which made their use on many crops uneconomic. The farmers therefore have welcomed the synthetic insecticides which can be produced in unlimited quantities and which, at first at any rate, seemed the perfect answer. The dangers from their use are described in later chapters.
Insect pests of crops were soon recognised as such, even if little was done about the problem until very recently. The importance of insects as vectors of disease has only been understood for about seventy years, though man and his habitations have provided niches for troublesome parasitic species in the same way that wild animals have supported their own parasites. Man has sometimes controlled insects of medical importance effectively without understanding the problem. Malaria was formerly widespread in Britain, but it was almost eliminated long before man knew the parasitic organism concerned or that it was carried only by the Anopheles mosquito. This was because man avoided marshy areas, thinking that malaria was caught from the “bad air,” and so he kept away from the breeding places of the mosquitoes. He also drained the swamps, usually to produce better agricultural land, but in so doing he got rid of the insects which carried the disease. Incidentally naturalists are now very concerned at the continued draining of marshes and swamps, which are now the last refuges of many species of wild life. This is just one of the ways in which non-chemical pest control can have effects which have end results which may be as disastrous to wild life as the most indiscriminate use of chemicals.
In the Middle Ages most people of all ranks of life harboured body lice on their persons, and as recently as 1940 the majority of the girls in our industrial cities had lousy heads. Personal cleanliness could eliminate these pests, except where conditions were grossly overcrowded, but in war and after any disaster infestation, and the risk from louse-borne typhus fever, grew. Persistent insecticides now control these insects, and properly applied to man and his clothing they present little danger to any other organisms.
Pest insects which attack man, but depend on his having a permanent home, include fleas and bedbugs. Fleas are not only a nuisance, but also carry plague, a disease which died out in Britain many years before effective insecticides were discovered to control the vectors. Improved hygienic conditions rather than chemicals have made fleas uncommon insects.
Human bedbugs are very similar to the species which attack bats and swallows, and primitive man may have become infested first when he also lived in caves. Bugs were common from the earliest times in the warmer parts of the world. However, Britain had no bedbugs before the sixteenth century, possibly because the houses were too cold. Bugs certainly existed in Italy in classical times. When bugs arrived in Britain, they soon spread through overcrowded slums, and before the 1939 war most houses in our cities, except detached surburban villas, harboured at least a few. However, they were only common in unhygienic and overcrowded dwellings, and improved conditions soon reduced their numbers. Modern insecticides, particularly those which put a persistent film in the cracks which the bugs haunt, control the insects effectively without seriously contaminating the environment.
When his numbers were few, pollution was not a serious problem to man. Many pests, both plant and animal, have become common only because man has produced suitable conditions. In some cases pests have been controlled with little harm to the environment, in others pest control has become a new and potent form of pollution. The great difficulty is to assess accurately just how much pollution affects the environment and the plants and animals it contains. We are seldom able to give simple answers. Sometimes an animal has obviously been killed, perhaps from the effluent from a factory, perhaps by accidental contamination with a pesticide. Generally, however, we have to depend on circumstantial evidence of damage, and this is the reason for the controversy which so often surrounds our subject.
Toxicology is difficult and complicated. The results of analyses of animals’ bodies, where traces of poisonous substances are found, are not easily interpreted. In the case of well-known poisons like arsenic, strychnine or cyanide the situation may be less mysterious if large amounts are found. We know, for instance, that if a man eats five grams of lead arsenate he is likely to be fatally poisoned. If the pathologist finds ten grams of lead arsenate in the stomach of a corpse, he will be almost certain that this poison caused death. If he finds only a few milligrams, he will be almost certain that death was due to some other cause. The finding of intermediate amounts makes diagnosis difficult. Consideration must be given to the site where the poison is found in the body, and to losses due to vomiting or excretion. The situation is even more complicated where poisons are broken down in the body, either as part of the process of damaging the victim, or due to post mortem changes. If we do not know accurately how toxic a chemical is to a particular animal, and if we are not fully familiar with these chemical changes, we cannot usually say for certain whether a small residue of poison in a live or dead specimen has any significance.
It is generally fairly easy to establish the acute toxicity of a substance, that is the amount which, in a single dose, is lethal. Experiments with rats, chicks or fish are commonly made. Groups of animals are given different doses, and the least amount of poison which kills is found. Usually different individuals of a species show a somewhat varied susceptibility, and instead of determining the amount which kills them all, the so-called LD50, that is the amount which kills half of a batch, is determined. In most instances few animals die from a single dose of half the LD50, and twice the LD50 is likely to kill almost every individual. However, this is not always the case. Sometimes a population contains a few individuals which can survive relatively large doses of certain poisons; under certain circumstances these may be selected out and may breed a strain which is more resistant than the normal to a toxic substance. Resistance or susceptibility to poisons is not necessarily correlated with unusual or subnormal “vigour,” and this type of chemical selection may leave a species less well adapted to normal environmental conditions.
Although acute toxicity is not difficult to determine in the laboratory, it can only be done with a limited number of species, and values for others (including man) can usually only be inferred. Also the effects of a specific poison may differ even with the same batch of the same species depending on how it is administered, e.g. neat, in suspension, in oily solution, on an empty stomach, through the skin, by inhalation and so forth. These difficulties have usually meant that, at least where man is exposed, a fairly large “safety factor” has been applied. Thus if work with rats suggests that the LD50 for substance “X” is 50 milligrams per kilogram, it could be assumed that half of a group of 50 kilogram men would probably die if they ate one gram each of “X.” It would generally be found that a single dose of one hundredth of this amount, i.e. of 10 milligrams, would be unlikely to be harmful. In many cases this assumption is quite justified but contamination of food to this extent would not normally be tolerated.
While there are sometimes difficulties in establishing the effects of single, large doses of poisonous substances, the study of the effects of repeated small doses, each of which would probably be harmless, spread over long periods, presents even more serious problems. Poisons which are unstable are unlikely to be very dangerous under these circumstances. Those which are stable, particularly if they are stored in the body, may present great risks even if they are not acutely poisonous in single doses. All these factors are borne in mind when, for instance, new insecticides are tested. Their action on a number of insects, particularly pests, is determined. Then long-term experiments, lasting over severa, years and a number of generations, are then made with rats, chickens and other animals. It is obviously impossible to include more than a few species in such trials, so it is not surprising that sometimes a desirable species of bird, or mammal, is found (too late) to be unexpectedly susceptible. The effects of chronic exposure to low-level industrial and urban pollution is even harder to study. Some, impressed by the complexity of the situation, fear that the ecological effects of pesticides may bear little relation to their gross toxicity.
Everyone wishes to abolish the damage which may be caused to man and to wild life by pollution from every source. As, however, we are not always agreed as to when damage is being caused, or how exactly some obvious damage arose, an easy solution will not be found. Man has always polluted his environment; he has always suffered from pests, but because
