Arthropod vectors  Tsetse flies
Tsetse flies
Author: Dr Reginald De Deken
Licensed under a Creative Commons Attribution license.
SAMPLING AND COLLECTION METHODS
Sampling of tsetse flies is often used to assess the apparent density, which is related to:
1. The true density: the number of tsetse flies per surface unit. True density depends on the renewal
of the population (influenced by the development rate and the number of females one or two
months before), the mortality (particularly important during harmful climatic conditions and
shortage of hosts) and the rate of immigration and emigration.
2. The availability of the tsetse fly which varies according to:
-
the behaviour of the tsetse flies with respect to the method used to capture the flies. Each
capture method gives a more or less significant bias while capturing only among certain parts
of the population of tsetse flies (e.g. fly rounds generally give an over-estimate of the number
of males and non-teneral males present).
-
the activity of the tsetse flies which varies according to the moment the density is monitored
(influenced by climatic conditions); the place where the sampling is carried out (depending on
the season and their physiological state flies prefer some places and types of vegetation); and
the sex, the age and some cyclic phenomena such as hunger or pregnancy.
Thus longitudinal surveys will assess essentially the population dynamics and behaviour of the tsetse
fly with respect to the spatial (ecological zones) and seasonal variations of the environment.
Cross-sectional sampling of adult tsetse flies is often carried out to study their distribution, determine
their trypanosome infection rate or the effectiveness of control measures.
Sampling methods
Both vision and odour detection is used by the tsetse fly to locate its host. These visual and olfactory
stimuli attract tsetse flies respectively at short (<15m) and long range (<100m for the odour of a single
ox). The different sampling methods make use of these stimuli.
Fly-round
Two fly catchers carry a black coloured flag on a tour which is representative for the tsetse fly
habitat. The black screen is often baited with a sachet containing a tsetse fly attractant
(butanone or octenol).
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Arthropod vectors  Tsetse flies
Fly round Zambia
The fly catchers move along this path, stop at well-defined points and capture during a fixed
period the tsetse flies which are attracted. This method gives by no means an idea of the true
fly density but makes it possible to observe fluctuations in the density of the flies over time. This
method captures much more males than females (notwithstanding female tsetse flies live
longer than males and thus a population will comprise more females than males). With this
method teneral males are often underrepresented and teneral females relatively
overrepresented. Instead of a black flag to attract the flies an ox can be used as a bait to
increase the catch.
Traps
They attract and capture tsetse flies. The colour of the trap is important: blue attracts tsetse
flies while black encourages them to land.
Different tsetse traps
Several types of traps were developed of which the efficiency may vary according to the
tsetse fly species. The biconic trap of Challier Laveissière is often considered as the golden
standard.
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Arthropod vectors  Tsetse flies
Biconic trap
Performance of the trap will depend on its location, visibility and the use of chemical odours
(CO2, acetone, octenol and phenols) or natural odours (cow urine) which attract tsetse,
especially those of the savannah group. In general teneral flies (especially nulliparous females)
are underrepresented in the samples coming from the traps, but the ratio males/females in the
sample is generally more representative than in case of fly rounds. Most flies captured by a trap
are hungry and have low fat reserves.
The electric screen
The electric screen: It is made up of a metal frame and parallel electric wires between which a
high voltage is applied. Any fly colliding with the screen is electrocuted and falls into the
collecting device. Electric screens are used to evaluate the effectiveness of traps, target
screens, odours etc.
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Arthropod vectors  Tsetse flies
Electric screen to study tsetse flies
If the tsetse population would be in equilibrium and no immigration or emigration would occur,
an estimate of the real population density may be obtained using mark / release / recapture
experiments. Hereby, a random sample of the population is marked, released and recaptured
at a later date. The population density (X) can then be calculated since
X
Y
=
M
R
Where Y = the number of all recaptured flies (marked and unmarked), M = number of all
marked flies, R = number of marked flies recaptured.
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Arthropod vectors  Tsetse flies
Marked tsetse fly
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