1.1.1 Introduction The conditions under which the tsetse lives make up its environment. It includes conditions of climate, vegetation, animal life, soil, and the effects of human activity. All these may be called environmental conditions or environmental factors.
The study of the tsetse in its environment is called ecology.
The particular place where the tsetse lives is called its habitat. For example, gallery forest, thicket, miombo woodland, and mopane woodland are different kinds of tsetse habitat.
Where one type of vegetation gives way gradually to another, that is called an ecotone. For example, as one passes from thick woodland to open grassland, one may walk through an area of smaller, more scattered trees. This area is an ecotone.
1.1.2 Climate (see also Volume I, 10.2) Sunshine, rainfall and humidity, temperature, atmospheric pressure and wind make up the climate of a particular area. Temperature and humidity are the most important of these for tsetse, and light can also be important.
Very restricted areas in the habitat may have a special climate of their own, called the microclimate (see 220.127.116.11).
18.104.22.168 Temperature Glossina lives well at 25–26°C and most laboratory colonies are kept at that temperature. If the temperature goes much higher or much lower than that, there may be some damage to the fly. Here are some examples:
- death occurs rapidly (in 5 minutes) if G. morsitans adults are kept at 46°c
- death occurs if G. tachinoides adults are kept at 44OC for 1 hour
- death occurs if G. morsitans adults are kept at 40°C for 1 hour
- death occurs if G. fuscipes adults are kept at 40°C for 3 hours
- around 14°C and below, Glossina adults cannot fly
- death occurs if G. morsitans adults are kept at -4°C for 6 hours.
In general, a temperature above 38°C is damaging to the adults; and a temperature below about 17°C will not allow the adult to live a normal active life.
For pupae, 32°C is the upper limit for normal development, and 16°C is the lower limit.
At the northern limit of tsetse distribution, high temperature and dryness limit the spread of the fly.
In the south of the continent, the limits of tsetse distribution may also be set by high temperatures and dryness (in the hot dry season). But in some areas, seasonal low temperatures are more important. Gold winter months may delay the development of tsetse pupae for so long, that they cannot produce adults successfully.
Temperature also affects the rate at which the fly lives.
- shorten the time from emergency to the production of the first larva
- shorten the time from the production of one larva to the production of the next (interlarval period)
- shorten the period spent as a pupa (pupal period)
- shorten the life of the adult
- shorten the period the adult can last without a blood meal.
Low temperatures have the opposite effect in each case.
22.214.171.124 Rainfall and humidity Rainfall probably does not have any direct effect on tsetse, but does so indirect-ly by:
A humid atmosphere allows tsetse to spread away from sheltered habitats, so long as the temperature is not too high. So in the wet season G. morsitans disperses away from its dry season home (riverside woodland) into more open country, in the northern savanna areas.
In the southern parts of its range, G. palpalis can live away from free water, probably because of the humid atmosphere.
In high rainfall areas, G. tachinoides is also able to live away from free water.
Both G. pallidipes and G. longipalpis have dry season habitats in well-sheltered thickets, but wet season dispersal areas in less densely wooded country.
Humidity of the soi 1 is important to the sur-vival of tsetse pupae.
- Glossina longlpennis, G. morsitans morsitans and G. swynnertoni pupae can develop in very dry soils (0–10% R.H.).
- Glossina morsitans submorsitans, G. pallidipes and G. tachinoides pupae can develop in fairly dry soils (30–40% R.H.)
- Glossina austeni and G. fuscipes pupae require rather mare humid soils (40–50% R.H.)
- Glossina palpalis, G. brevipalpis and G. fuscipleuris require very humid soils (about 70% R.H.).
- The humidity is affected by wind, especially in West Africa. Here, humid winds blow during the rainy season from the south-west; during part of the dry season a dry dusty wind (Harmattan) blows from the north-east.
