3.1.1 Environment
3.1.1.1 Climate The northern limit of distribution of Glossina palpalis gambiensis follows closely the 1000 mm rainfall limit from Mali to Togo. In the region of Dakar, Senegal, the fly lives in an area of low rainfall and high humidity, due to humid winds coming from the sea.
The southern limit of distribution is set by thicker vegetation, in which G.p. gambiensis is replaced by the very similar G.p. palpalis.
The distribution area of G.p. gambiensis covers different climatic zones with large variations in
- rainfall (1000–2000 mm)
- temperatures (mean temperature of the coldest month more than 2Co and the daily maximum temperature may reach 40°)
- relative humidity (5–100% according to season and latitude)
- winds (in the rainy season wind blows from south-west and in the dry season the Harmattan blows from the north-east).
Species of the palpalis group need high humidity and deep shade. The best conditions are about 25° and 80–85% R.H.
Glossina palpalis gambiensis living in a wide range of climates in the savanna zone of West Africa is able to find suitable microclimates in dense vegetation along watersides with a closed canopy. A thick mat of vegetation to each side prevents dry winds from penetrating the gallery forest. Shade allows temperatures to remain lower in the daytime and higher during night time than in the surrounding savanna. The presence of water nearby (either free or underground) provides the required high humidity.
During the host dry season maximum temperatures are very high but flies find resting places close to water, where conditions are more suitable than in the gallery forest as a whole.
3.1.1.2 Typical habitats Glossina palpalis gambiensis is distributed along gallery forests of the dry Sudan savanna zone and the more humid Guinea savanna zone.
In the dry season most small streams dry up especially in the Sudan savanna, but in some streams a few waterholes remain. In others with a deep riverbed, humidity remains quite high because of underground water. When small open gallery forests become too dry, tsetses leave and concentrate along the remaining humid areas.
3.1.1.3 Other habitats
- Sacred groves. These are small patches of vegetation consisting of large old trees, dense creepers and shrubs, often around a water hole or flooded area. They are often very close to villages and are protected by the villagers. Vegetation is very dense and provides suitable shelter for tsetse.
- Niayes. This Wolof word is used for landscape typical of the Dakar region, Senegal. The dominant tree is the oil palm (Elaeis guineensis). Rainfall is low (600 mm) but humidity is high due to sea winds and underground water.
- Mangrove swamps. The seashore of West Africa, from Senegal to Liberia (at the western limit of G.p. gambiensis) is lined with mangroves, especially at the mouths of rivers. In Senegal this vegetation (dominated by Rhizophora and Avicennia) is occupied by tsetses, but not as a breeding area.
- Lakeshores. If there is dense vegetation there, the lakeshore provides tsetse with the same sort of shelter as the gallery forest.
3.1.1.4 Effect of animal life on G.p. gambiensis
the host animals available to G.p. gambiensis are
- species always available: monitor lizard (Varanus) and crocodile, that live permanently at watersides
- species sometimes available: bovids (especially bushbuck), warthog, when these animals come to drink
- man, when visiting the habitat for fishing, washing, cutting wood and cultivating gardens near rivers.
Blood meal identifications from G.p. gambiensis at Kou Forest (near Bobo-Dioulasso, Upper Volta) gave the following results: reptiles 58%, man 24%, other primates 2%, bushbuck 2%, other bovids 12%, other mammals 2%.
Predators. The predators of G.p. gambiensis are the same as those of other tsetse species. In Poa Forest (near Bobo-Dioulasso, Upper Volta) spiders (Theriidae and Clubionidae) have been seen holding tsetses.
The action of predators appears to be greatest in the rains, especially in May.
Parasites. In the rainy season some nematode worms live inside a few tsetse flies, but seem to do the flies no harm. No pupal parasites have been found so far.
3.1.1.5 Action of man on the environment
Man-made habitats.
- Dams. These are being built in West Africa, but it is still too early to judge for certain their effect on vegetation, but probably the artificial lakes that are formed will encourage vegetation growth favourable to tsetses.
- Plantation. Many mango plantations are grown along riversides. These provide tsetse flies with suitable shelter, particularly so in the case of old trees with low branches.
- Hedges. The soil beneath hedges of Euphorbia balsamifera planted for fencing off gardens, is used as tsetse breeding sites.
Bush clearing.
There was an important campaign in 1938–1939 clearing bush from 500 localities from Senegal to Niger. This resulted in a retreat of G.p. gambiensis from its original northern limit.
