To the general public a pesticide is merely something to be used to rid us of those lower forms of life which cause us inconvenience. The attitude of the average person towards pests is one of impatience rather than of real concern. To those with a wider knowledge of the problem, however, the question of combating the ravages of insect and other pests has a much wider significance.
Although the human race maintains its position at the top of the biological tree, the chief challengers of this position are the lower forms of life. These more primitive biological forms probably inhabited the earth aeons of time before the first human being appeared. They outweigh all other forms of life, and if they were all in direct competition with us for food we should fare very badly. In the event of an atomic disaster they would probably survive us.
Some of the greatest calamities in history have been caused by attacks by the lower forms of life on the health of man, or on his food supplies, for these attacks were of a type against which he could do nothing. Fires could often be extinguished or controlled, enemies could be repulsed, floods could often be stemmed, but the farmer who saw a cloud of insects descend on his crops could merely stand hopelessly by and watch a disaster which he could not control, and await a famine which he knew to be inevitable.
The agriculturalist of earlier days was even more perplexed by the attacks of organisms which he could not see. All he knew was that his crops failed and his stock sickened and died; and his remedies against these disasters were pathetically few.
The first advance in insect control was made about the middle of the last century, and it came not through the invention of new pesticides, but through an improved knowledge of entomology. This new knowledge made possible more effective application of the then known pesticides, limited though they were.
The next major advance in pest control came a little over 30 years ago with the discovery of the insecticidal power of DDT. The chemical itself had been discovered over 60 years earlier but had lain unnoticed, more or less as a laboratory curiosity, until its unique insecticidal power was discovered by Dr. Paul Muller.
The name DDT is, of course, derived from its chemical designation which is Dichloro-Diphenyl-Trichlorethane and this chemical gave results in insect control which had never before been dreamed of. It appeared to be ideal, for it was safe in use, it was not harmful to vegetation and it was effective against most insects.
It immediately found use not only for the control of insect pests on crops but also against insect carriers of disease. So effective was it against malarial mosquitoes that malaria was reduced to negligible proportions in countries like Greece where previously it had been endemic.
One of the most spectacular successes of DDT was achieved in Naples in 1944 when well over two million people were dusted with DDT to rid them of body lice which threatened to spread a typhus outbreak to epidemic proportions. So well was the louse infestation controlled that the epidemic was prevented.
DDT went from success to success and it seemed that most of our troubles from insects, and other of the lower biological organisms, were almost at an end.
Naturally enough efforts were made to improve on DDT and a number of other chemicals appeared on the market. All of them had this in common with DDT, that they belonged to a chemical group known as the chlorinated hydrocarbons. That is to say that they consisted of organic molecules to which carbon atoms had been added. Some of them were even better than DDT for special purposes.
Amongst the chemical pest destroyers which followed DDT were materials now familiar to most people-Aldrin, Dieldrin, Chlordane, Benzene Hexachloride, Lindane and many others. The demand for chlorinated hydrocarbon pesticides became immense and manufacturers were hard pressed to keep up with it. However, the spate of new chemicals brought to light certain dangers which had not been foreseen.
In the first place though DDT was quite safe in use, many of the chlorinated hydrocarbons which followed it were not. Most agriculturists had been brought up in the belief that pesticides were harmless if one did not actually swallow them. After all, they were used to arsenic and strychnine, and similar materials, and when DDT came on the market it too was safe to handle. But nearly all the chlorinated hydrocarbons which followed DDT were capable of being absorbed through the unbroken skin. Users just could not accept this new idea that skin absorption was not only possible but highly dangerous, and many cases of poisoning occurred before users realised that careless handling of the chlorinated hydrocarbons could be dangerous through inhalation or skin absorption.
One of the most important tasks of Public Health authorities in the earlier days of the chlorinated hydrocarbons was to spread the doctrine of the necessity for protective clothing, and of the avoidance of skin contact when handling these materials.
Medically, these chemicals are known as cerebral convulsants and when absorbed into the body, they can cause symptoms which may vary from irritability to severe convulsions.
Certain species appear to be especially susceptible, such as fish which are killed in concentrations of only a few parts per million by certain of the chlorinated hydrocarbons. Calves are also very likely to be killed by small concentrations of benzene hexachloride, and animals such as cats which lick their fur are easily affected by a deposition of the material on their coats.
In spite of these disadvantages however, the chlorinated hydrocarbons are efficient pesticides and they held the field for many years as the most effective weapon we possessed against most pests, but a few years ago they fell into disfavour for two reasons.
