Arizona State University College of Liberal Arts and Sciences


Microbial Insecticides

(Chapter 15)

Pesticidal proteins

Insect infestation can cause a significant decrease in crop productivity. Such infestations usually were difficult to fight without the use of nasty chemicals with high toxicity to other life. However, more environmentally sound approaches are being developed using biotechnology. One approach involves the Bacillus thuringiensis toxin protein. This protein all by itself is harmless, but is converted to a potent toxin in the gut of certain kinds of moths (depending on the bacterial strain the toxin was isolated from) and of mosquito larvae. The gene coding for this protein has been cloned from various Bacillus thuringiensis strains, and has been incorporated into several plants. The moth against which the toxin is active dies after eating from the transformed plant. The drawback of this case of biological pest control is that the plant is resistant to that particular moth (even though the beast has first eaten from it before it is killed), but not to most other species of moths. Another drawback is that resistance of the moth to the toxin may develop (the only thing the moth has to do is to learn to not convert the original toxin protein in its midgut). The first cases of resistance to Bacillus thuringiensis toxin in caterpillars already have appeared. To minimize this problem, farmers now plant some "wild type" crop (not producing the toxin) in a corner of their plot, and the toxin-producing crop of the same species on the remainder. In this way, toxin-resistant insects may not have such a huge advantage over their toxin-sensitive siblings that the toxin-resistant ones quickly become the prevalent species in the ecosystem.

The major advantage of the Bacillus thuringiensis (Bt) toxin is that it is harmful to only a few species of insects, while it is essentially harmless to other animals and humans. These biological pesticides also degrade rapidly in the environment. Thus, the use of such biological pesticides appears to be a significantly more environmentally safe solution to pest control than the classical (synthetic chemical) pesticides. Indeed, the majority of the remaining cotton fields in Arizona have been planted with transgenic cotton plants producing a Bt toxin that is particularly effective against the pink bollworm, the primary pest on cotton in Arizona. At least 4% of the fields planted with transgenic cotton is set aside to be planted with the non-transformed strain. This is part of the strategy to minimize the ecological survival advantage of Bt-resistant bollworms that may develop or that may "immigrate." Cotton seeds carrying a Bt gene have been commercially available since 1996.

Other ways of biocontrol

Another approach towards biological pest control is based on an original and rather devious idea: as many male insects are attracted to females through chemicals (pheromones) the females excrete in minute quantities, one can spray the fields with pheromones, thus profoundly confusing the males about where to find their partner. The genes for pheromone biosynthesis have been cloned from various insects and expressed in bacteria, thus paving the way for making enough pheromones to spray the fields with. Note, by the way, that pheromones by-and-large are innocuous compounds and need to be sprayed only in minute concentrations. Preliminary evidence indicates that this approach is highly successful.

Other molecular-genetically based techniques of environmentally responsible pest control are: (1) Sterilize a large number of male insects (for example, by irradiation), and release them in the field. They will mate, but no progeny will result. (2) Clone genes for the synthesis of juvenile or anti-juvenile hormones from insects, produce the hormones in large quantities, and spray on the fields. An excess of juvenile hormone will prevent maturation of the insects, and an excess of anti-juvenile hormone will result in premature maturation and sterility. These hormones generally are specific for certain groups of insects and are not toxic to others, thus minimizing the impact on the environment.

However, presently biological pesticides still have a rather modest market share compared to the total pesticides marketed. The answer to this apparent paradox is that the narrow spectrum of control makes biological pesticides unattractive for some applications. However, the low market share is also due in part to the fact that the development of biological pesticides is relatively new (of the last dozen years), and that industry has been slow with catching on to the idea, and has not yet spent considerable resources on development of better biological pesticides. However, the public opinion currently is strongly in favor of ecologically acceptable methods of insect control, and this will impact the setting of priorities in product development by industry.

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Instructors | Aims
Lecture Part: Schedule | Expected Background & Textbook Info | Historical Perspective
Intro to Biotechnology | DNA, RNA and Protein Synthesis | Chemical Synthesis, Sequencing, and Amplification of DNA |
Directed Mutagenesis and Protein Engineering | Vaccines | Antibiotics & Proteins | Bioremediation |
Microbial Insecticides | Plant Genetic Engineering: Methodology | Plant Genetic Engineering: Applications | Transgenic Animals
Human Molecular Genetics | Regulatory & Ethical Aspects | Biotech Inventions | Additional Materials
Lab Part: Aims and Expectations | Schedule

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