Settlement and farming along river banks has resulted in the progressive destruction of gallery forest, for firewood.
3.1.2 Behaviour
3.1.2.1 Choice of breeding site Females deposit their larvae in shaded places. Pupae are found at 3–4 cm depth
- in sandy soils
- in humus with coarse particles
- voider logs at the foot of large trees with buttress roots
- under culverts
- between aerial roots
- under dry leaves on the ground
- in soil under low overhanging branches
In the dry season breeding sites are concentrated along dried-up river beds. These may be flooded at the first heavy rains; if so, many pupae are destroyed.
3.1.2.2 Resting sites (as at Poa Forest, Bobo-Dioulasso, Upper Volta).
Day resting sites (December).
These have been studied by tagging flies with radioactive materials (a special technique not described in this Manual). 95% of flies prefer the underside of the long woody parts of plants such as branches, twigs, roots and trunks; very few rest on leaves.
85% of flies select twigs, branches and trunks with a diameter less than 10 cm, as resting sites; just over half of the flies (51%) prefer twigs and branches 1–2 cm in diameter.
Flies often choose resting sites in the river bed close to water (29%) or within 0.50 m from the river bank (40%). Hardly any flies are found more than 3.5 m away from the edge of the stream.
Flies rest near the ground: 80% rest lower than 0.50 m and none rest higher than 1 m from ground level.
Night resting sites.
Most (90–95%) of the flies choose leaves as places on which to rest, especially green leaves of small plants (79–80%).
Other resting sites are grass stems, branches, twigs and exposed roots. Very few flies (less than 0.1%) rest on the ground.
In the early dry season (May–June) 96% of males rest less than 3 m away from the river bank; 93% rest below 0.50 m; 60% rest below 0.20 m.
During the hot dry season (March) flies are seldom found higher than 0.50 m, and none more than 2 m from the river bed.
Nearly 50% of flies rest vertically, and most of the remainder rest horizontally. The usual habit is to rest on the tip of a leaf or on the end third of the leaf blade. The fly lies parallel to the main vein of the leaf with its head towards the stem. On dry twigs fallen to the ground the fly stands on the highest point.
3.1.2.3 Activity In Glossina palpalis gambiensis the daily activity pattern depends on temperature and light intensity.
Activity starts shortly before dawn, at a temperature of about 16°.
Males show a high activity level in the morning (10.00–11.00 hours), low activity at midday (12.00–13.00 hours), followed by a second smaller increase in activity in the afternoon. Female activity is similar except that greatest activity is generally in the afternoon (13.00–15.00 hours).
Activity declines rapidly from 16.00 hours and ceases altogether at sunset (18.00 hours in the dry season).
The activity pattern varies according to season.
During the cold season (December–February) activity starts from 08.00 to 09.00 hours, increases rapidly, and stays at a high level for six hours before decreasing.
At other seasons activity starts at sunrise then rapidly increases; but the early afternoon decrease varies according to the season.
In the hot dry period (late February–March) there is an obvious early afternoon drop followed by a second peak.
In April–May there is a rather steady activity.
In the rainy season, for both sexes the morning high activity level slowly decreases until at 17.00 hours it decreases more rapidly.
3.1.2.4 Dispersal Glossina palpalis gambiensis generally disperses along the river line, following river beds or the edges of gallery forests, exceptionally covering up to 4 km in one day. It can also cross open spaces without vegetation and can even cross from one river system to another (across a watershed).
In the rainy season, this subspecies can also disperse through savanna. In the dry season, it concentrates along parts of gallery forests only where flowing, still or underground water occurs. These are the permanent habitats.
3.1.3 Populations
3.1.3.1 Breeding sites Breeding sites may dry out in the dry season, causing death of pupae placed there. In the early rainy season breeding sites along river beds are often flooded (see also 3.1.2.1).
3.1.3.2 Physiological state Males are active not only when hungry but also when they have partly digested the bloodmeal in their gut. Very few fully engorged flies are caught. Hungry flies are caught in hand nets in greater numbers in the middle of the day whereas flies in intermediate hunger stages are caught throughout the day.
Soon after larviposition females urgently need to take a blood meal. A second meal is taken when females bear a first or second instar larva. Females carrying a third instar larva rarely need to feed.
Teneral flies generally rest at the breeding sites for one or two days before taking their first blood meal.