The first was that certain insects and other pests had become resistant to them. In certain situations flies, mosquitoes, and cockroaches were no longer eradicated by them and other materials had to be used.
A second, and more potent reason was that the chlorinated hydrocarbons as a class are very stable chemicals and maintain their chemical integrity for years. Sprayed or dusted on a crop, they may be washed off it into the ground and taken up by another crop. If an animal eats a sprayed crop they are absorbed into the animal's tissues and stored in the fat, and if a human eats the fat of such an animal he too, absorbs the chemical and it becomes stored in his fat.
A cow may eat a sprayed crop and the chlorinated hydrocarbon is excreted in her milk. The chances are that when we use a chlorinated hydrocarbon on an edible crop, it will come back and be stored in our own bodies.
Investigations have shown that, because of the widespread use of these chemicals, most people in the more developed parts of the world have some chlorinated hydrocarbon stored in the fat. In spite of this, however, no disease pattern has yet been associated with it.
Public Health authorities are unwilling to take the view that, because nothing has happened so far, nothing ever will, and they feel that the fat storage of a chemical, which should not be normally in the human body, is to be deprecated. This view has been endorsed in the U.S.A. to the point where they are unwilling to accept for consumption meat which contains over a certain specified concentration of these chemicals. Tolerances have also been established in the U.S.A. and in other countries, setting out what quantities of chlorinated hydrocarbon residues are acceptable on vegetable or animal flesh intended for human consumption.
It seemed then, that some cracks had appeared in the shining armour which a few years earlier had seemed to be such a complete protection against insect pests.
About 1940 a group of German chemists was working on the production of an efficient war gas, and by "efficient" they meant a gas which would be absorbed through the skin, and which would incapacitate those contaminated by it. In the course of their work they came up with chemical materials which, though not suitable for warfare, seemed to have the characteristics of efficient insecticides. These chemicals are known generally as organic phosphates. Structurally, they are organic molecules to which atoms of phosphorus have been added and sometimes sulphur as well.
These chemicals vary considerably in potency but they all have a common mode of action in that they inactivate cholinesterase, the enzyme which controls muscular action, and it is because cold blooded animals are less able to survive such a situation than are higher animals, that they are highly successful insecticides.
There are a large number of organic phosphates in use and they vary considerably in toxicity and in their mode of absorption. Some act by surface action, others by being absorbed into the plant so that they are lethal to insects which suck the plant juices. Parathion is one of the most toxic of these organic phosphate pesticides and malathion the least toxic and between these two extremes lie a large number of intermediate toxicity. As already mentioned, the toxicity of these compounds depends on their ability to inactivate the enzyme, cholinesterase, which controls muscular activity, and this results in a condition of spasm of the muscles, commencing with those of the bronchi and intestinal tract, and finally spreading to all the muscles.
In cases of poisoning this muscular spasm is evidenced by nausea, vomiting, tightness in the chest and, later, by general convulsions and finally death.
It seemed almost providential that the organic phosphates came to light just about the time when certain misgivings about the chlorinated hydrocarbons were becoming general.
They resembled the chlorinated hydrocarbons in being good and efficient pesticides but they had the important advantage that they did not persist so long as residues. In a matter of weeks as a rule they lost much of their toxicity and if a reasonable time were left between application and harvest, the danger of absorption was negligible. It is true that they were fraught with the same dangers to the user, in that they would be dangerous through skin absorption as well as by ingestion or inhalation, but even here they had an advantage, for it is possible. by a relatively simple blood test, to tell to what extent the blood cholinesterase has been depleted.
From the point of view of public health these blood tests on users of organic phosphate pesticides are important. If it is found that a user has a blood cholinesterase which is 40 per cent below normal he should be removed from further exposure; if the depletion amounts to 60 per cent he is close to symptom level and further exposure should be prohibited.
From what has been said it will be clear that the organic phosphates have certain advantages as pesticides. They undergo degradation relatively quickly, they are not absorbed and stored in the body as the chlorinated hydrocarbons are, and in the event of contamination of the operator, it is easy by means of a blood test to determine the degree of poisoning which has occurred. This is not possible with the chlorinated hydrocarbons for which there is not any generally applicable test.
Another point in which the organic phosphates score is that, in the event of poisoning, specific and effective treatment is available. The administration of atropine will very often check the symptoms, and injection of P.A.M. (9-pyridine-2-aldoxime methiodide) will often effect a dramatic cure. It is not, however, equally effective against all organic phosphate compounds.
