Insecticides for tsetse and trypanosomiasis
control using attractive bait techniques
1: Insecticides for tsetse control
past trypanosomiasis control policies of national control organisations,
significantly supported by donor agencies, have placed emphasis on the
eradication of the tsetse vector (F.A.O. 1991). Although often successful
in achieving their objective of controlling trypanosomiasis within the
project area, these actions have usually not produced sustainable results
because of the inability to consolidate and protect in the longer term
against reinvasion. Financial continuity has also rarely been adequate.
meeting of the F.A.O. Inter-Secretariat Group of the Programme for the
Control of African Animal Trypanosomiasis and Related Development, held in
Rome in December 1991, recommended that, in consideration of the problems
experienced, the technical approach and the policy of the programme be
revised to more accurately take into account the current dynamic situation.
They argued for a change in emphasis from vector eradication towards vector
suppression. However, whereas eradication implies a once and for all
solution and cost to the problem, suppression is an ongoing process
demanding a recurrent financial commitment. It can only be justified if it
can be achieved and maintained at a favourable cost:benefit ratio and is
the last 30 years, tsetse control and eradication attempts have largely
involved the use of either selective ground spraying or the sequential
aerosol application of insecticides. These techniques are considered more
suitable for eradication and cannot be justified for continuous on-going
vector suppression because of high recurrent costs and environmental
the success of long term control is largely dependent on the sustainability
of the actions undertaken, the techniques used for long term vector
suppression must be efficient, economically justified, simple to apply and
environmentally acceptable. The recently developed bait techniques meet
these prerequisites, in most circumstances, with many of the tsetse species
concerned. They also offer the possibility of being adapted for
implementation through the active participation of farmers and livestock
control can be carried out using artificial attractive devices, either with
or without odours and/or insecticides, depending on the local situation and
the tsetse species. Where the conditions are favourable, the use of
livestock, regularly treated with an insecticide, may be used, either alone
or in conjunction with artificial devices. Both methods may also be used to
form barriers and so prevent, or retard, tsetse reinvasion.
4 in the series of FAO training manuals for tsetse control personnel deals
with the practical and theoretical aspects of control using attractive
devices. These guidelines are intended to be of further assistance to the
same target group through providing current information on the insecticides
suitable for use with these techniques.
High levels of toxicity to various tsetse species
Whilst this is a fairly obvious prerequisite, it has to be remembered that
all insecticides will kill tsetse, provided they are exposed to an adequate
amount. However, in practice, small doses of high toxicity are desirable
because this reduces what may otherwise be the very high cost of transport
and the number of times the spraying has to be halted to refill the spray
equipment. The ideal insecticide would be one that needed only very small
quantities to kill tsetse and had virtually no effect on any other animal
at these doses. Unfortunately such an ideal does not exist. However, it is
fortunate that tsetse are generally very sensitive to most of the known
insecticides and it is possible, therefore, to apply relatively low dosages
of any insecticide for tsetse control as compared to the use of, for
example, the same insecticide in agriculture.
Ease with which they can be made into suitable formulations
Whilst the trap/target and/or livestock baits serve to attract the tsetse,
they are only a part of the delivery system by which sufficient numbers are
brought into contact with the insecticide. The efficiency of the chemical
and the degree to which it can be adapted for practical use depends largely
on the ease with which it can be processed into a usable form.
of the insecticides under consideration are concentrated solids in their
primary manufactured state. They then have to be converted into an
acceptable form (or formulation) for field use and so meet the following
easy to transport in sufficient quantities to remote and relatively
isolated areas where, if necessary, they can be diluted to the required
easy to apply evenly and adhere to the relevant surface, be it cloth or
readily transferred to the tsetse, often by a very brief contact. The
insecticide should be sufficiently toxic that a lethal dose is transferred
to the fly even by such a brief contact;
rapidly absorbed through the cuticle and into the insect so that death or
"knockdown" occurs relatively quickly;
once applied, be relatively stable in a variety of climatic conditions.
They must not, for example, be rapidly degraded by sunlight or easily
washed off the target surface by rain. Obviously the longer they persist
and remain lethal to the tsetse, the more cost-effective they will be;
is therefore extremely important and it has to take into account many
factors, some of which are not easy to reconcile. The subject of
formulation is discussed in more detail later.
Low levels of mammalian toxicity
In the large scale use of insecticides, such as often required for tsetse
control, it is not always practically possible for suitably trained
technicians to closely supervise and monitor their use. Under such
conditions, highly toxic chemicals would pose a severe risk not only to the
handlers but also to local inhabitants and the environment in general.
Also, insecticides with a high mammalian toxicity could not be considered
for direct application to livestock as not only would this affect their
well-being but may also be transferred to humans through milk and other animal
products. The selection of a suitable insecticide must not, therefore, be
based on its toxicity to tsetse alone but should take into account these
other equally important considerations.