3.1.3.3 Density and distribution In the dry season, particularly in March, low humidity and very high temperatures are unfavourable to tsetse. At this period of the year vegetation is not dense enough to provide adequate shelter and adult flies die from excessive water loss.
After the recent drought in the Sahel zone the northern limit of G.p. gambiensis in Niger and Upper Volta is now 50–100 km further south of that in 1953.
3.1.4 Epidemiology and control
3.1.4.1 Epidemiology With respect to human trypanosomiasis (Gambian sleeping sickness), G.p. gambiensis is the most important vector in West Africa because of its wide distribution over the savanna zones.
Most villages lie near water courses. Man-fly contact is very close in the dry season when flies concentrate at waterholes or in the moist parts of riverine vegetation. Thus places such as bridges, fords, watering sites, riverine fields and gardens are important points where the disease might be picked up.
The subspecies is also an important vector of animal trypanosomiasis. Cattle going to streams and rivers to drink come in close contact with tsetse at such places.
3.1.4.2 Control Work carried out in the past to control tsetse by riverine bush clearing has already been mentioned (3.1.1.5).
The fact that G.p. gambiensis is a riverine species resting near the ground and near water helps in the selective application of residual insecticides.
Only a narrow and low strip of riverine vegetation has to be treated. Also, the dry season is long enough (4–5 months) for a single application of insecticide to be sufficient.
3.2.1 Environment In savanna areas the ecology of G.p. palpalis is very similar to that of G.p. gambiensis and will not be redescribed in detail.
Glossina palpalis palpalis extends from Ghana in the west to Nigeria in the east, then continues to the south as far as Angola.
Forms transitional between this subspecies G.p. gambiensis occur in southern Ghana, Ivory Coast and Liberia.
Glossina palpalis palpalis occurs in a wide range of vegetation zones. In the north, in the Sudan savanna vegetation zone, it occurs in vegetation closely associated with rivers and streams. In the Guinea savanna vegetation zones it is also associated especially with rivers and streams, but in the much more humid vegetation of the rain forest, or in secondary vegetation produced by man's activities in the rain forest zone, it can be found well away from surface water.
Glossina palpalis palpalis often occurs with G. tachinoides but does not extend so far into the dry north.
3.2.2 Behaviour In savanna areas, pupae of G.p. palpalis are found especially in denser, forest-type vegetation in low shade provided by shrubs, creepers, low branches, fallen logs, overhanging river banks, etc.
In rain forest, breeding is widely dispersed but still mainly in heavy shade. The pupae are most easily found in sandy soil but also occur in soil containing much decaying vegetation (humus).
In very wet areas, pupae have been found up to 2 m above ground in forks of trees or in tree rot-holes. Pupae are difficult to find in the wet season, which may be due to the use of less specialized breeding sites and/or to the difficulty of handling wet soil.
Daytime resting sites in gallery forest are mainly on the underside of thin branches and stems with diameters up to about 2.5 cm. Flies rest mainly at heights of 0.3 to 2mabove the ground; they rest lower on the vegetation at higher temperatures. Tree trunks are rarely used as resting sites.
At night flies rest higher, especially on the upper surfaces of leaves. Actual resting heights depend on the vegetation; resting flies have been found up to 18 m above the ground.
Dispersal occurs especially along water courses and G.p. palpalis can fly at least 1.6 km (1 mile) in 24 hours.
3.3.1 Environment Glossina fuscipes has a very wide distribution throughout much of Zaire and nearby countries, extending to the east as far as the eastern shore of Lake Victoria. The present range includes the rain forest area as well as secondary forest and thickets in Zaire, northern Angola, southern Congo, western Tanzania, western Kenya, Uganda, western Rwanda, western Burundi, and southern Sudan. It does not extend into river systems draining into the Indian Ocean.
In Ethiopia and southeastern Sudan there are some isolated belts.
The species is divided into three subspecies: G. fuscipes fuscipes, G.f. quanzensis and G.f. martinii. In this section all the subspecies are dealt with together unless the subspecies is named.
The fly usually lives close to water, in linear forest vegetation along rivers and lakes. In addition, flies may live in less humid habitats some kilometers away from water.
3.3.2 Resting sites By day: in S.W. Central African Republic flies rest on tree trunks, foliage and low branches, with 94% resting between 1 and 1.5 m above the ground.
In S.E. Uganda in the dry season in thickets not near water, 92% rested between 1.5 and 3 m above ground, and 87% were resting on the underside of twigs and branches.