There is another class of pesticides in use at present — the carbamates — which act in a manner similar to the organic phosphates but they are much less toxic to humans. They are mentioned here mainly for the sake of completeness.
The two large classes of pesticides, the chlorinated hydrocarbons and the organic phosphates, have been dealt with at length because they are so extensively used in agriculture at the present time. They are used alone and in combination with other agents and the number of proprietary preparations in which they appear is legion.
There are other materials which also require mention. DNC and DNOC possess all the dangers of those already mentioned, but they have a certain safety factor in their intense yellow colour. People tend to avoid contact with them for this reason, and consequently, poisoning is seen less frequently. They act by accelerating the body chemistry which gives rise to extreme fatigue and debility.
Fluoroacetate or 1080 is used extensively as a rabbit poison and anxiety is often felt about it. Fluoroacetic acid and sodium fluoroacetate are extremely toxic materials and have to be handled with the greatest of care. The question usually asked about them is whether the eating of a rabbit poisoned by them could cause poisoning in humans. Experimental work indicates that human poisoning would not be caused in this way. Nevertheless there are aesthetic objections against eating poisoned rabbits. Dogs, however, can easily be poisoned in this way, not only because they are susceptible to the poison but also because they eat the viscera of the rabbit where the poison may be concentrated.
Arsenic deserves special mention from the public health point of view. It is one of the oldest agricultural poisons and in spite of the toxicity of some of the newer compounds, arsenic ranks high as a cause of accidental death. Such accidents usually occur because the arsenical solution, which sometimes has a sweetish taste, is left within the reach of children, or because it is kept in a soft drink bottle and is taken inadvertently.
The relationship of the Public Health administrator and the user of pesticides is implicit in what has already been said. Those engaged in public health work recognise that protection of the public follows along two lines.
First, it is necessary to educate the user of modern chemicals to know what he is using to inform himself about its potential dangers and to take adequate precautions. Much work in this way has been done by those of us in the Department of Public Health whose special interest lies in this sphere. It is felt too that by educating the user of modern agricultural chemicals in safety measures, that a good deal has been achieved and that, by and large, the average agriculturalist is now using toxic chemicals with more care and intelligence than he did, say, five years ago.
This educational programme has been carried on by lectures, radio talks, television appearances, and by the issue of Departmental literature covering the safety measures to be observed by users. Articles at a more technical level have been written for the information of medical men who may have to deal with problems of poisoning.
It is sometimes suggested that a more positive approach to the safe use of toxic chemicals would be to enact legislation strictly governing their use. Legislation of this kind is notoriously difficult to formulate and still more difficult to enforce. People, moreover, generally react more favourably to moral suasion and education and it is felt that more can be achieved by such means.
After all, no legislation can force users to read labels and to understand the nature of the materials which they are using but much has been achieved in this way by systematically instructing users. The necessity for protective clothing, for avoiding skin contact, for washing the skin should such contamination occur, for the use of respirators when conditions demand it, have gradually become appreciated.
There will always be people who will not heed advice just as there are many who will not stick to safety precautions when driving a car, but those who defy sense in such matters will also ignore legislation.
The second safety measure from the point of view of Public Health is to try to ascertain whether consumers are being subjected to toxic residues. This problem has solved itself to some extent in the case of the chlorinated hydrocarbons as they are not used now on crops for human consumption, nor are they used on animals whose flesh is to be consumed by humans. In some countries such as the U.S.A. legal tolerances are laid down. Here, however, though we accept similar standards, they are not legally defined tolerances. The Department of Agriculture does a considerable amount of work in this field and advises agriculturists as to the time which should elapse between the application of individual pesticides and harvesting. If these instructions are followed, with the materials now in use on food crops, no residue danger should exist.
Doubts and fears, regarding the remote effects on humans as the result of the widespread use of chemical pesticides, exist in the many people, and every now and then there is a stirring of public uneasiness about the worldwide use of chemicals on the food we consume. "Why" it is asked "must our food be contaminated by toxic chemicals?"
The answer is that only by means of chemical pest control can the world's food supply be maintained, and even now it is not adequate. If chemical pest control were abandoned at this stage, starvation would be world wide, and insect-borne diseases now under control would once more become epidemic.
Chemical methods of pest control may one day be superseded by more efficient methods, but at present they are the only methods which are successful and for that reason we must learn to live with them.