Photo : Messenger
Pesticides are toxic substances that help kill insects. Chemical insecticides are mainly used to control insects. Its application destroys the eggs and larvae of insects. Pesticides are applied in medical, industrial and household activities including agriculture. In the 20th century, the use of pesticides as a major regulator of agricultural productivity growth is believed to have expanded.
Foods mixed with pesticides often cause harm to humans. Also, almost all types of pesticides used are thought to have significant impacts on biodiversity. Many types of pesticides are also harmful to humans. Some pesticides also affect the food chain.
Nicotine, a naturally occurring insecticide derived from neem, is used against insects. Nicotine-based insecticides are still being used globally, including in the United States and Canada. But the use of such pesticides has been banned in European Union countries. Non-herbicide pesticides are formulated with metallic compounds that often contain sulfur.
Pesticides can be inorganic or organic substances and fall into three general categories - food poisons, contact poisons and smoke poisons. Fumes are usually most effective against pests of products stored in enclosed spaces such as houses, barns, or greenhouses. Some substances like repellents, attractants, chemical disinfectants, pheromones etc. are also used in pest control, but their modes of action are different. Usually, these chemicals are not toxic.
In general, inorganic pesticides are effective only as food poisons and are currently used mainly as baits. Most organic pesticides are synthetic or plant-based and are effective as contact poisons, and in some cases as smoke poisons. The history of the emergence of pesticides is not very long. The first widespread use of insecticides began in the 1800s when potato fields were infested by the Colorado potato beetle in Mississippi, USA. An arsenic-containing substance called Paris green proved so effective as a plant preservative that growers then began using it to destroy apple moths.
Paris green became very popular by 1900 and spread throughout Europe and America. Apart from Paris green, other inorganic salts, such as lead arsenate, cryolite, mercuric chloride, sodium fluoride and sulfur are the oldest insecticides used. Some of these are still in use. Tropical aborigines knew the toxicity of the roots and parts of certain plants, such as Tephrosia, Deris, Lonchocarpus, etc., which they used for centuries as fish poisons for killing and catching fish.
This later gave rise to plant bio-pesticides commonly known as botanicals or alkaloids, such as rotenone, nicotine sulfate, and pyrethroids. It is known that Rotinone was first used as an insecticide in 1848 to control leaf-eating caterpillars. However, it was not possible to isolate the active ingredient of rotenone before 1902.
The current trend in the discovery of new pesticides is almost entirely towards synthetic bio-chemicals. The mid-20th century was a period of revolution in the discovery of synthetic insecticides for pest control. The discovery of DDT and its successful use in controlling widespread pests, including flies, inspired chemists and the chemical industry to discover and market hundreds of new insecticides.
Synthetic bio-pesticides can be classified in various ways on the basis of chemical conjugation such as chlorinated hydrocarbons, cyclodiene compounds, carbamates, organophosphates etc. Among the chlorinated hydrocarbons, DDT, methoxychlor and lindane have been widely used for many years. Its use is now banned in many countries, including Bangladesh, as its residues remain active for a long time.
Cyclodiene compounds are highly chlorinated cyclic hydrocarbons. These include chlordane, heptachlor, aldrin, dieldrin, endrin etc. Most of these pesticides are more effective against soil pests. Endrin has been banned in Bangladesh since 1962 as it is highly toxic to fish. Carbamates are a unique class of pesticides.
Organophosphorus insecticides are widely used to kill almost all kinds of pests. As a result of this research, thousands of insecticides of all types have been discovered. Malathion, Diazinon, Bydrin, Dimicron, Azodrin, Nogos, Nexion etc. are some of the most known organophosphates. Most of these pesticides have multiple trademarked names.
Most organophosphorus pesticides are systemic in action. This is an outstanding feature because it is absorbed and transported to lethal levels in various parts of the plant, and herbivorous insects die as soon as they consume them. Insecticides of these qualities are now essential for the control of stem-boring caterpillars.
Before 1956, no pesticides were used to control harmful insects in Bangladesh. That year, the government first imported pesticides. Until 1974, the government distributed pesticides to farmers free of charge. In 1979, the pesticide business was transferred to the private sector. Pesticide use declined in the early 1980s. But in 1999 usage increased. The main reasons for the increase in the use of pesticides today are the increase in the amount of land used for high-yielding rice cultivation, the increasing enthusiasm among farmers to use pesticides for pest control, and the excessive and sometimes unnecessary use of pesticides by farmers due to ignorance.
Microbial insecticides are the control of harmful pests by infecting them with pathogens and their by-products. Like chemical pesticides, they can be stored for some time, marketed in drums, diluted with water and sprayed with spraying machines. Microbial pesticides have some noticeable advantages over chemical pesticides. They are effective against specific pests, safe and free of toxic residues.
Moreover, microbial insecticides kill the common enemies of harmful insects and do not induce resistance in harmful insects. Some are not antagonists of chemical pesticides, but are adjuncts and often used in combination. However, these pesticides also have some disadvantages. Due to excessive specificity, their production and marketing are limited. They may prove ineffective in terms of temperature and light intensity. Such pathogens are usually effective at high host population densities.
Beuveria bassiana and Metarhizium anisopliae are very effective against cabbage caterpillars and soil-dwelling harmful insects respectively. Several species of viruses, bacteria, protozoa, and roundworms enter the stomachs of their feeding microbes and host insects to cause effective infection. The best spore-forming bacteria in the insect body are Bacillus popilliae and B. thuringiensis. Protozoa, especially Nosema are being used with success against some moths.
The use of microbial insecticides for controlling harmful pests in Bangladesh is very limited. It has been proved to be effective against rice borer weevil. Scientists of Bangladesh Agricultural Research Institute have identified many such microbes and prepared a list of them. Not all pesticides are equally effective for all insects. Specialized drugs are often useful for controlling specific types of pests. The success of insecticides in controlling pests depends on several conditions. If the right medicine is administered in the right dose at the right time, then the expected results will be obtained. But all pesticides are deadly poisons. So, they should be used with caution and wisdom.
Nowadays, emphasis is placed on managing harmful pests by combining various control methods rather than relying solely on drugs. Such a method is called integrated pest management method. Since the use of pesticides definitely pollutes the environment and disrupts the balance of the environment, the sooner alternatives are adopted to reduce their use, the better for humans.
The writer, an official at Southeast University in Dhaka, is a columnist
Messenger/Disha
