Future status of pesticides development

By Dr Sohail Ahmed

Maximum productivity and efficiency in agriculture will depend on the development and the use of improved practices, such as growing varieties with disease-and-insect resistance , application of proper kinds and amounts of fertilizers, efficient use of water, and labour-saving machinery and equipment. But these advantages will be lost without the use of crop protecting chemicals.

There are pest problems today for which no satisfactory control methods exist to replace the use of chemicals, and pesticides will be the most dependable weapons of the applied biologist unless and until more acceptable techniques can be developed.

The chronological evidence, from the discovery of the DDT to the present day pyrethroid insecticides and similar development in many other potent, selective and safe pesticides, has shown not only increased production and consumption but awareness and sense of judgment about hazards and risks as well.

At present, the developing countries are using relatively little in the way of chemicals to control pests in the majority of crops. In developed countries the use of crop protecting chemicals have played a major role in the increased and more efficient production of food because pesticides have provided immediate effective control at practical and affordable costs.

Many people, when considering chemicals, including pesticides, fail to distinguish between the toxicity and hazard. Toxicity, on the one hand, refers to the ability of a chemical to cause poisoning when administered in adequate quantity through specified routes.

Hazard on the other hand means the probability that a substance will cause harm in the circumstances of usage. Two distinct types of toxic effects (acute and chronic) can be observed from many poisons.

Since, for any one poison, there may be little or no relation between either the mechanism of these two kinds of toxic action or the magnitude of the concentrations which evoke them, the two effects must be carefully distinguished.

An acute toxic response is one, which occurs shortly after application of a single dose of the poison. It is determined by the intrinsic toxicity of the substance to the organism and can often be traced to some specific disruptive effect at the biochemical level.

A chronic effect, on the other hand, is one, which sometimes occurs when an organism is exposed to repeated small and non-lethal doses of poison over a considerable period of time.

Most pesticides are not highly dangerous materials. Occupational pesticide poisoning is uncommon, most cases being due to accidental or intentional ingestion of large amounts.

Safety problems are mainly confined to a few very highly poisonous substances that are so effective or so economical that at present they are unlikely to be supplanted. The hazard of a pesticide varies with its mode of formulation (e.g., solution, emulsion, powder, gas, granule, pellet, etc.), and with its mode of application, which may be by dispersal as a spray, dust, smoke, gas, incorporation in soil, or by release from aircraft.

Many people may confuse accidents caused by direct exposure to chemicals with the possible hazards presented by pesticide residues in air, water and food. There is no evidence that our drinking water supplies and food are contaminated to a level that would be injurious to masses.

Pesticides have made possible a greater abundance and variety of agricultural products at far less cost to the consumer than would otherwise be the case. They have been a major contributor to the upsurge in agricultural productivity over the past three decades.

But from this productivity came the great surpluses, and the farmers’ temporary gains from increased efficiency have often been erased by lower prices. In another vein, the use of pesticides to eradicate disease-carrying insects throughout the world has sharply reduced the death rate and thus has been a substantial factor in the population explosion.

The concept of the benefit-risk equation has a compelling logic which all accept in principle. But going from principle to practice always is attended with disagreement and conflict.

This occurs because a diverse society generates a diverse range of material and aesthetic interests and values. Conflict occurs even though all concur with the ultimate goal of the promotion of the public good.

Although, all of us are consumers and all have a stake in an expanding and prosperous economy, our attitudes on pesticides and their regulation differ; the wildlife conservationist and the chemical manufacturer approach the subject from different perspectives, the housewife buying vegetables and fruits at the market has different concerns than the farmer who grows the food.

The public debate over pesticides is but one facet of a wider debate that reflects a greater sensitivity to the fundamental questions raised by the continuing and accelerating pace of man’s modification of his total environment.

Pesticides are but one factor and we are increasingly aware that our environment is being altered, perhaps even more dramatically, by air and water pollution from sources other than pesticides, atomic fallout, and the population explosion.

It is not generally known that without the aid of pesticides, without the technology of the agricultural chemical industry, we would not have a large exportable surplus. We would in fact, be barely able to feed our present population – and certainly not in the manner to which it has become accustomed.

Pollution is assuming alarming proportions all over the world, and looks like becoming one of the major problems of our time. Pesticides are only a small part of alarming proportions of pollution all over the world but an important one.

All reasonable efforts must be made to reduce these dangers as far as possible. It is fair to add that investigations to date have not shown any significant damage to any form of life on planet for which pesticides can clearly be held responsible.

Critics of persistent chemicals as influenced by Rachael Carlson mythical approach have not fully addressed alternates originated as a result of public outcry. One such example is remarkable advancement to research on biopesticides.

Biopesticides are certain types of pesticides derived from such natural materials as animals, plants, bacteria, and certain minerals. For example, canola oil and baking soda have pesticidal applications and are considered biopesticides. At the end of 2001, there were approximately 195 registered biopesticide active ingredients and 780 products. Biopesticides fall into three major classes:

(1) Microbial pesticides consist of a microorganism (e.g., a bacterium, fungus, virus or protozoan) as the active ingredient. Microbial pesticides can control many different kinds of pests, although each separate active ingredient is relatively specific for its target pest[s].

For example, there are fungi that control certain weeds, and other fungi that kill specific insects. The most widely used microbial pesticides are subspecies and strains of Bacillus thuringiensis, or Bt. Each strain of this bacterium produces a different mix of proteins, and specifically kills one or a few related species of insect larvae.

While some Bt’s control moth larvae found on plants, other Bt’s are specific for larvae of flies and mosquitoes. The target insect species are determined by whether the particular Bt produces a protein that can bind to a larval gut receptor, thereby causing the insect larvae to starve

(2) Plant-Incorporated-Protectants (PIPs) are pesticidal substances that plants produce from genetic material that has been added to the plant. For example, scientists can take the gene for the Bt pesticidal protein, and introduce the gene into the plant’s own genetic material.

Then the plant, instead of the Bt bacterium, manufactures the substance that destroys the pest. The protein and its genetic material, but not the plant itself, are regulated by the EPA.

(3) Biochemical pesticides are naturally occurring substances that control pests by non-toxic mechanisms. Conventional pesticides, by contrast, are generally synthetic materials that directly kill or inactivate the pest.

Biochemical pesticides include substances, such as insect sex pheromones, that interfere with mating, as well as various scented plant extracts that attract insect pests to traps.

Because it is sometimes difficult to determine whether a substance meets the criteria for classification as a biochemical pesticide, the EPA has established a special committee to make such decisions.