126.96.36.199 Microclimate Meteorological instruments (see Volume I, 10.2.6) are usually placed in rather open places, to give information about the general climate of the area, but the local climate (microclimate) of small sheltered parts of the habitat (microhabitats) may be very different.
The following are some of these microhabitats:
- At the river bank, especially under an overhanging bank. Here the air remains very humid even in the dry season, and it is a favourite resting place for palpalis group flies, in the far northern limit of their distribution. In this area, nearly all the palpalis group flies are found within 5 m. of the river bank.
- In rot holes, or in hollow trees, or in animal burrows. For example, in the Zambesi valley Glossina pallidipes enters rot holes in trees in the daytime, during the hottest season. Here it is cooler and more humid than outside.
- On a tree trunk, especially low down and in cracks in the bark, or between buttress roots. In the day it is cooler and more humid here than in the more exposed parts of the habitat.
- At the top of trees, at night. Here it can be wanner than nearer the ground, because the ground may lose much heat by radiation at night, especially in the cold season and when there is no cloud.
The fly's behaviour (see 1.2) can bring it into these places where it can survive better than if it had to suffer the general climatic conditions of the area.
1.1.3 Vegetation zones and tsetse habitats
188.8.131.52 In West Africa The following are some of the main vegetation zones in West Africa starting from the coast and working inland, and some of the tsetse species to be found in these zones.
- Mangrove swamps. These are found along some parts of the coast especially in the river mouths, where the water is brackish (part river water, part see water). They are habitats of Glossina palpalis and G. caliginea (in Nigeria).
- Fresh water swamp forest. Some G. palpalis and G. caliginea are found here also.
- Rain forest. An area of very high rain fall, very tall trees, high humidity and dense shade at ground level. Where this forest is well-developed, these species may be found: G. tabaniformis, G. haningtoni, G. fusca and (for instance in Nigeria) G. nigrofusca, as well as other less cannon fusca group flies. Glossina pallioera and G. palpalis are also found here. Further north, where the forest is slightly drier, the trees are less tall and more often lose their leaves in the dry season, the forest edge species G. medicorum and G. nigrofusca (in Ghana) live.
- Derived savanna. This is a forest zone that has been largely cut down and burned to make farms. It is sometimes called farm bush, especially where there has been re-growth of trees. It is therefore country that has patches of forest on poor land, with farms and savanna areas in between. The forest patches have G. medicorum, G. fusca, G. longipalpis and G. palpalis. Of these, G. longipalpis may spread out into savanna areas, especially in the wet season.
- Southern Guinea savanna. This has a dry season of 3–4 months. The trees are 15–18 m. (40–50 ft) high, sometimes taller, broad leafed, and forming an open woodland with tall grass. Glossina longipalpis lives in the southern parts of this zone. Further north, G. morsitans replaces it. Rivers and streams along which evergreen trees grow form the habitat of G. palpalis and G. tachinoides.
- Northern Guinea savanna. This has a dry season of 4–5 months, and an annual rainfall of about 1000 mm. (approx. 40 inches) and over. The dominant trees are doka (Iso-berlinia doka, I. dalzielii), members of the family Caesalpinaceae. The trees in this zone are about 13 m. (40 ft) high, and the woodland has a very uniform appearance. Grass is shorter than in the southern Guinea savanna. The zone is infested with G. morsitans.Along rivers and streams, G. palpalis and G. tachinoides are found. Glossina medicorum inhabits vegetation along the Komoe and the Black Volta rivers.
- Sudan zone. The dry season here lasts about 7 months. Annual rainfall is 500–1000 mm. (20–40 inches). It can become very hot. Characteristic trees are Acacia, baobab (Adansonia. digitata), doum palm (Hyphaene thebaica); there are also thorny thickets. This zone is inhabited by G. morsitans, but the species is based mainly cm patches of forest, dispersing a little in the milder rainy season. Glossina tachinoides and G.p. gambiensis can live here, but only along river banks. For this zone, G.p. gambiensis is limited to the area Senegal to Togo.
