Bacterial Control of Mosquitoes and Blackflies: Biochemistry, Genetics and Applications of Bacillus thuringiensis israelensis and Bacillus sphaericus

Abstract

Owing to problems associated with the use of synthetic insecticides to control mosquito and blackfly vectors of disease, interest in biological control approaches has been stimulated over the past two decades. Although many strains of the entomopathogenic bacterium Bacillus thuringiensis have been isolated over the years the known strains were only effective in killing insects affecting crops and trees and their use as control agents had therefore been limited to agriculture and forestry. In the mid-1970s efforts to study and develop new and existing biological control agents resulted in the isolation of B. t. israelensis (BTI). This showed high toxicity to mosquito and blackfly larvae and stimulated great interest in its role as a microbial insecticide. Over the preceding decade or so multiple isolates of B. sphaericus (BS) had also been obtained which had been shown to be effective against mosquito larvae.Both these sporulating bacteria produce a crystalline parasporal inclusion body containing proteins with insecticidal action. After ingestion by aquatic larval stages of the vector the inclusions dissolve in the digestive juices releasing the toxins which act as gut poisons. These act rapidly causing the midgut epithelium to swell and burst thus inflicting severe damage to the gut wall and death of the larva.One of the principal practical problems for the widespread use of these bacterial toxins is their relatively short persistence in breeding sites. Considerable research interest has therefore focused on the molecular genetics of the genes coding for the toxins. Already several BTI DNA fragments containing toxin genes have been cloned in different expression systems and the different toxins produced have been characterized. This work has now been extended to examine the possibilities of cloning the genes coding for the toxins into blue-green algae which are permanent occupants of many breeding sites and act as larval food sources. Successful cloning and expression of the gene encoding larvicidal activity from BS into a cyanobacterium was described in 1987.This book reviews this intensive work of the late 70s and 80s on these agents from characterization through to operational use. Part 1 of the book deals with information on BTI in 12 chapters covering discovery; characterization; structure, protein composition and toxicity of parasporal body; mechanism of action of parasporal body; genetics of BTI; cloning of BTI mosquito toxin genes; transfer of mosquito toxin genes into mosquito larval food sources; potential for improved formulations of BTI; usage of BTI against mosquitoes; progress in control of blackflies with BTI in temperate climates; BTI usage for onchocerciasis control in West Africa; and mammalian toxicity testing of BTI.In Part 2 a similar format is followed for B. sphaericus in 9 chapters; classification of BS strains and comparative mosquito toxicity; biochemistry of larval toxin; genetics of BS; local production of BS; persistence and formulation of BS; formulations and persistence of BS in Culex quinquefasciatus larval sites in tropical Africa; field trials of BS for mosquito control; and mammalian safety.The book is concluded in Part 3 by a perspective for the next decade that reviews the supportive factors that may encourage the use of these bacteria such as resistance of vectors to chemical insecticides and the public's attitude to insecticide use, etc. Counterfactors that complicate usage are also considered such as efficiency and cost effectiveness and the need for improved formulations.Although the book may already be a little dated in that most references are no later than 1987, overall its content is reasonably well balanced and it is realistically priced at #45. It provides a range of useful information and references of general interest and particularly for those concerned with mosquito control or those more interested in the use of microbial genetics and molecular biology approaches.newline˜C.J. Leake