By Dr. Mohd. Isa Abdul Majid
FROM A HISTORICAL PERSPECTIVE, different natural substances have been used as pesticides. Initially, salts of metal, sulphur, natural oils and tobacco products were utilised.
During the last 50 years or so, chemical synthesis of pesticides has increased considerably. Now, there are more than 55 classes and 1,500 individual substances produced in more than 100,00 formulations of pesticides.
Basically, pesticides are created to control or destroy pests. Insecticides control insects, herbicides control weeds, fungicides control fungi such as mould and mildew and rodenticides control rodents. In additions, pesticides are also defined by their method of dispersal (such as fumigant) or mode of action (such as ovicide which kills eggs of pests). Some of these chemicals are applied to control pests that reduce crop yields or to protect the nutritional value of our food. Others are used for cosmetic purposes to enhance the appearance of fresh food.
Pesticide use in agriculture, forestry, industry, public health and households make them one of the most common type of chemicals coming into contact with all groups of a population. They are widely used in all countries due to their proven effect in vector control and their effectiveness in agriculture.
However, pesticides represent a very serious health and environmental problem. Preventing their eventual adverse effects is much more difficult than is the case with other substances used in industries.
The contribution of pesticides to health and the economy is closely interrelated. They contribute directly to our health through control of certain vector-borne diseases; they contribute directly to the economy through increased production of food and the production of many materials during storage. In some countries, greater and more dependable production of food has eliminated famine and thus pesticides contribute as much to health as to the economy.
One of the main contributions of pesticides to the control of human diseases is in the control of diseases spread by arthropods and the other vectors. Outbreaks of malaria, louse-borne typhus, plague and urban yellow fever - four of the most important epidemic diseases in history are controlled by an organochlorine, DDT.
Other diseases that have been controlled to some degree by this compound include filariasis, dengue, various virus encephalitides, louse-borne relapsing fever, trench fever, murine typhus, shigellosis amoebiasis, leishmanuasis, bartonellosis, onchocerciasis, sandfly fever, trypanosomiasis, yaws, infectious conjunctivitis, cholera. Chaga's disease, scrub thyphus, scabies, rickettsial pox and tick-borne relapsing fever. Other pesticides have made some contribution, but the chlorinated hydrocarbon insecticides, especially DDT, are the most important in the control of insect-borne disease in humans.
As the resistance of vectors and other factors began to interfere with the use of chlorinated hydrocarbon insecticides, it became necessary to use several other classes of compounds. A variety of compounds ranging from DDT to pyrethrum have contributed greatly to human comfort through control of household insects which, although not known to carry disease, are an annoyance.
Systemic use of rodenticides helps to prevent the transmission of diseases acquired by humans from rats and other rodents.
However, any effort of an epidemic of plague may lead to increased spread of the disease. This is because infected fleas that have left dead rats may attack humans instead. During epidemics, DDT has proven valuable by killing fleas before they can leave rats or have a chance to contact humans.
Pesticide use is only one of several factors that have permitted maintenance of our food supply despite a continued increase of the population and decrease of area suitable for agriculture.
The maintenance of our food and fibre supply is largely due to a reduction in the losses caused by pests. food and fibre are subject to damage by pests in every stage of production, transportation and storage. It has been reported that weeds cause average losses of 22% to 44% in the production of soybean, corn, small grain, flax and forage compared to weed-free or weed-controlled fields of the same crops.
In developed countries in the temperate zone, an estimated 10% to 15% of the total value of agricultural and forest products is affected by weeds. The expense of chemicals and the measures to control weeds and actual losses in yield and quality are about equal. Losses from weeds are greatest in tropical countries. In some instances, weed control may tripple the yield of rice.
Some examples to indicated the extent of damage if control is not exercised include the damage inflicted by various grain insects to food, damage by moths to wool and damage by termites to wood. If we take the last example, houses will have to be rebuilt every few years because the damage caused by termites have made them unsafe.
Through the use of preservatives, wood may be protected even if it is left in the open. This in turn results in lesser demand for sawn timber from our tropical forests.
When new pesticides as well as increased usage of them were introduced in the late 1970s, a fundamental shift in agricultural practice was also introduced. As an example, the practice of traditional crop rotation, used to maintain soil nutrients, was abandoned. A single crop rotation was practised instead.
The abandonment of rotations has many ramifications, including increased insecticide and chemical fertiliser dependence.
Insecticide applications are often needed to kill the insect pests that are formerly easily controlled through rotations. In addition, the farmers must now depend on synthetic fertilisers rather than rotations to maintain soil nutrients.
Herbicides also play a significant role in shifting tillage practices. The conventional practice of ploughing was changed due to the concern of soil erosion and new systems were developed based on herbicides. The use of alternative tillage systems such as "No-till" resulted in large amounts of herbicides used to replace mechanical cultivation.
This is turn led to increasing reports of grounds water contamination by herbicides and health problems associated with the use of excessive herbicides. This is now a real problem in a number of countries.
Other similar problem resulting from pesticide use is the phenomenon of pest resistance. In 1938, scientist knew mites that were resistant to pesticides. But by 1984, this number escalated to a a shocking 447 species of insects and mites.
Similarly herbicide resistance in weeds was virtually unknown before 1970 by the 1984, at least 48 species of weeds had developed resistance against some herbicides. Resistance to pesticides has also been encountered in over 100 species of plant pathogens, two species of nematode worms and three species of rodents. This has led to the need for stronger pesticides applied in more frequent doses - a phenomenon known as the pesticide treadmill. Farmers find themselves forced to buy larger quantities of pesticides to ward off pests.
As problems with broad scale use of pesticides as the sole method of control increased, a new ecologically oriented concept was introduced to lessen these problems. This concept, known as integrated pest management (IPM), relies on natural mortality factors such as utilising the natural enemies of pests to eliminate pest problems.
This concept also allows the use of pesticides as a last resort when systematic monitoring of pest populations and natural control factors indicate a need. It is hoped that IPM, which has been implemented in certain agricultural sectors in Malaysia, will lead to a reduction in pesticide usage.
In the next issue, a full description of the hazardous health effects of pesticides, especially occupation-related health problems, will be highlighted.
The writer is a pharmacist and head of the Toxicology Laboratory at the Officer at the National Poison Centre, Universiti Sains Malaysia, Penang.