184.108.40.206 Other vegetation zones The vegetation of East and Central Africa is not set out in such a clear manner as that of West Africa. Some extensive vegetation types are:
- Miombo woodland. This closely corresponds to the northern Guinea zone of West Africa. The dominant trees are Brachystegia and Isoberlinia (Caesalpinaceae) of which there are several species. It is one of the main habitat of G. morsitans.
- Mopane woodland. This occurs along the valleys of Zambesi, Limpopo and Luangwa rivers. The dominant tree is mopane, Colophospermum mopane (Caesalpinaceae). It forms an important habitat for G. morsitans in the Luangwa and parts of the Zambesi valleys.
- Baikiaea woodland on Kalahari sands (gusu woodland). This occupies a large area of western Zimbabwe, Zambia (much of Western Province) and part of Botswana. It is not much used by tsetse, probably because the sandy soil becomes too cold for too long a period in the winter months. It is also a dry soil.
- Manga woodland. This contains much Acacia, Albizia, Combretum and Terminalia. It forms only a marginal tsetse habitat in Zambia.
- Dry thorn bush (nyika) is a drier type of Acacia dominated woodland, with Commiphora and Combretum, and occupies much of Somalia, Sudan, Kenya, Uganda and northern Tanzania. It is the home of Glossina swinnertoni and G. pallidipes.
- Patches of evergreen forest, not as extensive in East Africa as the other vegetation types mentioned, are the habitat of G. brevipalpis and G. austeni.
- Itigi thicket is not favoured by tsetse.
220.127.116.11 Less typical habitats Under 18.104.22.168 and in Chapters 2, 3 and 4, descriptions are given of the typical habitats in which various Glossina species may be found. But these may not include all the places where a particular species occurs. If a spraying programme misses out these unusual habitats, then the sprayed areas will quickly be re-infested.
Many of the less typical habitats are man-made ones, for example in and around villages, especially in the rain forest belt of West Africa, where the original vegetation has been cut down to make farms and plantations.
From these places, flies can attack man and village livestock, especially pigs and to a lesser extent cattle, so that these populations of flies can be very important from the point of view of disease transmission.
Because they are around places where people live, these tsetse populations have been called peri-domestic (‘around the home’).
1.1.4 Effect of other animal life on tsetse
22.214.171.124 Host animals Lists of the hosts that are used by different species of tsetse are given in Volume I, Chapter 6. Some additional information will be found in the following chapters of the present volume.
Although the tsetse is completely dependent on the host animals for their food, these animals are the most difficult environmental factor to study in tsetse ecology. When a worker goes out into the field, he may at once frighten away the host animals and may attract a few of the tsetses to himself. Therefore, the natural situation is completely altered. The field worker should remind himself of the dependence of tsetse on host animals, even where these seen to be scaroe.
The dependence of tsetse on wild game animals is also shown by:
Rinderpest is a disease affecting many kinds of wild animals. It particularly affects buffalo, giraffe, eland, bushpig and warthog, but also to a lesser extent affects reedbuck, wildebeest, kudu and giant forest hog. Wild animals that are very much affected by rinderpest provide 80–90% of tsetse feeds in East Africa. When the disease killed off many of the African animals in the period 1889–1896, the tsetses were greatly reduced in numbers, and their distribution became restricted to quite small areas.
When the main outbreak was over, the game animals recovered in numbers and the fly belts expanded, eventually to reach their present size.
Examples of game control to eradicate tsetse are given in Volume III, 2.2.
Sometimes, tsetses use domestic cattle for the food supply, rather than wild animals (for example, in the Koalib Hills, Sudan, where an isolated belt of G. morsitans used to live). In other places, such as at the northern fringes of G. palpalis distribution, populations of flies may depend for their food on people visiting a water hole.
126.96.36.199 Predators and parasites
Predators Figure 1.1 illustrates some of the predators known or believed to feed on tsetse adults and pupae.
