Insecticide

Insecticide (kitnashak) material used to kill insect pests by disruption of their vital processes through chemical action. Insecticides may be inorganic or organic molecules, and are grouped into three general classes- stomach poisons, contact poisons, and fumigants. Fumigants or poison gases are generally the most effective insecticides to use when the insects and the commodities they are damaging are in a tight enclosure, such as a house, storeroom, or greenhouse. A few other groups of chemicals known as repellents, attractants, chemosterilants, insect growth regulators, and pheromones are also used in insect control programmes, but their mode of action is different. As a rule these chemicals are non-toxic to the target organisms.

In general, inorganic insecticides are effective only as stomach poisons, and they are used now mostly in baits. Most organic insecticides, either synthetic or of plant origin, act as contact as well as stomach poisons, and in certain cases as fumigants.

The history of the development of insecticides is not a very long one. In the 1860s, the invasion of the potato fields of the Mississippi, USA by the Colorado Potato Beetle gave rise to the first general use of an insecticide. An arsenical known as Paris Green was used so successfully as a plant protectant that orchardists next adopted it in their fight against the codling moth. Paris Green became so popular that by 1900 its use became widespread in the USA and Europe. In addition to Paris Green, several other inorganic salts, including lead arsenate, cryolite, mercurous chloride, sodium fluoride, and sulphur were probably the earliest used compounds for pest control. Some of them are still being used at the present time. Natives of tropical countries knew about the toxic nature of the roots and different parts of certain plants such as Tephrosia, Derris, Lonchocarpus and others, which they used for centuries as fish poison to kill and collect fish. This practice subsequently led to the development of the organic insecticides of plant origins, often known as botanicals or alkaloids, such as rotenone, nicotine sulphate, pyrethroids, etc. The earliest recorded insecticidal use of rotenone was against leaf-cutting caterpillars in 1848. However, it was not until 1902 that the active principle of rotenone was isolated.

Recent trends in the development of new insecticides have been almost entirely towards synthetic organic chemicals. Perhaps the first use of such a chemical was in 1892, when the potassium salt of 4,6-diritro-o-cresol was marketed in Germany as an insecticide. The first large-volume use of a synthetic organic insecticide began in 1932 with the advent of -butoxy-'thiocyanodiethyl ether. However, it was the discovery of DDT in 1940, during World War II, which conclusively proved that a synthetic compound could be superior to inorganic and natural products for many insecticidal uses.

The middle decades of the twentieth century have been the years of revolution of synthetic organic insecticides in the field of pest control. The development of DDT and its successful applications against a number of pests, including the house fly and mosquitoes, inspired chemists and chemical industries to bring into use hundreds of new insecticides of various types.

On the basis of their chemical composition, synthetic organic insecticides are classified in various ways, namely chlorinated hydrocarbons, cyclodien compounds, carbamates, organophosphates, etc. Of the chlorinated hydrocarbons, DDT, Methoxychlor and Lindane were widely used for many years. Because of long residual effects, use of DDT has been restricted in many countries including Bangladesh.

The cyclodiene compounds are highly chlorinated cyclic hydrocarbons which include Chlordane, Heptachlor, Aldrin, Dieldrin, Endrin, etc. Most of these insecticides are very effective against soil insects. However, Endrin is highly toxic to fish and therefore its use in Bangladesh has been prohibited since 1962. The carbamates represent a unique class of insecticidal compounds of considerable diversity. Like the organophosphates they are irreversible inhibitors of the cholinesterase enzymes of the neuromuscular system. The commonly used carbamates are Sevin and Baygon. Sevidol, a common granular insecticide used in Bangladesh is a mixture of Sevin and Lindane (gamma BHC).

From the standpoint of pest control, the organophosphorus insecticides are extensively used for the control of almost all types of insect pests. Research in this field has resulted in the discovery of thousands of compounds with insecticidal properties of every description. Malathion, Diazinon (Basudin), Bidrin, Dimecron, Azodrin, Nogos, Nexion etc are few of the best known organophosphates. Most of these insecticides have more than one trade name.

Many of the organophosphorous insecticides are systemic in action, that is to say, they have the unique property of being absorbed and translocated to various plant parts in amounts lethal to insects feeding thereon. This property makes them useful in controlling stem borers and similar pests feeding on internal tissues.

Although Bangladesh is a pest prone area, insecticides were not used till 1956, when the Government imported 3 m tons of pesticides to control pests. Up to 1974, the government promoted the use of pesticides by supplying them free of cost to farmers (providing 100% subsidy). The subsidy was reduced to 50% in 1974. The government withdrew the subsidy completely in 1979 and the insecticide business was handed over to the private sector. Then to cope with emergency situations, the government, however, maintains a buffer stock of 15-20 m tons of pesticides.