By night: in S.E. Uganda, flies rest up to 6 m, but with 88% resting below 4 m; 68% were on or under twigs less than 2.5 m diameter.
3.3.3 Host animals In the case of G.f. fuscipes in East Africa, feeds were as follows: 38% from bovids, mainly bushbuck; 34% from reptiles; 18% from man; 3% from bushpig; 7% from other manuals.
In one study made on G.f. fuscipes on islands in Lake Victoria, the flies fed on crocodiles and monitor lizards, but after these host animals had been chased away, the flies fed on the party of men.
Around Brazzaville, some G.f. fuscipes populations nearly always feed on man, and have become 'village flies'.
3.3.4 Parasites and predators At Lake Victoria, 1.5% of G.f. fuscipes pupae were found to be parasitized by Syntomosphyrum (Hymenoptera). This parasite was bred and released in large numbers in an effort to control G. fuscipes on an island in Lake Victoria, but only 5% of the wild population of pupae became parasitized.
Predation of pupae by ants has been observed.
3.3.5 Behaviour At Lugala, Uganda, G.f. fuscipes activity started about dawn, with greatest activity occurring near the middle of the day. Activity level fell rapidly in the evening, and there was no activity at night. The pattern is more or less the same whatever the season, or whether the flies are males or females.
At Kinshasa, Zaire, G.f. quanzensis daily activity began when the temperature reached 20° or 21°, and increased rapidly above 23°-24O. Male activity began before female.
When resting flies are disturbed, they fly up but often return to exactly the same spot.
3.3.6 Breeding sites In East Africa, breeding sites are dry sandy beaches under the shade of dense vegetation. Pupae may also be found in dry leaf fragments under overhanging rocks, and in coarse dry sand sheltered by ferns and by the forest canopy.
3.3.7 Adult populations In some areas G.f. fuscipes adults decrease in numbers during the dry season. In other areas the numbers remain roughly constant throughout the year.
In Kinshasa, Zaire, G.f. quanzensis density was found to be highest at the start of the rains, and lowest at the start of the dry season. The percentage of females caught was highest when the total density was highest.
Males may move up to 300–400 m per day in riverine habitats (Uganda).
Males have a four-day hunger cycle. Results of hunger staging studies depend largely on the number of stage 4 (very hungry) flies caught.
3.3.8 Epidemiology and control Glossina fuscipes was the vector responsible for up to 200,000 deaths from Gambian sleeping sickness between 1895 and 1910, in Uganda. This epidemic followed an outbreak of rinderpest which reduced game animals and caused flies to feed from man in greater numbers.
Glossina fuscipes is also a vector of Rhodesian sleeping sickness, on a much smaller scale.
Infection rates with trypanosomes is generally lower in G. fuscipes than in other tsetse flies living in the same areas. This is probably because of the large number of blood meals taken by G. fuscipes from reptiles, which are not reservoirs for Trypanosoma vivax, T. congolense or T. brucei. Infection rates in flies living in sane remote places may even be nil. In other places, vivax type is the commonest infection, and the infection rate is highest in or immediately after the wettest months.
Mechanical transmission of T. gambiense by G. fuscipes has been studied experimentally, and probably occurs naturally. This would help to explain how G. fuscipes can have very low rates of T. gambiense infection even in sleeping sickness areas.
3.4.1 Environment
3.4.1.1 Climate Glossina tachinoides is usually found within gallery forests, and its distribution pattern corresponds to the system of drainage lines and streams along which these gallery forests grow.
This thick vegetation lessens the severe effect of the general climate and makes microclimatic conditions (of temperature, humidity and light) that are better suited to the life of tsetse.
A wide variety of host animals live in or near the same habitat, providing permanently available food supplies.
Unusually harsh climatic conditions may greatly alter the environment. Thus the occurrence of a long drought, involving the reduction of riverine vegetation and the drying up of streams, destroys some of the habitat and some of the animals that are essential for the survival of G. tachinoides.
3.4.1.2 Typical habitats According to the season, G. tachinoides may come to occupy different parts of the general habitat.
The habitats that are permanently occupied or visited frequently, consist of vegetation types in which generally one species of tree or shrub is dominant (Morelia, Mitragyna, Mimosa).
The density of the vegetation and the canopy or cover formed by the leafy branches are important in the ecology of G. tachinoides. They provide a refuge for the flies, allowing them to find the conditions required for their survival and their breeding.