The most important predators are probably ants feeding on tsetse pupae, and asilids (robber flies), waspes and spiders feeding on the adults.
Insect parasites Included here are insects that attack the pupa of Glossina from within.
Fig. 1.1 Predators of tsetse A. Bembex wasp; B. Ant; C. Asilid (robber fly)
Fig. 1.2 Parasites of tsetse pupae A. Syntomasphyrun (Hymenoptera); B. Mutilla (Hymenoptera) ; C. Thyridanthrax (Diptera)
- Syntomosphyrum (Hymenoptera) (Figure l.2 A) This is a tiny black wasp about 1 ran. long. It is easily reared in the pupae of various fly species, and millions of the adults have been released into the field in various control attempts which, however, have had no lasting effect. The female wasp lays its eggs inside the tsetse pupariun. Several or many of these wasps emerge from each infected tsetse pupa. Natural rates of infection are normally about 0.2% or less.
- Mutilla (Hymenoptera) (Figure 1.2 B) Ihis is a larger insect, about 4 mm. long. The male is winged but the female is wingless. The female lays only about three eggs in each tsetse pupa, and of these only one developes all the way to become a wasp. Six times as many females emerge as males. Natural rates of infection may be as high as 40% in the warmer seasons (G. morsitans). But the period spent in the tsetse pupa is much longer than pupal period of the healthy tsetse, so that infection rates can be overestimated. The parasite is not easy to rear in large numbers, and it has not been used for attempts at control.
- Thyridanthrax (Diptera) (Figure 1.2 C) This is a fly, slightly smaller than the tsetse it parasitises. Eggs are laid on the soil surface, and the tiny larvae burrow searching for pupae. Only one fly emerges from each parasitised tsetse pupa. Locally, rates of parasitism may be as much as 20% or more, and it is probably one of the most important of the tsetse parasites. The pupal period may last much longer than the 4–5 weeks that is normal for tsetse or it may be shorter. It is a difficult insect to breed and no field releases have been made.
There are many other species, particularly Hymenoptera, that have been recorded as emerging from tsetse pupae. So far as is known, they are not as important as the ones described above.
Other parasitic organisms Other types of organisms that have been known to attack tsetse (pupa or adult) include bacteria, fungi, protozoa, viruses and nematode worms. Their relative importance is not yet clear and more work has to be done on them.
For both predators and parasites of tsetse, it is not yet known how important they are in keeping down the numbers of tsetse in the field.
1.1.5 Human activity (see also Volume II, Chapter 2) Human settlements, with crop farming and tree-felling, may locally prevent tsetse from existing in an area, at least in large numbers. The reasons are that
One of the most important results of human activity is fire. Fires late in the dry season change the environment by destroying young trees and bushes and thus encouraging vigorous grass growth. Fire also makes the area drier, and tsetse are therefore pushed into places of shadier, thicker vegetation where the air is moister.
Fires early in the dry season may have the effect of making the habitat more wooded, but this change is brought about very slowly, over many years.
1.2.1 Flight Only about 15–30 minutes of each day is spent in active flight. A single flight does not last longer than about 1 1/2 – 2 1/2 minutes. The speed of flight may be 3–6 m/sec (11–24 km/h = 7–15 miles/h), but it is much slower immediately after a meal.
After taking in a large blood meal a short, slow flight is made away from the host animal to a resting place such as a tree trunk. Here, the weight of the fly is quickly reduced by primary excretion (some drops of water are passed out through the anus). More short flights may then follow, taking the fly to a safer place.
1.2.2 Best and resting sites Tsetses spend most of the day settled. The places where they remain settled for long periods are called resting sites. These are different from the places where flies may perch temporarily, for example when they settle out from a following swarm, or when they are looking out for the next meal.
It is important to know about resting sites (sometimes called 'true resting sites') because these are the best places for spraying persistent insecticides for killing the tsetse. Insecticides placed at the resting sites and only at the resting sites will have the maximum effect against tsetse and minimum effect against other animal life.
