Transactions of RHASS Volume 1940 - Page 025
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| Year | 1940 |
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38 THE CONTROL 014‘ I’ESTS OF FARM AND GARDEN CROPS. (1) Plant-breeding.——This method has been known since 1815, when Thomas Andrew Knight advocated the planting of wheat which was resistant to rust. More recently potatoes immune to scab and turnips resistant to finger-and-toe have been introduced. This method of controlling disease is becoming more and more important, and is perhaps the most economic of all methods of pest control. Up to the present it has been applied chiefly to the control 0f fungoid diseases. (2) Proper Nutrition.—It has been known from the time of Liebig (circa 1840) that badly nourished plants are more susceptible to attack than healthy robust plants. Where certain forms of disease are prevalent it cannot be too strongly emphasised that greater attention should be paid to properly balanced manuring. Many failures of field crops can be traced to malnutrition. A heavy dressing of a nitrogenous manure, for example, makes potatoes more susceptible to blight, and wheat to mildew and yellow rust, eSpecially when the nitrogen is not used in proper balance with phosphates and potash. Similar results have been obtained with a great variety of plants, and the generally accepted view is that heavy nitrogenous manuring leads to an increased liability to disease, especially when unbalanced by other manurial substances. The lack of available phosphate and potash also induces a decline in the plant’s resistance to disease. The plant’s vitality is also lowered by unfavourable soil conditions —for instance, too sour or too alkaline conditions, bad drainage and bad aeration of the soil, and it is rendered an easier prey to pests. These conditions may be detected by an examination of the soil conditions, and, if necessary, an analysis. The cure consists of the application of a properly balanced manure, with drainage and liming where necessary, and efficient cultiva— tion. In recent times we have learned that certain plant diseases may be promoted by lack of certain of the minor elements of the soil, such as Boron or Manganese, and can be prevented or cured by small applications of Suitable com- pounds of these elements. (3) Application of Fungicides and Insccticidcs.——The applica- tion of a noxious substance to the plant surface to kill or repel insects has been tried since quite early in the history of agriculture and horticulture. The first insecticides used are said to have been decoctions of bitter or strong-smelling herbs, which were applied to the plants in the hope that the offending insect would be either deterred or driven away by the objection- able taste or offensive odour. It was noted by early explorers that the natives of the Amazon basin and the Malay Peninsula made use of certain plant juices against insects, and some of these juices have been shown by modern science to be most effective means of dealing with certain insect pests. THE CONTROL OF PESTS OF FARM AND GARDEN CROPS. 39 One of the first substances used in modern times in Europe was a mixture of sulphur with soap-Suds, which was probably introduced about 1820. Little progress was made in the chemical control of pests until about 1870, when Paris Green, an arsenic compound, was used in America against the Colorado Potato Beetle. Since 1880 great advances have been made, and to-day we have at our disposal a large number of substances for combating insect and fungoid pests. It is not possible to deal in this paper with all the substances which have been tried as inSecticides and fungicides; only the more important ones are described, and Special attention is paid to those which have been introduced during the last twenty years, which has been a very active period in investiga- tions upon this subject. The pest control industry is now a very important one in this and other countries. The amount of damage done by the various plant pests is estimated to be at least £20,000,000 a year in this country alone. In the United States, where there is a much greater area and a more favourable climate for pests, the value of the insecticide trade alone was estimated to amount to over £30,000,000 in 1936, and we are indebted to that great country for much modern investigation into the use and value of various chemical substances for controlling crop pests. CROP PESTS. The successful treatment for all parasites must be based upon the life-history of the pest, and the attack concentrated upon the weakest part of its defences. Where the pest is open to attack by the direct application of a chemical prepara‘ tron, then it may be treated by spraying or dusting. Sometimes the most easily attacked stage is the egg, and for this purpose a special group of insecticides is used, called ovicides. For the destruction of insect pests in other stages there are three important. groups of insecticides—namely, contact insecticides, stomach insecticides, and vapours or gases. The contact insecticides are generally used for insects which damage the plant by piercing the cell walls and sucking the cell sap. They either paralyse the insect by action on its nervous system or close its spiracles and suffocate it. The stomach insecticides are used for insects which eat parts of the plant to which they can be applied. This type of insecticide consists of pomonous chemicals, which poison the insect when eaten by it. ' The third class consists of poisonous vapours and gases which are breathed in by the pest and thus poison it. For treating diseases caused by either fungi or bacteria ’we use a group of chemical preparations known as fungicides. l‘hese are generally applied by spraying or dusting. |
| Title | Transactions of RHASS Volume 1940 - Page 025 |