3.4.1.3 Other habitats Special circumstances such as forest clearing, or drought, may greatly alter the microclimate and scatter the host animals. Glossina tachinoides nay then take refuge in other (atypical) habitats, next to villages and in cultivated areas. Eventually they may become established and breed there.
These atypical habitats may be several kilometers from the nearest water; they can be of semi-artificial or artificial type:
Semi-artificial habitats. These consist of various kinds of plantation (such as oil palm, mangoes, bananas, cola nuts, cacao, coffee); cattle control posts may also be used.
Artificial or peri-domestic habitats (peri-domestic = around the hone). These are found in the high rainfall, southerly regions of G. tachinoides distribution. They include villages where the domestic pig and other livestock are kept (see 3.4.1.4).
3.4.1.4 Effect of animal life on G. tachinoides
Animal hosts. Glossina tachinoides feeds on man and a variety of mammals and reptiles (see Volume I, 6.1). The origin of the blood meals depends on the chance or opportunity of the tsetse feeding on one animal host species rather than another: in the natural habitat there is probably not much selection of one host species rather than another, on the part of the tsetse. This is called opportunistic feeding.
In the typical habitat, the host animals most often used are:
- bovids (especially bushbuck)
- suids (especially warthog)
- reptiles (especially monitor lizard and snakes).
These are fed upon in various proportions according to area, season and habitat.
The reptiles are more fed upon in the humid savanna areas, making up more than one-third of the blood meals.
In the dry savanna areas antelopes provide the greater number of blood meals (more than 75%).
Man is only an incidental (occasional) host, and the amount he is fed upon depends on how often he visits the habitat.
In the semi-artificial habitats (see 3.4.1.3) wild hosts are less abundant, and here man, domestic livestock, as well as wild hosts, are fed upon.
Tsetse populations living in artificial peri-domestic situations, have become very specialized in their feeding habits. They feed mainly on domestic pigs, a little on cattle, and only rarely on man. This host dependence is so strong that the removal of the pigs brings about a large reduction of the fly population. This is no longer opportunistic feeding.
Natural enemies. The natural enemies of G. tachinoides have most often been studied in the gallery forest. They are mainly predators and parasites belonging to the Arthropoda (see Volume 3, Appendix I).
Predators active against adult tsetse:
- Rabber flies (Asilidae, Diptera). They perch on look-out places on isolated twigs,within the habitat or at its edge, and cath in flight the tsetses that come within reach.
- Spiders (Araneae). These have their home on the tree trunks which tsetse use as their resting sites. The flies are caught by the spiders and immediately wrapped up in a web cocoon.
Predators active against tsetse pupae:
- Seed-eating birds, while scratching in soil for their main food, may discover and eat tsetse pupae.
- Small mammals may eat tsetse pupae along with a lot of other insects in the soil.
- Other soil arthropods (especially beetles and ants).
It is usually impossible to actually see these predators eating tsetse pupae in the field, so the exact importance of the various predators is not known.
Parasites of pupae (see 1.1.4):
- Thyridanthrax (Bombyliidae, Diptera)
- Syntomosphyrum (Hymenoptera)
- Mutilla (Hymenoptera)
Usually, parasitism is observed only in the hot season, when the pupae are fairly concentrated.
In general, predators or parasites are not restricted to tsetses. The action is seasonal. Rather high rates of parasitism do not necessarily lead to low tsetse numbers.
3.4.1.5 Action of man on the environment Man has an effect on the population of tsetse by
altering the habitat
reducing the numbers of host animals
This effect may come about simply by people cutting down the riverine vegetation where tsetses and wild animals are living. It may not be part of a deliberate control programme, but it can have the same effect.
3.4.2 Behaviour
3.4.2.1 Choice of breeding site Pupae of G. tachinoides are to be found in the habitat of the adult, especially in places protected by a vegetation cover, giving permanent shade and humidity, and cooler conditions Pupae are often found in or near the disturbed soil of animal tracks and animal resting places.
The breeding site can vary according to the seasons, in relation to the change of temperature and humidity. This variation is greatest in regions where the differences between seasons are greatest, and where flooding occurs (gallery forest of dry savanna). In such places the breeding sites are concentrated in the dry season and dispersed during the rains and floods.
The breeding sites also vary according to the general area. In the regions where G. tachinoides occupy atypical habitats (in the southern part of their distribution) the pupae can be found:
- in plantations
- in soil beneath parts of the foliage hanging away from the trunk (bananas, mangoes, etc.)