Resting sites have been investigated (see Vol. I, 7.4):
- by searching the vegetation carefully for resting flies
- by enclosing whole trees and bushes in a net cage and releasing fed flies within the cage to see where they rest
- by marking ('tagging') flies with fluorescent or reflecting paint and looking for them at night using lamps.
Flies have also been tagged with radio-active materials and searched for later in the bush. This technique is not described in the Manual because it requires very specialised instruments and safety precautions in which the research chief will give training.
Besting sites may vary according to:
- the time of day or night
- the climate: and season
- the species of tsetse fly
- the vegetation
- the resting places of host animals (e.g. bushbuck).
During the hottest part of the day (visually early to mid-afternoon), the true resting sites are lowest down on tree trunks, and on the underside of shaded, fallen logs. At copier times of the day, and in cooler seasons, the flies rest higher up tree trunks, and on the underside of branches. At night, some flies go up into the canopy of trees and rest on the leaves or twigs.
Woody living parts of the vegetation are often chosen for day time resting, but some species (e.g. Glossina palpalis gambiensis) may prefer leaves.
The resting habits of different species are described in Chapters 2, 3 and 4.
1.2.3 Response to host animals Usually a fly detects a host animal by its sense of smell, from up to 100 m away. Larger host animals or more of them, are more attractive to tsetse than smaller host animals, or single individuals.
When it smells the host, the fly moves up-wind, which brings it closer to the host animal. Then the fly is able to see the host (at 50 m or more).
Flies may land on a greater variety of host species than is indicated by the blood meal list. However, some hosts prevent flies from feeding by flicking their tails and twitching the skin.
Many flies, especially non-hungry males, are attracted to hosts even though they are not going to feed. They make up the 'following swarm', a party of males that settles on the ground or low vegetation close to a moving host. A male in a following swarm may fly on to a virgin female as she comes for her first meal, and mate with her.
1.2.4 Daily activity Under average conditions most activity is in the early to mid-morning, and in the late afternoon.
Under very hot conditions, activity during the middle of the day may be stopped almost completely, with flies finding the cool places in which to shelter from the great heat.
Under cool conditions, there may be one period near the middle of the day when tsetses are most active in the field.
Some facts concerning the life of tsetse cannot be described for individual flies, but only for a group or sample of flies. Examples are the percentage of females caught by a trap, the density of tsetse living in an area, and the distribution of resting sites.
When gathering this information we are usually trying to learn more about the population of the flies living in an area.
1.3.1 Pupal population
188.8.131.52 Importance of the pupal population In general, more than half of the total population of tsetse in an area are present below ground, as pupae.
Experience shows that in some seasons (particularly in the rains) pupae are very difficult to find.
It is therefore impossible to work out what number of tsetse are living in a given area, from the number of pupae found there.
But the collection of pupae can give information about:
- the changes in the breeding areas according to season
- what parasites the pupae suffer from
- where tsetse are living.
184.108.40.206 Breeding sites (larval deposition sites) Ways to search for pupae are given in Volume I, 7.1.
The following table shows the sort of places in which tsetse pupae might be found. These are called breeding sites.
