**3. Insecticide use in Botswana**

any heritable changes but selects favourable mutations that allow the insect to survive the treatment [3]. The resistant strains thus develop through the survival and reproduction of individuals possessing one or more of many possible mechanisms that allow survival after exposure to an insecticide, each controlled by one or more resistance (R) genes. Strains tend to revert to susceptibility in the absence of insecticide exposure unless they have become homozygous for the R genes [1, 4, 5]. This makes insecticide resistance to be a natural phe‐ nomenon controlled by genes that bring about the biochemical, physiological, or behavioural

Resistance can shorten the long-term effectiveness of a particular insecticide against a species population prompting the use of an alternative insecticide to which there is no resistance; but unfortunately, this often becomes a temporary solution. The development of cross-resistance may occur to compounds within a group with a similar mode of action, especially if their

Cross-resistance can also occur between groups of insecticides with different modes of action and can be mediated by a single gene, i.e., be monogenic due to a single defense mechanism operating against two or more toxicants. It can also be polygenic where multiple mechanisms are available, which may not act equally against different toxicants. Since multiple resistances involve multiple genes, it can be a most serious development, should it occur in the field [6].

Resistance to insecticides by insect pests has been documented for over 75 years, but its greatest impact has occurred during the last 30 years following the discovery and extensive use of synthetic organic insecticides [7]. Insect resistance was first observed in 1908, reported by Melander [8] in the San Jose scale insects *Aspidiotus perniciosus*, found to have become insen‐ sitive to lime-sulphur. Thirty years later, there were further reports of insect resistance towards

When dichlorodiphenyl-trichloro-ethane (DDT) was introduced in 1946, insect resistance to the compound appeared quickly and worldwide. The first sign of resistance towards DDT was shown in the housefly *Musca domestica* [9]. Thereafter, cases from different locations were reported: *Aedes sollicitans* in Florida, *Culex pipiens* in Italy, and *Cimex lectularius* in Hawaii [1]. New insecticides that were later introduced did not last long with regard to their usage as the number of species showing resistance to one or more toxicants doubled every six years between

A number of resistant species are also reported in other agriculturally important orders such as Lepidoptera (67 species, representing 15%), Coleoptera (66 species, representing 15%), Acarina (58 species representing 13%), Homoptera (46 species, representing 4%), and Heter‐ optera (20 species, representing 4%) [11]. However, studies have shown that resistance develops faster in insects with many generations per year rather than only one, at higher selection pressures than at lower ones. Sawicki [12] noted that resistance is regarded as a problem only when the cost of control becomes unjustified or when excessive use of the control

changes on which resistance is based.

264 Insecticides Resistance

metabolism and their target site attachment are very similar [6].

**2. History of resistance to insecticides**

agent presents health and environmental hazards.

numerous other pesticides.

1948 and 1983 [10].

The economy of Botswana is mainly dependent on agriculture and mining. The agricultural sector in Botswana covers both crops and livestock production. The industrial growth has brought about awareness in farming systems for both livestock and arable farming. However, this has also brought about an increase in the use of chemicals for pests on animals and crops. Insect pests are very important in crop production because they pose a serious problem to farmers. They reduce the yield and quality of crops resulting in lower prices for the crops and lower returns to the farmer.

Since the introduction and use of DDT in Botswana in the 1950s, other types of insecticides such as organophosphates, pyrethroids, and carbamates have been used in various aspects of agriculture. In crop production, these were used to target pests diamond back moth, aphids, locusts, and armyworms; fruit flies, diamond back moth, aphids, and leaf miners; American bollworm, diamond back moth, aphids cutworms, and bagrada bug, respectively [13].

From the results of experiments carried out during the 1970s in Botswana, carbaryl proved to be the most effective insecticide against *Helicoverpa armigera* on cotton, sorghum, and cowpea when tested against insecticides such as DDT, endosufan, monocrotophos, and tetrachlorvin‐ phos [14]. However, the current pest management option for *H. armigera* in Botswana is the use of pyrethroids from recommendations based on the information from manufacturers and recommendations from other countries [14, 15].

Organophosphates are commonly used for the control of infestations of parasites for livestock and may also be applied as sprays and dips in form of acaricides. The same application of organophosphates has extended to spraying of the quelea birds by the Plant Protection Unit of the Ministry in Botswana [16].

Several control methods have been employed in the management of tsetse fly in Northern Botswana, and all of these methods involved the use of chemicals (Table 1). After the spraying of 2001 and 2002 in the Okavango Delta and 2006 in the Kwando-Linyanti systems, tsetse fly has not been found [17]. There were reports, however, that the deltamethrin spraying nega‐ tively affected other nontargeted organisms such as *Cyrtobagous salvinae*, with recovery in abundance after spraying [18].


**Table 1.** Insecticides used for the different control methods for tsetse fly.
