The terms biorational pesticide and biopesticide are rapidly gaining popularity in the current climate of environmental awareness and public concern. These terms are derived from two words, biological and rational, referring to pesticides of natural origin that have limited or no adverse effects on the environment or beneficial organisms.
There is however, actually no legally clear, absolute definition of a biorational pesticide. The Environment Protection Act of United States considers biorational pesticides to have different modes of action than conventional or traditional pesticides, with greater selectivity and considerably lower risks to humans, wildlife and the environment. A biorational pesticide is derived from a variety of biological sources, including bacteria, viruses, fungi and protozoa, as well as chemical analogues of naturally occurring biochemicals such as pheromones and insect growth regulators (IGRs).
They are considered third-generation pesticides that are environmentally sound and closely resemble or are identical to chemicals produced by insects and plants. Biorational products are quite different from conventional, broad-spectrum products. They do not control pests in the same way as most broad spectrum products. They are typically target-specific and have little to no impact on non-target organisms.
Most biorational insecticides are much more effective against some insect pests than others. As a result, proper identification of a target insect pest is essential. Many biorational insecticides have relatively short residual activity compared with conventional products. Thus, biorational products must be applied when the pest is in its most vulnerable life stage. Otherwise, applications may be ineffective, and applications of a conventional product may be necessary.
Classification
Biorational insecticides are classified into two distinct groups: biochemical and microbial. Biochemical products include hormones, enzymes, pheromones and natural insect and plant growth regulators. Microbial products originate from biological organisms such as bacteria, fungi, nematodes, protozoa and viruses.
Biochemical products
Insect pheromones are chemical substances that are given off by insects and cause a specific reaction upon reception by other insects of the same species. Releaser pheromones are fast acting and are used by insects for sexual attraction, aggregation, dispersion, oviposition (egg laying and alarm or warning.
Primer pheromones are slow acting and cause gradual changes in growth and development. Of these types of pheromones, sexual attraction pheromones offer the greatest potential for insect control. There are four primary uses for sex pheromones in present insect-control programs: male trapping, movement monitoring, detection and population monitoring and confusion.
Insect growth regulators (IGRs) are chemical compounds that alter the growth and development of insects. IGRs disrupt an insect's growth and development in two basic ways: as juvenile hormones and as chitin synthesis inhibitors. Juvenile hormones prevent insects from maturing, they force an insect to remain in its juvenile life stage (immature, caterpillar or grub stage). Much of the exterior (skin) and even some of the internal parts of an insect are constructed of chitin (a polymer). Chitin synthesis products inhibit the production of chitin, resulting in the inability of an insect to produce new exoskeletons (skin). And, without an exoskeleton, insects cannot survive. A chitin synthesis inhibitor would be an appropriate control product for the many insect pests of agricultural and stored grain products.
Microbial products
Bacillus thuringiensis (Bt) is a spore forming bacterium that produces toxins that, after ingestion, cause a rapid gut paralysis and death of certain insects, especially caterpillars. Bt is a naturally occurring soil bacterium, but chemical manufacturers have developed a process to formulate it and make it commercially available as a result, Bt is the most widely used microbial insecticide against agricultural pests.
However, Bt is not a "silver-bullet" like all control products, it has limitations. Bt has a relatively short-residual activity, is readily degraded by direct sunlight; has slow action, limited-contact activity and is unable to kill larger larvae. Success with Bt depends on close monitoring and application when insects are small or in their most vulnerable life stage. Bacillus popilliae is a spore- forming bacterium, the causal agent of milky disease in white grubs of the Japanese beetle and other scarab beetles.
Several strains of milky disease bacteria infect various species of white grubs, each strain tends to be specific for that type of grub. The bacteria are harmless to wildlife and humans as well as beneficial insects. Like Bt, milky' disease spores have been formulated and marketed. Beauveria bassiana is a naturally occurring fungus that infects several insects. Spores of the fungus adhere to the insect skin and upon germination, penetrate the body wall, eventually killing the insect.
Beauvaria bassiana has limitations as well. Similar to other fungi, it typically requires hot, moist conditions to be effective. More research is needed on fungus-based insecticides to better understand how they may be used pest management program. Entomopathogenic nematodes are microscopic roundworms that attack and kill insect caterpillars and grubs, and they continue their life cycle by reproducing within the dead host. These types of nematodes are beneficial organisms that naturally occur in most soils and are not considered a threat to plants and wildlife other than insects. They pose no threat of contamination to streams, lakes or groundwater. Unfortunately, factors, such as poor or short shelf-life, sensitivity to heat and sunlight, and speed of kill have limited the use of nematodes.
Spinosad is in a relatively new class of insecticides called naturalytes that contain two fermentation-derived substances produced by the bacterium Saccharopolyspora spinosa. This bacterium is a naturally occurring microorganism that acts as both a contact and a stomach poison, but is more effective if eaten by the insect pest. Spinosad in similar to conventional synthetic pyrethroids in that it is effective at very low use rates and has a fairly short residual activity. Spinosad, however, affects the insect nervous system by causing tremors, rapid paralysis and death. Because of the aforementioned characteristics and its low toxicity to humans and wildlife, spinosad may become an important resource for the management of pests of agriculture.
Biorational pesticides will become important resources for farmers and their pest-management programs. The farmers know their effects and uses. The government will also give some priorities in the registration and other procedures. The demand will likely grow for these new tools in future.
Extracted from Agrigold Swarna Sedyam
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