (sebum) on the behaviour of tsetse flies, Glossina spp., was investigated in the field using electrified targets, some of which operated intermittently, and by direct observations of flies landing on treated and untreated cloth. As the off-period of an intermittently operating electrified target increased, the catch decreased both with and without the sebum present. Targets with sebum always caught more flies than targets without sebum, but there was no evidence to suggest that sebum increased the duration of stay on a target. Direct observations of flies on cloth targets revealed that for both species the presence of sebum reduced the duration of contact and for G. pallidipes the number of return contacts was increased. The results from direct observations were used to predict the number of repeat landings that would need to be made by flies in order to account for the catch of tsetse at intermittently electrified targets.     

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.