- in places even within villages, such as underneath water storage vessels, at the base of fences, under heaps of firewood or yams stored against the walls of village huts.
At the breeding site itself, temperatures vary from 19° to 33°: this causes the duration of the pupal period to be from 38 to 23 days. It is therefore about 1.6 times longer in the cool season than in the hot season. The humidity there is always between 50% and 80% R.H.; the pupa may die if the humidity goes higher or lower than these limits.
3.4.2.2 testing sites As with other tsetses, G. tachinoides spends its life between long resting periods and brief moments of activity.
The flies settled at true resting sites can be regarded as being asleep, and digesting their last meal.
Under extreme climatic conditions (of heat or cold) they sit quietly and are not disturbed by things going on around them.
The true resting sites are of two kinds, day resting sites and night resting sites.
Day resting sites.
These are on the woody living parts of the vegetation, sheltered from the sun and wind, such as tree trunks, branches and exposed parts of roots.
Glossina tachinoides usually occupies the zone bordering the vegetation, above all when it is of a bushy type (Mimosa habitat); it practically never strays deeper into the vegetation.
The resting sites are widely dispersed in the cold wet season; but they are concentrated in the hot dry season. The tsetses come closer to the water and to the ground when the temperature and dryness are both severe.
The horizontal concentration is best seen when the gallery forest is wide; as in the humid savanna regions when near to 90% of flies are at less than 16 m from the waterside, and 75% are less than 8 m, in a gallery forest of 100 m width.
The vertical concentration is caused by the tsetses searching for more shaded places, during which they always find a; microhabitat having a lower temperature (by as much as 10°) and a higher humidity than the general environment. At temperatures less than 30° the tsetses use all the woody structures to be found under foliage. When the temperature rises, the resting sites come closer to ground level. In the hot period, 99% of flies observed at rest are less than 0.80 m above ground and 80% of them are less than 0.30 m. In northern regions, at hours when the general temperature can be as high as 40°–41°, virtually all of the tsetses are then settled at heights less than 0.30 m from the ground.
Night resting sites.
These differ from day resting sites both as regards the plant parts that are used, and their exact location.
The great majority of tsetse rest on the green parts of the vegetation. They also ate found in higher, more scattered sites than during the day.
3.4.2.3 Activity Glossina tachinoides is active only during the day, when temperatures are between 18° and 41?. Activity is greatest at 31°. It rises rapidly as this temperature is approached, then drops very rapidly above this temperature.
The pattern of daily activity therefore varies according to the season.
- There is a single maximum of activity at midday in the cool season.
- There are two maxima (in the morning and evening) in the hot season.
- There is fairly steady level of activity in intermediate seasons (notably the rainy season).
- On very hot days, midday activity is totally absent.
Variations of humidity do not seem to have much influence on the amount of activity; but when humidity is high flies may be active a little longer than usual.
Variations of light intensity: when the temperature conditions are right, flies may be active in poor light, but a strong light intensity reduces activity.
Tsetses are drawn towards the light when the temperature is lower than 31°, but above that temperature they look for shady places.
3.4.2.4 Dispersal Individuals of Glossina tachinoides can move around (disperse) within their, habitat or even beyond.
This may be done by flight in the normal way, or it may be done by the flies being carried along by a vehicle, an animal or a man, on which the flies may settle temporarily.
Dispersal within the normal habitat is done throughout the days of the cool or intermediate seasons, but only in the mornings and evenings of the hot season. By ordinary flight, G. tachinoides can spread 1 km per day, following tracks cleared in the undergrowth.
In the hot hours, when activity is low, flies move towards shadier places in which to find resting sites,or to deposit larvae.
Dispersal away from the main habitat occurs when the temperature and light are favourable. That is the case early in the morning or late in the afternoon of the hot season. It also happens during the rainy season throughout the day, when the rains have reduced the temperature, and when mists or cloud reduce the brightness of the sun.
It allows a wider scattering of the resting sites at night at this season. As a result of such dispersal, a certain number of tsetse may be carried well away from their usual homes and come to occupy other habitats, lying sometimes several kilometers distant.
3.4.3 Populations The breeding sites of pupae are described under 3.4.2.1.
3.4.3.1 Sex ratio This is often written as the percentage of females in a sample.
- At emergence: the females make up 50% of the emerging flies.
