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the web of life in southern Africa

Family: Glossinidae (tsetse-flies)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Ecdysozoa > Panarthropoda > Tritocerebra > Phylum: Arthopoda > Mandibulata > Atelocerata > Panhexapoda > Hexapoda > Insecta (insects) > Dicondyla > Pterygota > Metapterygota > Neoptera > Eumetabola > Holometabola > Panorpida > Antliophora > Diptera (flies) > Brachycera > Muscomorpha > Eremoneura > Cyclorrapha > Schizophora > Hippoboscoidea

Tsetse-flies are found in Africa and transmit Trypanosoma parasites to vertebrates, thus causing sleeping sickness in people, and nagana in cattle. They have a fascinating reproductive biology because the entire egg and larval development occurs in the female. Females lay the mature larva on the ground and it then pupates within its own skin. Each female is able to produce a maximum of only 8-10 offspring in her lifetime.

Diversity and distribution

Glossina is the only genus in the family and contains 23 species (see Jordan 1993 for keys to adults and puparia). Tsetse-flies are found in the lowland rainforest and savanna regions of Africa south of the Sahara and a small part of the Arabian Peninsula. They are absent from most of South Africa except for northern KwaZulu-Natal. They occur only where there is woody vegetation and they are not found in higher altitude areas because winter temperatures are too low. 

Four species of fossil Glossina, thought to date back to the Oligocene (38-26 million years BP), have been found in sedimentary shales in Colorado, USA, indicating that Tsetse-flies once had a much wider distribution.

The nearest living relatives of Tsetse-flies are the louse-flies in the family Hippoboscidae, which are also blood-sucking.

Distinguishing characteristics

In the field, as far as Tsetse-flies are concerned, it is a case of "don't find us, we'll find you". My experience of Tsetse-flies in Mkomazi Game Reserve, Tanzania, is that they tend to be localised. They were absent from most of the reserve but, when driving through certain low lying areas, they would suddenly fly through the windows and immediately settle on the occupants of the vehicle and start biting. Slapping them is not enough to kill them because they are adapted to survive a hard impact from above - you need to slap and then pull your hand sideways.

From a morphological point of view, the following are useful features for identification:

  • Wings are held closed over the abdomen, so that they are fully overlapping one another.
  • They have a piercing proboscis which sticks out horizontally from the front of the head.
  • Compound eyes are widely separated.
  • Unique to flies is that the discal medial cell of the wing is shaped like a butcher's cleaver and is sometimes referred to as the 'hatchet cell'.
  • Unique to flies is that the hairs on the arista of the antenna have further hairs branching off them.

Life cycle

Tsetse-flies have a form of reproduction called adenotrophic vivparity where the egg hatches within the female and the larva develops in the female by feeding on food from modified accessory glands. During her life-span a female can theoretically give birth to only a maximum of 8-10 offspring (in reality much lower), so tsetse-flies are rather like human beings in that they make a large investment per offspring so that juvenile mortality is low. However, this means that they can't produce many offspring. 

The egg hatches in the female.  Eggs develop sequentially in the female, alternating between the four ovarioles: after the female is about 9 days old, the first egg passes into the uterus from one of the two ovarioles in the right ovary.  After 9-10 days, there is the second ovulation from one of the two ovarioles in the left ovary, and so on. In the uterus the egg is fertilised by a sperm from the spermatheca (gained during earlier mating with a male). After 3.5 days of development in the egg, the 1st instar larva breaks out of the egg case.

The larva develops in the female. The larva develops in the female's uterus by feeding on food from modified accessory glands. It passes through 2 moults  to reach the 3rd instar and it is then 'larviposited' by the female. 

The larva is laid onto the ground (larviposition). The female finds a suitable place to lay the larva. In the wet season or wet regions such as rain forest, where there is general dampness everywhere, females tend not to concentrate their larviposition in particular areas. However, in dry areas larviposition takes place mainly in well-shaded spots so that there is an aggregation effect in these places. 

