**Author details**

### N.M. Makate\*

**5.1. Microbial and spinosyns**

insecticides [41].

270 Insecticides Resistance

to humans.

Botswana.

**6. Conclusion**

*5.1.1. Resistant crop varieties*

use in combination with a low dose of insecticide.

Reports in Botswana have indicated that most insect pests found on agricultural crops have been subjected to chemical control. Diamondback moth *Plutella xylostella*, which is a pest of cabbage, is one such example of insect pest whose control relies heavily on the application of pyrethroids. It has also been demonstrated that DBM quickly develops resistance to many new

Studies were conducted using *Bacillus thuringiensis* (*Bt*) [41] and Spinosad [42] as alternatives to insecticides to demonstrate their efficacy on the diamondback moth (DBM). *Bt* is a soil dwelling bacterium and is largely used in agriculture worldwide. It is a natural insecticide that produces crystals protein (cry proteins), which are toxic to many species of insects but nontoxic

Spinosad is derived by fermentation from the soil actinomycete and is effective by both contact and ingestion to numerous insect species [43]. Bioassays using both natural insecticides were carried out on the eggs and 2nd instar larvae of DBM. The results using *Bt* indicated that *Bt* was effective against both the eggs and the larvae, whereas spinosad was shown to be more effective against the eggs than against the larvae. These results were able to demonstrate that both natural insecticides used in the experiments can achieve effective control of the develop‐ mental stages: eggs and larvae of DBM. However, more bioassays still remain to be done on other insect pests that cause damage to various agricultural crops commonly grown in

One other option to using insecticides on crops is to plant crop varieties that are found to be insect resistant. However, resistant varieties are usually only resistant to one or a limited number of insect pests. Genetic engineering has been able to allow the transfer of desired genes from one species to another, resulting in a quicker development of pest-resistant varieties or transgenic crops. On the evaluation of nine cabbage varieties for resistance to the cabbage aphid, Munthali [44] concluded that the most resistant cabbage variety would be the one that has a combination of low aphid numbers and low percentage of damaged leaves per plant. Notwithstanding, the use of these partially resistant varieties would also be recommended for

The levels of resistance in the two strains of Gaborone and Molepolole for both esterases B1 and A2–B2 are yet to be elucidated by carrying out bioassays against the susceptible strains. This approach will help to determine whether there is any correlation between esterase levels and insecticide resistance in these strains. This will also give an indication to the kind of resistance mechanism that may be conferred in these strains. It is at the DNA level that we can

be able to trace the origin and the migration path of these esterases into Botswana.

Address all correspondence to: makaten@mopipi.ub.bw

Department of Biological Sciences, University of Botswana, Botswana
