Tsetse flies (Glossina spp.), also called tik-tik flies are a family of bloodsucking Dipteran insects of Africa. They are very common in tropical and subtropical regions south of the Sahara. They are vectors of human and veterinary trypanosomiasis (sleeping sickness) and this makes them one of the most harming pests in Africa, especially for cattle.
There are about 30 species of the genus Glossina, each one with a different distribution and prevalence, whereby various species can share a common environment. There are three major subgroups based on morphological, molecular and behavioral features:
- savannah testse flies, from the subgenus morsitans. They live in savannah-type and other dry environments and attack mainly large mammals.
- forest testse flies, from the subgenus fusca. They live in tropical forests, preferably in areas with dense vegetation rich in shady and humid places close to water flows.
- riverine tsetse flies, from the subgenus palpalis. They live close to rivers and lakes and attack mainly reptiles and large herbivores.
Besides humans and wild animals, tsetse flies attack mainly cattle, horses, camels and pigs. Other domestic animals such as sheep, goats, dogs and cats are occasionally affected. Interestingly, indigenous breeds are mostly tolerant to trypanosomiasis, i.e. they are infected but not severely harmed.
Biology and life cycle of tsetse flies
Adult tsetse flies are medium-sized (0.5 to 1.5 cm long). Males and females suck blood, although they also feed on plat juices. Most tsetse flies are diurnal, i.e. they are active during the day. They find their preys visually and through odor. They suck blood every 2 to 3 days. Each blood meal lasts 1 to 10 minutes. Flies are active only for about one hour a day. In the meantime they rest in the surrounding vegetation.
Tsetse flies are active the whole year through. Adult females do not lay eggs but are larviparous, i.e. the eggs hatch inside the uterus, where they are nourished with the secretions of a specific gland. There is always only one larva developing in the uterus. It needs about 10 days to reach maturity (stage III). Then the female deposits the larve on dry soil below the vegetation. The larva crawls into the soil and pupates without further feeding. The pupal stage lasts 3 to 13 weeks. An adult fly lives for about 3 months. There are about 4 generations a year.
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Harm and economic loss due to tsetse flies
The major harm of tsetse flies is not blood sucking, but trypanosomiasis. Trypanosoma spp is a protozoan (i.e. single celled) blood parasite. There are several Trypanosoma species, each one with a variety of possible hosts and vectors. Some species do not seem to be very harmful for cattle. Others are indeed very harmful.
Infected cattle show a substantial reduction in growth, weight gain, and milk production, and fatalities are not uncommon. This is the case for imported European cattle breeds. Indigenous breeds (e.g. N'Dama, Boule, Muturu, Maasai zebu, etc.) are rather tolerant although not unaffected. The problem is that indigenous breeds are significantly less productive than European breeds, and consequently they cannot be productively raised in regions where tsetse prevails. It has been estimated that the eradication of tsetse flies would allow increasing the cattle population in Africa by over 100 million heads.
Prevention and control of tsetse flies
There have been and still remain two basically different approaches to tsetse control: large area control and farm animal control. Large area control is investigated, implemented and financed by governments and large international organizations and aims for eradication of trypanosomiasis from whole countries. Farm animal control is driven and financed by the single farmers and aims for protection of their own livestock.
Various strategies have been followed:
Land clearing, i.e. destroying the brush and wood vegetation where the flies spend most of the time between blood meals. It has not been very successful because it is hardly sustainable. Maintaining the cleared areas free of brushes and trees is too labor intensive and/or requires massive use of herbicides. And nowadays such violent interventions in the environment are mostly unacceptable. But in fact, normal "spontaneous" urbanization of former rural areas has shown precisely this effect in numerous parts of Africa, leading to a substantial reduction of tsetse flies in those regions.
Slaughtering of wild animals. Wild animals being the major reservoirs of Trypanosomas, reducing or eliminating them has a significant impact on the trypanosomiasis transmission to livestock and humans. It was tried regionally during the last century with moderate success. However, it is not sustainable for the simple reason the other animals will sooner or later replace the slaughtered ones. And such a strategy is nowadays unacceptable as well.
Large area insecticide treatments. This approach started early in the last century but accelerated in the 1950's when numerous modern highly effective insecticides become available. Massive ground and aerial campaigns were carried out. Obviously, such insecticidal campaigns do not distinguish between tsetse flies and hundreds of other beneficial insects and invertebrates. And they are not sustainable either, because flies from untreated neighboring areas will sooner or later re-infest the treated regions.
Release of sterile male flies. The release of billions of industrially produced sterile males was extremely successful in the eradication of screwworm flies in North America. This approach has also been tried against tsetse flies. It has had some success, e.g. in Zanzibar, an East African island. Whether it can be successful in continental Africa remains open, considering that there are about 30 different tsetse fly species, all are difficult to rear in the laboratory, and billions of sterile males of different species had to be produced and be released in the right place at the right moment.
As a general rule, large-area control has shown to be unsustainable. It was successful in a few islands, but there are no lasting results in continental Africa. Among other reasons because it relies on continuous funding by governments and international organizations that can be interrupted any time for political reasons.
Traps that catch flies or insecticide-impregnated screens can reduce the fly population, but are certainly insufficient for its elimination. They can be useful as barriers against re-infestation of already cleared areas. Their efficacy strongly depends on design (color, shape, size, etc.) and location. Fly attractants (e.g. acetone, cattle urine, etc.) are frequently used to enhance trap attractiveness for tsetse flies.
Farm animal control
There are not too many products that ensure a reasonable protection of farm animals against tsetse flies. Best results have been obtained with synthetic pyrethroids (cypermethrin, deltamethrin, etc.) administered in the form of dips or sprays, or as ready-to-use pour-ons. Footbaths or leg spraying can provide satisfactory control of tsetse species that prefer to land on the cattle legs. Protection of dips and sprays last for about a week, whereas pour-ons may ensure up to one month protection.
So far there are no effective biological control methods agains tsetse flies. Learn more about biological control of flies and other insects.
|If available, follow more specific national or regional recommendations or regulations for horn fly control.|
Insecticide resistance of tsetse flies
So far there are no reports on serious problems of tsetse fly resistance to insecticides.
This means that if a particular product does not achieve the expected control, it is most likely because the product was not adequate or it was not used correctly, not because the flies have become resistant.