The larva pupates within a puparium. The freshly-laid free-living larva is fully fed, and after expelling the waste-products it gained while developing in its mother, it burrows into the soil where its skin hardens and blackens into a puparium and within the puparium, pupation and metamorphosis take place. The puparial period can range from 20 days (at 30ºC) - 47 days (at 20ºC). Development in the puparium is generally unsuccessful below about 17ºC and above about 32ºC. The entire life cycle from egg to adult usually takes about 30 days.

Adults mate. In most species, mating seems to take place on or near the host, but this is not the case in Glossina pallidipes. There is no evidence of volatile sex pheromones being produced but females do have species-specific cuticular hydrocarbons which induce a copulatory response in males of the same species. Mating takes an hour or two during which time a spermatophore is formed within the female's uterus using secretions from the male. Just before copulation ends, the male ejaculates sperm into the spermatophore. Within the subsequent few hours, the sperm moves from the spermatophore up the paired spermathecal ducts into the paired spermathecae. These sperm serve the female throughout her life so she does not have to mate again.  Males are able to mate a number of times with different females. 

Tsetse-flies gain energy for flight through the partial breakdown of proline, an amino acid gained from the blood meal. They are therefore different from most other flies which derive their energy for flight from plant sugars (e.g. through nectar feeding). 

Diseases transmitted

All tsetse-flies feed on blood, mainly from mammals but also from reptiles and birds. Trypanosoma blood parasites can be transmitted during feeding and for indigenous African mammals this does not pose a problem as they are adapted to handling such infections. However, domestic animals and people can be badly affected.

  • Trypanosoma brucei. Causes sleeping sickness in people.

    • Trypanosoma brucei gambiense. Produces usually a chronic infection. Occurs in West and Central Africa from Senegal to Sudan in the north and down to Angola and Zaire in the south. People are the usual host; wild and domestic animals can be reservoir hosts but usually are not part of the transmission cycle. Three Glossina species, all in the palpalis group, are vectors of this disease: G. palpalis, G. fuscipes and G. tachinoides. The disease has a very patchy distribution and there are large areas where one or more of the vector species are present but not the disease.  

    • Trypanosoma brucei rhodesiense. Produces usually an acute infection. Occurs in the savanna woodlands of east Africa, extend down south to the northern parts of Botswana, Zimbabwe and Mozambique. Unlike the other subspecies, the main hosts are wild ungulates, people being usually only accidental hosts. Four Glossina species, all in the morsitans group, are vectors of this disease organism: Glossina morsitans morsitans; G. morsitans centralis, G. swynnertoni and G. pallidipes.  Life T. brucei gambiense, the disease has a very patchy distribution with large areas having the vectors but not the disease. 

  • Trypanosoma vivax and T. congolense. Important parasites of domestic animals, particularly cattle, causing animal trypanosomosis or nagana. About 10 million square kilometers of Africa is prone to animal trypanosomosis which seriously limits farming efforts and negatively affects regional economies. However, cattle farming does occur in tsetse-infected areas but usually requires that cattle be constantly dosed with drugs to treat and protect them or that insecticides be applied to the cattle. 

  • Trypanosoma simiae. An important parasite of pigs.

Control

The nature of the life cycle, with few offspring per female and a large investment per offspring, means that a tsetse population cannot survive under sustained regular mortality above natural levels. It has been calculated that an extra 4% mortality of females per day over a sustained period will cause extinction of a tsetse population. While it is possible to achieve erradication of tsetse in particular areas, these attempts often fail in the long-term because of invasion of tsetse-flies from adjacent regions. It is up to individual countries, or countries working together, to ensure that tsetse-flies are controlled sufficiently or eradicated. "The availability of the technology to reach a successful conclusion is usually not the limiting factor" (Nevill 1997a).

An essential part of a control programme is to understand the biology and ecology of the Glossina species involved. Especially important is to understand their movement, density and distribution and to use trapping methods to monitor what is going on as the control campaign progresses.

The main control methods are as follows.

