Chemical insecticides have helped keep our agricultural fields free of pests for centuries. However, the toll they have taken on our health and on the environment cannot be ignored any longer. Alternate pest control methods are the way to go if we want to preserve what is left of the Earth's environment, at the same time safeguarding the health of our families. One of the alternate methods of pest control that is gaining popularity is microbial insecticides. They employ microbes as insecticides- what can be more natural and environment-friendly? They are completely non-toxic to other animals and to humans, when compared to chemical insecticides.

Microbial insecticides are also called as biological pathogens or biological control agents. They are basically made up of microbes like bacteria, viruses or fungi that produce toxins that are harmful to the pest. Bacterial insecticides function only when ingested by the pest. They may kill a variety of related pests or just one. For example, Bacillus thuringiensis var. Kurstaki can kill in the caterpillar stage of development, a range of moth pests. But, Bacillus papillae can kill only the Japanese beetle larvae but not closely related species.

These insecticides are formulated in such a way that they can be easily applied to the plant just like chemical pesticides. They are available as sprays, powder and in the granule forms. Some microbial insecticides also employ nematodes as biological agents.

The Action of B. thuringiensis var. Kurstaki on Caterpillars

The B. thuringiensis insecticide (Bt) has been in use since the 1960s. The components are produced in industrial fermentation tanks under ideal conditions. It was discovered that the Bt strain that had undergone sporulation was toxic to larvae. This led to the conclusion that some toxic substance must be produced during the sporulation process. This toxin was discovered to be crystalline inclusion bodies with a large bipyramidal structure and a smaller cuboidal inclusion.

When, the susceptible crop is sprayed with preparations of B. thuringiensis, the caterpillar pest will consume the leaf containing the endotoxin. Once in the gut, the toxin is activated by the alkaline conditions and it binds to the cell surface receptors within the gut. Whether the protein is toxic to the insect or not is determined by the presence of these receptors. The caterpillar stops feeding while the intestinal cells get paralyzed and destroyed. The intestinal wall disintegrates and the toxin enters the blood cavity. The pest itself comes under attack from spores and bacteria, which enter the body cavity. After a day or two, the caterpillar is killed by their action. Since the caterpillar stops feeding almost as soon as it starts, the crops are safe. The crop that is suffering from a caterpillar pest problem may also be genetically modified with the insertion of a gene that produces a crystalline endotoxin.

Advantages of using Microbial Insecticide

-It is completely natural and does not introduce any new or man-made chemicals into the environment.
-It is absolutely non-toxic and safe for the farmer and the consumer. Its toxicity, if any, is directed solely at the plant being treated with the insecticide. They do not affect beneficial organisms.
-Although they contain microbes, microbial insecticides are not pathogenic in nature and do not affect the other animals or plants that are not targeted by it. This is one of its strongest advantages.
-In some cases, the microbial insecticide can be sprayed along with the chemical insecticides- they are usually not affected adversely.
-They can be applied at almost any stage of the planting, even at the harvest stage. These pathogens may also help act as a control for the future.
-They are extremely stable if stored under the right conditions and can last years in storage.
-The crystals are not inactivated by UV light.

Disadvantages of using Microbial Insecticides

These limitations will allow choosing the most effective microbial insecticide for the control of a particular pest.
-The microbial insecticides are highly specific in nature. An application of a particular microbial insecticide may be toxic to a highly specific type of pest. So, to treat a combination of pests, a combination of microbial pesticides may be required. However, it may be pertinent to note that this is the case with most chemical pesticides too.
-Some environmental conditions may be unfavourable for the optimum action of microbial insecticides. These may include high heat and desiccation. It is of utmost importance to time the application of the insecticide precisely.
-Preparations of some formulations may be complex and the storage of some of them can be tricky.
-The specificity of the microbial insecticides limits their distribution and popularity. They may also be more expensive than chemical insecticides.
-The action of the Bt toxin may be inactivated under some weather conditions. A few repeated applications of the preparation may be required.
-Microbial insecticides are generally more expensive to produce than chemical pesticides. This may decrease with more demand.
-To ensure that the larva will ingest required amount of toxin, a large amount of the preparation may need to be applied.
-The speed with which they kill the pest may be slower than the chemical pesticides and this may lead to the supposition that they are less effective.

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