Pesticides and Pollution. Kenneth Mellanby

we do not yet understand, they suddenly increased in numbers so that a density of twenty animals to an acre was not uncommon, and as many as 100,000,000 carcasses were sold in a year, without noticeably depleting the numbers still at large. It was not until rabbits were wiped out in many areas by myxomatosis in 1954 that the extent of the damage they had done was recognised. Pre-myxomatosis control, by shooting and trapping, was rarely effective, and merely served to “crop” the population. The importance of myxomatosis is discussed in chapter 10.

      Fig. 1 Changes in the distribution of the buzzard in the British Isles. (from Dr. N. W. Moore with acknowledgement to British Birds).

      KEY: Black: Breeding proved, or good circumstantial evidence of breeding.

      ? on black: Circumstantial evidence suggests that breeding probably took place.

      ? on white: Inadequate evidence of breeding.

      White: No evidence of breeding.

      Fig. 2 a. The breeding population of the buzzard in 1954.

      KEY: Black: 1 or more pairs per 10 square miles.

      Cross-hatch: More than 1 pair per 100 square miles, but less than 1 pair per 10 square miles.

      Diagonal hatch: Less than 1 pair per 100 square miles.

      White: No breeding buzzards.

      + means that breeding density may belong to the category higher than that indicated.

      —means that the breeding density may belong to the category lower than that indicated.

      b. Game preservation in 1955.

      KEY: Black: 3 to 6 gamekeepers per 100 square miles.

      Cross-hatch: 1 to 2 gamekeepers per 100 square miles.

      Diagonal hatch: Less than 1 gamekeeper per 100 square miles but more than 1 per 200 square miles.

      White: Less than 1 gamekeeper per 200 square miles.

      G. Principal grouse-preserving areas. On these, and also on some very large estates, the numbers of keepers may be higher than shown on this map.

      Mice and rats invaded man’s home as soon as there was a home to invade. They also lived in his grain stores and farm buildings. Early man tried to make his granaries rodent-proof, sometimes with remarkable success. Control by trapping and poisoning was generally inefficient, and the rodent population in contact with man was roughly a measure of the amount of food he made available. A mouse-proof larder is more effective than an apparently efficient trap. The domestic cat was probably the most useful method of local control, though cats, like other “pesticides,” have had their side effects. Those that have escaped and become feral have important effects on other wild life. Modern methods of rodent control are much improved, but these animals still do much economic damage in our cities and on our farms to-day. It is perhaps surprising that the black rat (Rattus rattus), which is found mainly in towns, where it is particularly at home in hot-water ducts in tall buildings, was our “original rat,” though some think even it only arrived about A.D. 1200, and the brown rat (Rattus norvegicus), which is the species commonly found in the country, only arrived in Britain in the eighteenth century. The black rat was apparently driven from the rural haunts by the brown invader.

      Pigeons and sparrows are serious agricultural pests, against which no really satisfactory control measures have so far been devised. Pigeons become more numerous each year. This increase is probably due to the increased amount of winter food, particularly clover leys on farms, that is available. The recent fall in the number of hawks may also have had some effect. Organised shoots give some sport to the participants, but have negligible effects on the pigeon population. Work on poisoning or narcotising pigeons is progressing, but the danger to other and more desirable species of birds is difficult to prevent. Sparrows probably increase because suburban householders feed them in winter. This enables them to survive in cold weather, and the increased population does more harm on the nearby farms to which it migrates in summer and autumn. Again shooting and trapping has little effect.

      Farmers and others were then soon aware that wolves and other large mammals and birds might be pests, competing with them in various ways, even if they often overestimated the damage done by the carnivores and sometimes underestimated the amount of food taken by rabbits and other herbivores. They took active, if sometimes misdirected, steps to control these animals. On the other hand, they almost always underestimated the harm done to their crops and to their health by insect pests, and it is only in recent years that serious attempts have been made at control in this direction.

      In Britain we do not have plagues of locusts, which in many countries can consume the whole of a crop, but it is estimated that to-day some £300,000,000’s worth of food is lost each year because of pests of crops and insect damage to farm stock. This sum is almost exactly the same as is spent annually on the support of agricultural prices (“farm subsidies”). Some insects, like the caterpillars of Cabbage White butterflies, eat crop plants, reduce the yield and make many plants unsaleable. Other insects do little damage themselves but carry organisms which cause diseases. Thus aphids carry the virus causing virus yellows in sugar beet; this can seriously reduce the value of the crop. For many years farmers and gardeners accepted insect damage as something they could not prevent. They learned by experience that it could sometimes be avoided or reduced to a minimum by timing their operations carefully. Thus if broad beans in the garden are sown early, seed is set before aphids (“black fly”) are numerous and a good crop is obtained, but in two years out of three a late sown crop will be smothered by insects and prove a failure. Cultural devices such as this are valuable and important, but will seldom allow a late crop of broad beans to be produced. Field beans, which flower and set seed over a long period during the summer, cannot be successfully grown in a way to avoid attack in a bad aphid year. Resistant strains of crop plants have been recognised and used for many years, and in future are likely to prove very important, where resistance is linked with high yield and quality. Farmers generally prefer to be able to grow the most profitable crop at the most convenient time and wish to attack pests by any possible means. Before 1939 most effective insecticides (except general poisons very dangerous to man) could only be produced in relatively small quantities and at a price 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,