  • Removal of vegetation. In savanna areas, larviposition occurs in shaded places, so one control method is to remove trees and bushes so one is just left with grass. This method was used quite extensively with success in the past but is labour intensive and requires that there be reslashing of vegetation on an annual basis. The method fell into disuse with the advent of insecticides. However, removal of vegetation for fire wood and urbanisation has sometimes achieved the same effect.
  • Killing of wild animals. The object here is to remove reservoirs of infection in the wild animal populations. This method was used extensively in the past.
  • Spraying of insecticides. Two main approaches have been used: (1) spraying of residual insecticides that persist in the environment for at least 2-3 months; and (2) spraying of non-residual aerosols that kill adult tsetse at the time of spraying but which must be repeated at regular intervals in order to kill newly emerged adults. Both ground and aerial application methods have been used. Aerial methods are expensive but have been used with success. For instance, in Zululand (northern KwaZulu-Natal, South Africa), between 1946 and 1953, the savanna tsetse-fly species Glossina pallidipes was totally eradicated mainly through the use of aerially applied DDT and BHC. However, consider the detrimental environmental effects of using these residual insecticides. In addition, the tsetse-fly problem still remains in Zululand due to two other cryptic species Glossina austeni and G. brevipalpis (Nevill 1997b). Ground spraying of residual insecticides can be a feasible and economical control strategy if it is applied to selected sites where there are concentrations of tsetse-flies.
  • Drugs. Animals can be given drugs to kill Trypanosoma, hence supressing the disease within the region.
  • Trapping. A number of different traps have been developed for capturing tsetse-flies in large numbers. Traps are particularly effective in reducing Glossina populations that are isolated on islands or which occur in a linear riverine habitat. A popular recent trap design is the biconical trap which is a netting trap that tapers above and below so it looks like a cone with another inverted on top. Insecticide-impregnated cloth targets that are attractive to the flies, are also used.Host odour attractants are used on these traps.  The advantage of using traps is that there is no contamination of the environment with insecticide.
  • Sterile insect technique (SIT). This method involves breeding up thousands of male Glossina which are sterilised using radiation and then released at regular intervals, thus swamping the population with males that are unable to fertilise females successfully. Glossina austeni has been successfully eradicated from Zanzibar using this method. During this campaign, 60000+ irradiated male flies were being released per week. From 1995-1996, 5.5 million sterile males were released in total. To get this number of males per week involved rearing a colony of 700000+ female flies (Dyck et al. 1997)!

References (by date)

  • Du Toit R. 1954. Trypanosomiasis in Zululand and the control of tsetse flies by chemical means. Onderstepoort Journal of Veterinary Research 26: 317-387. [not consulted]

  • Gruvel J. 1977. Predators. In: Tsetse: the Future for Biological Methods in Integrated Control (ed. Laird, M.). International Development Research Centre, Ottawa, pp. 45-55. [not consulted but a useful source of information on interactions with other organisms]

  • Jordan AM. 1993. Tsetse-flies (Glossinidae). In: Medical Insects and Arachnids (eds R.P. Lane and R.W. Crosskey). Chapman and Hall, London, pp. 333-388.

  • Dyck VA, Juma KG, Msangi AR, Saleh KM, Kiwia N, Vreysen MJB, Parker AG, Hendrichs J, Feldmann U. 1997. Eradication of the tsetse fly Glossina austeni by the Sterile Insect Technique (SIT) in Zanzibar - could South Africa be next? In: Insects in African Economy and Environment (Ed. H.G. Robertson). Entomological Society of southern Africa, Pretoria, p. 168.

  • Nevill EM. 1997a. Tsetse and trypanosomosis - an African problem. In: Insects in African Economy and Environment (Ed. H.G. Robertson). Entomological Society of southern Africa, Pretoria, pp. 165-166.

  • Nevill EM. 1997b. Tsetse in South Africa - where are they now?. In: Insects in African Economy and Environment (Ed. H.G. Robertson). Entomological Society of southern Africa, Pretoria, p. 167.

  • Motloang M, Masumu J, Mans B, Van den Bossche P, Latif A. 2012. Vector competence of Glossina austeni and Glossina brevipalpis for Trypanosoma congolense in KwaZulu-Natal, South Africa. Onderstepoort Journal of Veterinary Research 79(1), Art. #353, 6 pages. http://dx.doi. org/10.4102/ojvr.v79i1.353

Text by Hamish Robertson