|1.||In soil under fallen logs and under leaning trees in deciduous woodland||G. morsitans, G. brevipalpis|
|2.||In soil under fallen logs in open country||G. longipennis|
|3.||Under fallen logs but on the soil surface||G. swynnertoni (wet season)|
|4.||Under overhanging rocks||G. morsitans, G. austeni, G. palpalis/fuscipes|
|5.||Under leaf litter in (a) thickets; (b) gallery forest||G. pallidipes, G.palpalis gambiensis|
|6.||In dry sandy beaches or river beds where shaded by dense vegetation||G. morsitans (dry season)|
G. palpalis/fuscipes (dry season)
|7.||Where there is high canopy cover as well as lower level cover given blow branches and creepers immediately over the site||G. palpalis/fuscipes|
|8.||At sandy spots, where there is high canopy cover||G. tachinoides|
|9.||Around the base of oil palms (Elaeis), where shelter is given by broken frond stems||G.palpalis/fuscipes G. longipalpis|
|10.||In rot holes in trees||G. palpalis/fuscipes, G. morsitans, G. swynnertoni (dry season)|
|11.||In animal holes in the ground||G. morsitans (dry season)|
|12.||Under Lantana and Euphorbia hedges locally in Western Kenya||G. fuscipes|
|13.||In plantations of mangoes, etc.||G. tachinoides|
Breeding areas are zones in which breeding sites may be found. They are more concentrated in the dry season (especially in deciduous woodland) and more widespread in the wet season.
1.3.2 Adult populations
220.127.116.11 Sex ratio The proportion of females amongst flies emerging from a collection of pupae is normally close to 50%.
Females usually live longer than males, so that a field population of flies will normally have more females than males.
But hand net samples taken on a fly round usually contain a low proportion of females, because the flies that come within reach of the nets are mainly males,
Samples taken in traps have a higher proportion of females, often in the range of 60–70%. This figure can vary according to location, time of day and year, and according to the species.
18.104.22.168 Age The fly that emerges from the pupa is hungry. After spreading and hardening its wings, its main problem is where to get its first meal. During this time, from emergence to taking the first blood meal, the fly is called a 'teneral' fly.
Ihe food obtained at the first meal is used to build up the flight muscles of the thorax, which are poorly developed at emergence.
In this way the fly gradually becomes a stronger flier. In the case of the female this is necessary for the later task of carrying the larva in the abdomen. In the case of the male it is necessary for catching and mating with virgin females, in competition with other males.
At each feed, there is a chance of the fly becoming infected with trypanosomes (this chance is greater with the earlier meals). Any trypanosomes picked up have to go through a cycle of development before the fly is permanently infective.
It follows that older flies are more important as disease vectors than younger flies.
Traps are able to catch older flies, especially older females, compared with other methods (e.g. hand nets used with men, vehicles or ox).
Males may live about 3 weeks in the field.
Females certainly live longer than males. Glossina pallidipes females in one population were estimated to live an average of 35 days or more.
Flies live for longer periods
- when it is cool (rather than hot)
- when it is humid (rather than dry)
- when there are many hosts available (rather than only a few).
22.214.171.124 Physiological state
Hunger The idea we gain of the hunger state of a population also depends on the type of sample examined.
Most flies taken during a normal fly round are non-hungry males.
Nearly all flies (G. morsitans and G. pallidipes) attracted to a stationary bait (live animal or animal model) are hungry, but 10–25% of flies visiting moving baits are not hungry.
With a man and an ox being used as bait together, the flies (G. morsitans) coming to feed on man are fewer and hungrier than those visiting the ox.
Mated status and pregnancy Nearly all non-teneral female flies are mated (but not in G. pallidipes; see 2.4.7).
Under the best conditions, the uterus of a female fly is seldom empty once ovulation has started. This is because very soon after the deposition of the mature larva, the next egg passes into the uterus and begins its development.
Pregnancy rates may differ according to whether the flies are caught as they come to a party of men, to a bait animal or in a trap.
126.96.36.199 Density The density of a tsetse population in a given area is never very accurately known unless all the flies are caught. This could only be done on an island or in a very isolated woodland or thicket.
Usually the density is estimated by a mark-release-recapture method (see Volume I, 7.7).
The terms Apparent Density and True Density are sometimes used. 'Apparent density' is taken as the number of non-teneral males (savanna flies) caught in 9000 m. (10,000 yards). It is a figure obtained from fly-round results. It does not necessarily give information about the 'true density', which is simply the number of flies per unit area. For instance, a high catch may indicate a hungry population, rather than a dense one.
When traps are used, apparent density is defined as the number of flies/trap/day (F.T.D.).