Insect Resistance

Chemical control of insect pests is both expensive and environmentally unfriendly. Worldwide expenditure on insecticides and the value of crop losses from insect predations have been estimated in Table .
Transgenic cotton, maize and potato crops are being grown commercially which express Bacillus thuringiensis (Bt) toxins to confer resistance to chewing insects. On sporulation, B. thuringiensis synthesizes S-endotoxin crystalline proteins encoded by Cry genes. On ingestion by an insect, prototoxins are cleaved in the alkaline midgut to the active toxin. This binds to specific receptors in the gut epithelial cells, which results in the formation of pores and eventually to the death of the insect. Some advantages of Bt toxins include:
(i) Specificity – each Cry protein is active against only one or a few insect species.
(ii) Diversity – many different Cry proteins have been identified.
(iii) Reduced or no detrimental effects identified on non-target insects or natural enemies of insects.
(iv) Very low mammalian toxicity.
(v) Easily degradable.
The production of transgenic Bt expressing insect resistant crops has been a high commercial priority, but effective production of Bt toxins required re-engineering of Cry genes for plant codon usage and removal of cryptic signals (e.g. splice sites, polyadenylation signals). These changes permit efficient expression of Bt toxins in plants, with both fulllength and truncated versions of Cry genes used successfully for insect

Table . Worldwide costs of insecticides and losses caused by insects

resistance. Now, more than 40 different genes containing insect resistance have been incorporated into transgenic crops with several commercialised in different countries such as the USA and Australia. Given the usefulness of Bt toxins for insect control, various management strategies must be adopted to delay development of insect resistance to Bt. These include:
(i) setting aside areas of non-Bt cotton as refuges to reduce the selection pressure towards insect resistance;
(ii) deploying different insect resistance genes (e.g. protease inhibitors);
(iii) using multiple Bt toxins which target different receptors;
(iv) use of spray inducible promoters to control expression of Bt genes;
(v) use of tissue-specific promoters such that insects can feed
unharmed on economically less important parts of the plant. It is mandatory to set aside non-transgenic refuges when growing Bt cotton, maize and potatoes. Even with additional costs associated with this and other agronomic management practices, the growth of Bt cotton gives higher returns to farmers, environmental benefits (50–80% less chemicals used, spraying reduced from 10–12 per year to 4–5 per year) and less occupational exposure of farm workers to sprays. There are other approaches under development for transgenic insect resistance,including those based on: protease inhibitors, a amylase inhibitors, lectins, chitinases, cholesterol oxidase, cloned insect viruses, tryptophan decarboxylase, anti-chymotrypsin, anti-elastase, bovine pancreatic trypsin inhibitor and spleen inhibitor.