Stability and Persistence
The inherent physical stability of any insecticide and its persistence once
applied in the field depend on the intrinsic stability of the actual
chemical molecule and also of the resulting formulation, under a wide range
of climatic situations. The formulation must be stable first in concentrated
form in storage and transport, and then must remain relatively stable and
toxic once diluted and applied in the field. Sunlight and extremes of
temperature can affect the chemical structure and thereby the effective
toxicity over time.
third major factor concerns the surface to which the insecticide is
applied. This can both react with the chemical and also bind the
formulation to it, thus sometimes affecting the availability and pick-up of
insecticide by the tsetse fly. Rainfall can also leach out the insecticide
from the surface. Dust and mud can also cover insecticide-treated surfaces
and so mask the insecticide from contact with the tsetse fly.
is important that these factors are borne in mind when planning control
operations and that bioassay and chemical weathering tests are carried out
to confirm the effectiveness of the application over a period of time.
Artificial tsetse attractive baits have been developed for maximum
attractiveness, both visually and by smell, to this one particular insect.
Also,as the insecticide is applied directly to the device, it is only those
insects which come into contact with the surface that are killed. It has
been shown that some individuals of other insect species, particularly
blood sucking flies, are also attracted, but not to the same degree as
tsetse. Similarly with insecticide-treated animal baits, it is only the
non-target species that are also attracted to the animal that will be
affected. In most cases the effects are again limited to nuisance and blood
sucking species which to variable degrees restrict animal productivity or
transmit other diseases : in this regard, their incidental control can be
regarded as a bonus.
the products should ideally have as little deleterious effect on non-target
fauna as possible whatever the technique used. They should also be rapidly
broken down into non-toxic derivatives once they are leached out or are
released into the wider environment. There is no known insecticide at
present which makes this completely possible but, with the choice of
insecticide and formulation, type of application and low dosage rates
required, it is possible, using the synthetic pyrethroids, to go a long way
towards achieving these ideals.
This is a paramount consideration when selecting the vector technique to be
used. If the costs involved are greater than the benefits derived, then control
action is not justified. When the objective is control, then action has to
be sustained over a long period, in which case even very modest annual
expenditure could eventually prove more costly than using a more expensive
eradication technique over a much shorter period.
The groups of insecticides
are classified in groups based on common factors of their basic chemical
composition. The following are the major groups of insecticides, with
comments as to their suitability for use with the current tsetse control
DDT, dieldrin and endosulfan.
are fairly active compounds with usually good stability. They are also
relatively cheap. DDT and dieldrin were the insecticides of choice for
selective application to tsetse habitats by ground spraying, because of
their long persistence. A single application remaining effective for 2-3
months, and possibly longer, especially in the dry season.
unlike other members of this group, is not so stable, and although it is,
therefore, not suitable for residual ground spraying, it has proven very
effective for sequential aerial application which uses very low dosages of
insecticides dispensed as a non-persistent spray of minute droplets.
and a specialised formulation of endosulfan have also both been used as a
residual deposit, applied by helicopter, in Nigeria and Cameroon.
as these insecticides have been, there has been an increasing reluctance to
use the more persistent members of this group, i.e. DDT and dieldrin,
because of their possible adverse effects on the environment and their
accumulation in food chains.
use of endosulfan as a low dosage aerosol applied sequentially by aircraft
has proven, through many ecological studies, to have minimal and in most
cases only transient side-effects on non-target organisms. This is mainly
because of the extremely low doses used and the relatively rapid breakdown
of the compound when applied in this manner. It is, therefore, a
recommended insecticide for use by this technique.
group in general lacks sufficient toxicity and persistence to be used for
tsetse control. Consequently, they have never been used for large scale
field operations and it is, therefore, proposed not to discuss them
the previous group, the carbamates also lack sufficient toxicity and
persistence to be used for tsetse control and again as for the
organophosphates, it is not proposed to consider them further.
deltamethrin, alpha-cypermethrin, cyfluthrin, lambda-cyhalothrin.
group has favourable chemical and physical properties which make several of
its members ideal for tsetse control, especially for use with the more
recently developed attractive bait techniques whether they be traps and
targets or insecticide-treated livestock.
naturally occurring pyrethrins, which are extracted from plants, are highly
toxic to tsetse but are so unstable as to make them of little use for
begun at Rothamstead Research Station in the United Kingdom, over 30 years
ago, led to the discovery of somewhat more stable synthetic pyrethroids
such as allethrin and bioresmethrin. These chemicals, although they showed
promise, were not ideal and it was only when a further generation of
synthetic pyrethroids were discovered and developed that exciting prospects
for tsetse control became apparent. This later group, or the "cyano
group" as they are known chemically, contains the now well known
insecticides such as deltamethrin, cypermethrin, cyfluthrin and
compounds all have extremely high levels of toxicity for most insects,
including tsetse, are very stable, and are only mildly toxic to mammals.
They are, therefore, relatively safe to handle and are less likely to have
undesirable effects on other non-target animals, particularly mammal and
is, therefore, this group of insecticides which at present has the best
potential for use with all attractive bait techniques including traps,
targets and livestock. The properties of the group, and of individual
members, which make them the most suitable of all known insecticides for
tsetse control will be discussed in more detail.