Control of insects by bacteria

Abstract

All bacteria in microbial insecticides are species of Bacillus and form spores since they have to survive in the environment and on the shelf. They can be formulated as wettable powders, suspensions and dusts for application with conventional pest control machinery. All are safe to man and virtually all non-target organisms. Development costs are relatively low, but host specificity greatly restricts markets, the largest being ca. 2000 tons per annum in the West for B. thuringiensis. All act only after ingestion, a disadvantage because there is no contact action and usually only larvae are attacked. Three main groups have special features that determine their commercial success.The B. popilliae group is produced only in vivo which limits production by three small firms. The Japanese beetle has been controlled in grassland in the warm parts of the USA by single applications of spores in heaps, spaced 2 m each way. The bacterium spreads slowly to untreated areas, is very persistent and kills only by infection.The B. thuringiensis group kills larvae of Lepidoptera, mosquitoes and blackflies, mainly by gut poisoning with a protein crystal toxin. It rapidly paralyses mouthparts and gut, stopping crop damage. It is readily produced by deep liquid fermentation, but does not persist and needs repeated application during the pest season. Products containing no beta exotoxin can be applied at unlimited dosage to food crops up to harvest. Only one application is needed for stored grain. After 20 years' use of strains against Lepidoptera, a different strain is now used commercially against mosquitoes and blackflies (only 5 years after its discovery), although improvements in formulation for aquatic application are needed. A recent new product based on the beta exotoxin is used in Finland only against flies in pig houses because it has some vertebrate toxicity.The B. sphaericus group is similar to B. thuringiensis, except that its proteinaceous toxin is different, is situated in the spore wall in strain 1593, and attacks only mosquitoes. Now at the pilot production stage, its commercial future depends on whether it is more potent than B. thuringiensis against certain species and whether it can recycle to give effective extended mosquito control in some environments.Intensive selection from natural isolates has improved potency 100 to 600 fold. This selective effort must be maintained and improved by genetic manipulation, which can be used to develop greater potential, particularly since DNA coding for the crystal toxin is carried on plasmids. This also gives speculative hope that the toxin may be incorporated into natural aquatic bacteria for mosquito control and into plants for protection against lepidopterous larvae. A great advantage is that these bacteria do not harm beneficial fauna to cause pest resurgence. At present, the main use lies in integrated pest control systems, although bacteria are not likely to supplant chemical insecticides on a large scale in the near future.