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 techniques :-
Examples; DDT, dieldrin and endosulfan.
These 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.
Endosulfan, 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.
Dieldrin and a specialised formulation of endosulfan have also both been used as a residual deposit, applied by helicopter, in Nigeria and Cameroon.
Effective 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.
The 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.
Examples; malathion, parathion.
This 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 further.
Like 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.
Examples; deltamethrin, alpha-cypermethrin, cyfluthrin, lambda-cyhalothrin.
This 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.
The 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 practical control.
Work 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 cyhalothrin.
These 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 bird species.
It 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 in Chapter 3.