**5.1. Microbial and spinosyns**

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 insecticides [41].

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 to humans.

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 Botswana.

#### *5.1.1. Resistant crop varieties*

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 use in combination with a low dose of insecticide.