- Among active flies captured by hand net: females make up on average about 30% of the sample. This value remains steady for most of the year, with some increase at the end of the rainy season and some decrease in the middle of the hot season. The figure only occasionally reaches 50%. In daytime captures, the figure increases as the light fades.
- Among flies captured by Challier-Laveissiere traps: a higher percentage of females is taken by these traps than by hand net collection.
3.4.3.2 Age The length of life of females is generally between a minimum of one month (in the hot dry season) and a maximum of 3 months (in the cool season).
3.4.3.3 Physiological state Sampling may allow the physiological state (hunger state, pregnancy state) to be studied, as well sex ratio and age.
The results differ enormously according to whether captures are made by hand net or by trap:
- traps and men with handnets both catch hungry males, but older flies are caught by traps
- traps capture a smaller proportion of teneral males, and a higher proportion of older females
- among the parous females (females that have already deposited at least one larva) hand nets take a higher proportion of females with empty uteri, than traps which take a greater proportion of gravid ones (females carrying an egg or larva in the uterus).
3.4.3.4 Density
Apparent densities vary in the course of the year and especially when the seasons are very marked, causing alteration of habitats.
In the northern regions of dry savanna, the apparent density passes through two maxima, one in May, the other in October, periods of the year during which the flies are concentrated in the only habitable places.
In intermediate seasons, the apparent densities are much less because the flies disperse.
Real densities, estimated from capture-recapture studies, also show changes in the year. In a habitat where tsetses are apparently abundant, one can estimate the densities of populations of about 1500 tsetses/hectare. Where populations are very thinly scattered, estimates are difficult or impossible to carry out.
Natural enemies of tsetse are thought to keep numbers down (but exactly how important predators and parasites are we do not know).
Host animals put a limit on the number of flies that can live in a given area.
Usually, any alteration to the environment leads to some thinning out of the tsetse population.
3.4.4 Epidemiology and control Glossina tachinoides is the most northerly of the tsetses in west and central Africa. In the northern part of its distribution area, where other species are not found, it is of course the sole tsetse vector of animal and human trypanosomiasis.
Its epidemiological importance is increased by
- its feeding on a variety of animals
- its need to take frequent meals
- its close association with man and domestic animals in the peri-domestic habitats of the south: risks of human sleeping sickness are obviously increased when domestic pigs are healthy carriers of Trypanosoma gambiense.
Knowing (a) the seasonal distribution and (b) the daily activity rhythms of G. tachinoides tells us how to avoid fly/man contact, or fly/livestock contact. Infested zones should not be entered in the middle of the day in the cool season, or in the mornings and evenings of the hot season, as these are the periods of greatest fly activity.
The control of G. tachinoides by discriminative and selective spraying of a residual insecticide takes account of variations in the resting places according to season and hour of day. These methods keep to a minimum the amount of surface that has to be treated, so saving cost and avoiding unnecessary pollution.
3.5.1 General information Not very much is known about either of these species.
In the case of G. caliginea the centre of distribution is southern Nigeria and southern Cameroon. The species spreads as far as Ghana in the west, to the Central African Republic in the east, and to Gabon in the south.
In the case of G. pallicera there are two subspecies. Glossina pallicera pallicera is distributed from Sierra Leone to Cameroon (except at the savanna gap that comes to the coastline in Togo and Benin). Glossina pallicera newsteadi is distributed in the Zaire river system.
Both species live in high rainfall, generally thickly forested areas.
Neither species is important economically as far as is known.
3.5.2 Glossina caliginea The most favoured habitat is near the coast in the freshwater swamps and the mangrove forest vegetation zones.
The fly can occur locally in large numbers in these zones.
It may penetrate inland for some distance along rivers and streams into the rain forest vegetation zone.
The natural hosts are not known. Man is probably not a favoured host as the proportion of females among flies attracted to man is very low. Male flies are known to feed on man.
It can be heavily infected with trypanosomes: 38% of flies dissected in Cameroon were infected.
3.5.3 Glossina pallicera This species is limited to the rain forest zone but can be found far from surface water. It is usually found along forest paths, or in open spaces in the forest.
It often perches on look-out sites such as fallen logs, and other low places, such as leaves, trunks and buttress roots.
It flies rapidly away from and back to the same spot.
A few blood meals have been identified from Nigeria and Cameroon. A wide range of hosts are fed upon, including bovids, suids, reptiles and birds. Man is not attacked.
Flies dissected and examined for trypanosomes had only a 2.5% infection rate.
