Genetic improvement of biocontrol agents for sustainable pest management

Abstract

Genetic improvement involves directed purposeful genetic alterations to enhance the efficacy of natural enemies for biological control. This may be achieved by conventional approaches as well as through recombinant DNA techniques. The conventional methods include strain selection, serial passage through hosts, mutation, conjugation, transduction, selective breeding, hybridisation, etc., whereas the genetic engineering approaches involve gene transfer utilising various methods. Entomophagous insects may be improved for climatic tolerance, sex ratio, host-finding ability, host preference, increased host range, increased pesticide resistance, etc. The main objectives in genetically altering microbes are to increase host range, virulence and persistence. The cry genes from Bacillus thuringiensis have been cloned and expressed in a wide variety of organisms (baculoviruses to cyanobacteria) as well as in plants in attempts to improve their delivery and efficacy against insect pests. Apart from B. thuringiensis, binary toxin from different B. sphaericus strains has been expressed in different hosts like Escherichia coli, non- or low-toxic B. sphaericus and crystal minus Bt israelensis as well as in Caulobacter crescentus or cyanobacteria Anabaena sp. Insect viruses, especially baculoviruses, are mostly specific viruses which can replicate only in hosts. The recombinant DNA technology has its current applications in inserting foreign genes into insect baculoviruses and achieving their rapid and efficient expression in the recipient host systems. Candidate genes for hyperexpression in the baculoviruses include those encoding insect-specific enzyme genes (juvenile hormone esterase gene), hormone genes (eclosion hormone gene, diuretic hormone gene) and insect-specific foreign toxic genes (scorpion venom toxin genes, predatory mite toxin gene, predatory spider toxin gene, parasitic wasp venom gene and Bt δ-endotoxin genes). It is worthwhile to mention that genetic engineering of BCAs is potentially very promising and has led to the development of more effective entomopathogens with desired pathogenicity, virulence, broad host range and persistence, providing a valuable tool for sustainable pest management.