Advantages and disadvantages of microbial control

The possibility of growing and distributing disease-producing organisms for insect control has as yet been restricted to a very few forms of insects. The organisms are restricted to a very narrow field of effective ness-either to a single species or closely related forms. Further expansion of the use of fungi for destroying insects consists not in finding such organisms, for there are many known species, but rather in learning how to produce, store, and distribute infectious material so as to produce insect epizootics at will.
Advantages of microbial control:
·         The relatively high degree of specificity of most pathogens tends to protect the beneficial insects.
·         When microbial pathogens applied through artificial dissemination for short term control of the pest the disease will persist and spread in the host population and initiate epizootics. They persist in the residual population and prevent pest resurgence. The microbial agents the target pest at low levels for several years.
·         Microbial agents are usually harmless and non-toxic to other forms of life because they do not possess any toxic residues.
Pathogenic insect organisms as control agents are not supplanting the use of insecticides, but rather supplement the chemical program. This gives an alternative way of control that operates without dangerous residues or building resistance. (no tendency to insect resistance has thus far been noted to any pathogenic organism). The micro organisms causing insect disease are not threat to the higher animals or plants in any but rare instances. Neither are the organisms thus far cultured dangerous to parasites or predators or to the pollinators (Steinhaus, 1952).
“on the basis of theoretical considerations, as well as experimental evidence, there appears to be little reason to believe that crystalliferous bacillus thuringiensis Berliner and its close varieties as they occur in properly prepared microbial insecticides are likely to mutate spontaneously into forms pathogenic for vertebrates. Similarly, on the basis of our present-day understanding of bacterial genetics, it appears that such mutations are not very likely with most of the truly entomogenous bacteria. At least, the possibilities in this regard don’t appear to be sufficient to preclude the use of adequately tested entomogenous microorganisms for pest-control purposes. Nevertheless, safeguards should be maintained to detect such events they occur.” (steinhaus, 1959).
 The possibility of microbial control of insects offers a new field to pesticide manufactures, either in manufacturing and distributing the product or to act as distributors for biological and fermentation houses. This is a natural field for agricultural chemical houses because many pathogens are compatible with a number of pesticides and fertilizers, and in some instances they are being applied together.
The microbial pesticide B. thuringiensis will be permitted to be manufactured and distributed under an experimental permit for one year by the department of agriculture. The material will be used against the eabbageworm, alfalfa caterpillar, and others of this type of insects. If no question arises and the material proves practicable, the manufacturer will be allowed to ask clearance for general use. Marketing will be permitted under a temporary exemption from the requirement of Food and Drug Administration tolerance.
This is the first use of such type of pesticide directly on food crops. The milky disease bacteria used against the Japanese beetle is applied to the soil and not directly to crops. 
source: E.R. de ONG.1960. chemical and natural control of pests. Rienhold publishing corporation: New York.