Parasites and Pests

By H. D. WALSTON

GARDENERS and fruit-growers have for long used chemical sprays as a means of combating insect pests, while on the Continent arsenical compounds have been used for -the past thirty years to protect potatoes from the Colorado beetle ; and for much longer than that it has been common practice to check mildew on potatoes and vines with copper sulphate. In recent years enormous strides have been made in this country both in the development of sprays and the machinery with which to apply them, so that now there are few farmers, fruit-growers or market gardeners who would not benefit from their use.

Sprays in common use today can be divided into two categories --those which are used against insects, viruses and mildews, and those which are used against weeds. For the general farmer it is the selective weed-killers which are likely to give the greatest help, and to cause the greatest change in technique. Ever since the introduction of the four-course rotation in the eighteenth century it has been a basic tenet of agricultural theory that every four years or so there should be a cleaning crop to allow the farmer to destroy the weeds which increase with corn crops. In the old days, when there was plenty of cheap labour, this opportunity for cleaning was not so important as it is today, because it was possible to hoe cereals by hand just as today we hoe roots, or at least to put a horse-hoe through the winter corn every year. More recently this practice has been given up, and because of shortage of labour it has even become difficult for many farmers to keep their roots clean.

One answer has been the bare fallow where the land is left empty for an entire year, being ploughed or cultivated regularly throughout the summer so as to destroy the accumulation of weeds ; but this, though effective from the point of view of weed- control, and not unduly costly when the rent of land is low and the supply of machinery adequate, is in direct opposition to the national policy of making the best possible use of every acre of land. For these reasons farmers have been quicker than they otherwise might have been to adopt new methods of weed- control, and the advent of selective weed-killers has given them opportunity. It is now possible, for an expenditure of £2 an acre, to dust corn 'crops with a powder which, if applied at the right time, will kill 90 per cent. of the commonest types of weeds ; or, if more intensive treatment is required, to use a special spraying apparatus in place of an ordinary manure distributor, and, at a slightly greater

cost, to put on an even more efficient liquid spray. It is claimed that where such weed-control is employed the yield of cereal is significantly above that of untreated crops; but there is not yet enough evidence to determine if this is due to some growth- promoting factor in the spray or merely to the destruction of com- peting plants which allows the corn crop freer access to soil nutrients, air and moisture.

One of the most dramatic effects of these selective weed-killers can be seen in the early summer as one drives through the spring

corn areas of East Anglia. A few years ago the eye of the non-

farmer was delighted and the heart of the farmer saddened by the sight of brilliant yellow fields contrasting vividly with the fresh green of winter wheat and oats and the pink of sanfoin just coming

into flower. These yellow fields were not, as, many casual passers-by thought, crops of mustard, but spring-sown barley infested with

charlock. Now less and less charlock is seen because it is highly susceptible to the new weed-killers, and, unless a new and resistant variant of the weed arises, it may well disappear entirely within a few decades.

It is not only in cereal crops but in root crops too that these weed-killers have their uses. Carrots, for instance, have during the past years attracted many growers, but the difficulties of keeping them free from weeds, and the high cost (£20 or £30 an acre for hand-hoeing alone) even when labour is available, have often turned what might otherwise have been a most profitable crop into an expensive failure. Now a simple spray has been produced which, while leaving the carrot itself untouched, kills most of the weeds -which grow round it and tend to smother it.

But with root crops it is the second type of spray, the biological rather than the botanical type, which is the more important. In the areas where sugar beet, celery and potatoes are grown as main cash crops, and are not grudgingly put in by cereal farmers in order to introduce an occasional root break, the intensity of production brings its own dangers, and biological pests appear which can utterly destroy an entire crop over many thousands of acres. in this case it is an economic proposition to spend £20 an acre or more on spraying a crop which, if successful, will bring in a gross, return of between £200 and £300 per acre, and it is this typc, of spraying in particular which has made worth while the construction of special machines, including even helicopters, to apply the spray without damaging the growing crop.

The greatest value of such sprays lies in those areas where highly specialised production, if not actual mono-culture, is taking place ; the more mixed the farming the less economic importance will there be in any one pest or disease, and the harder will it be for such pest to find sufficient materials on which to grow ; but in highly specialised areas of which we have only a few in England—and those mainly in fruit- and vegetable-growing districts—the danger is .

considerable. In other countries it is far greater. The disastrous outbreaks of phylloxera which swept through Europe nearly a hundred years ago showed the disaster which could befall whole regions of specialised viticulture as the result of one disease, and had it not been for Pasteur's discovery that European vines grafted 'on to American stock were resistant to the disease the disaster would have been infinitely greater.

Modern knowledge and modern techniques have made it unlikely that a calamity of a similar size will occur again in Europe, though Colorado beetle on potatoes and a whole series of insect pests on

fruit-trees are ever-present menaces. Outside Europe the position is more serious. Swollen shoot disease of cocoa is already causing

grave distress in West Africa, but there are grounds for thinking that suitable sprays may eventually be found which will bring. it under control without, as at present, going to the lengths of cutting down every infected plant ; while jaffid infestation of cotton in the Sudan is proving susceptible to spraying from the air with a conse- quent improvement both in quality and yield of fibre.

But there is still a great deal that we do not know about the control of pests, whether they be weeds or insects. Over thousands of years a balance has been established in plant and insect life, and by destroying by artificial means insects known to be harmful we may at the same time be robbing other insects, which are on balance beneficial, of their food. We already know that, if certain types of spray are used on fruit-trees at a time when the trees are in full flower, the insects which come to fertilise the plants will be killed and the fruit crop will suffer through lack of fertilisation. Some people maintain that, in areas where sprays have been widely used, the yield of honey has markedly diminished, and there can be no doubt that, in theory at least, the almost complete destruction of any one variety of plant or species of insect is bound to have a profound and disturbing effect upon the existing balance of plant and insect life. One cannot assert that this effect is automatically harmful, or indeed that the obvious short-term benefits are likely to be outweighed by long-term difficulties. But it is a factor to which we should not allow ourselves to be blinded by the dramatic success of the modern techniques of weed- and pest-control. On the other hand, the fact that Nature sometimes has a habit of getting her own back when we least expect it must not be used as an argument against new methods.