After the withdrawal of the subsidy, the use of insecticides declined during the early years of 1980s, but their use has now been on the increases reaching to over 13,000 m tons in 1999. Some of the reasons for the increase in the consumption of insecticides over the years are: (a) an increase in the area of rice cultivation from about 7.9 million ha in 1956 to about 10.3 million ha in 1998; (b) increase in the intensity of rice cultivation from below 60% in 1956, 68% in 1974, to 184% in 1999; (c) an increase in the area under high yielding varieties, in which farmers use high inputs; (d) sales promotion activities of insecticide dealers to motivate a large number of farmers to use insecticides for controlling their crop pests; and (e) excessive use of pesticides by farmers out of ignorance. Some farmers apply insecticides when they are not required at all.

At present, about 4 dozens of insecticides with over 150 trade names have been registered in Bangladesh. Many insecticides are being marketed under different formulations such as granular (Gr), liquid (EC), powder (WP, dust, SP), and aerosol.

Commonly used insecticides under different trade names include Carbofuran, Diazinon, Fipronil, Carbosulfan, and Chloropyrifos among the granular form; Cypermethrin, DDVP, Diazinon, Dimethoate, Monocrotophos, Malathion, Phosphamidon, Phenthoate, Fenitrothion, and Bidrin among the liquid form; Carbaryl, Cartap, and MIPC among the powder form; and Bromadiolone, Brodiofacum, and Zinc phosphide among the rodenticides.

Table Consumption of insecticides during 1998 and 1999 (in m tons).

Insecticide formulation	1998		1999
	Formulated products	Active ingredient	Formulated products	Active ingredient
Granular	9,139.10	534.63	11,192.61	684.35
Liquid	1,298.85	766.14	1,523.97	842.89
Powder	75.73	50.70	97.65	66.76
Miticide	31.38	16.52	25.67	12.54

Source Pesticide Association of Bangladesh.

Although the insecticide trade now looked after by the private sector under the supervision of the Pesticide Association of Bangladesh, a Technical Advisory Committee advises the government on technical matters pertaining to the import, formulation, repacking, sale, and distribution of insecticides under the provision of the Pesticide Ordinance, 1971. Nearly 30 companies/ organisations market insecticides throughout Bangladesh; among these are ACI Ltd., Alpha Agro Ltd., Jamuna Oil Company Ltd., BASF Bangladesh Ltd., Data Enterprises, McDonald Bangladesh (Pvt.) Ltd., Padma Oil Company Ltd., Novartis Bangladesh Ltd., Rhone-Poulenc Agrovet Bangladesh Ltd., Shetu Corporation Ltd., and the Agroproducts Ltd. [SM Humayun Kabir]

Microbial insecticide A pathogenic microorganism and its by-products used by man to suppress insect pest populations by causing a disease. Like chemical insecticides it can be stored for a period, diluted with water, and passed through a spraying machine.

Microbial insecticides have some striking advantages in contrast to chemical insecticides. They tend to be host specific, safe, and have no toxic residues. They also ensure the survival of natural enemies and unlikely stimulate resistance in target pests. Some microbial insecticides are compatible with chemical insecticides and can often be used in combination with them. However, microbial insecticides have some disadvantages too. Their high specificity restricts their production and marketing. Certain upper or lower limits of pH, temperature and light intensity might lead to their failure.

Fungi include some microbials which invade insects by spores, landing on the cuticle the germ tube penetrates the cuticle. Beuveria bassiana and Metarhizium anisopliae used against cabbage caterpillar, and soil pests, respectively, are most important. Several viruses, bacteria, protozoa and nematodes are ingested microbials and rely on ingestion by the host to initiate infection. Nuclear polyhedrosis and granulosis (used against lepidopteran and hymenopteran pests) and cytoplasmic viruses (against lepidopteran and dipteran pests) are very effective. The most important spore forming bacterium of insects is Bacillus popilliae (infects scarab dung beetles) and B. thuringiensis (attack lepidoptera, mosquitos and blackfly larvae). Protozoa, especially Nosema, have been used against moths.

In Bangladesh, use of microbial insecticides to suppress insect pests is very limited. Tests with B. thuringiensis for control of rice ear-cutting and swarming caterpillar have been found to be effective. A list of microbials has been prepared and identified by scientists of BARI. [Md Zinnatul Alam]

See also integrated pest management; insect.