Transactions of RHASS Volume 1940 - Page 026
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| Year | 1940 |
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| OCR Text |
40 THE CONTROL OF PESTS 0F FARM AND GARDEN CROPS. METHODS or APPLYING INSECTICIDES AND FUNGICIDES. Pest control chemicals may be applied by three methods, as follows :— (1) Spraying, where the material is applied as a solution or as a suspension or an emulsion in water. (2) Dusting, where the material is applied as a finely divided powder. (3) Fumigation, when the chemical is applied as a gas or vapour. Of these three methods the one that finds the widest applica- tion is spraying. In spraying, great care must be taken to select materials of a quality and kind Suitable for the purpose in view. In this country there are not at present any legal standards for poisons to be used as sprays. An arsenical spray of unsuitable quality, for instance, may injure the foliage of the plant as much as it does the pest. Then the application must be made at the proper time. The essential requirements of a spray are (1) the material must not damage the plant, (2) the spray must cover the whole surface upon which it is to act. A contact spray must make contact with the pest to be killed, while a protective spray must cover the whole of the surface it is to protect. Where the poison is to act as a stomach insecticide it must remain on the plant surface sufficiently long to poison the pest. If it is washed off by rain, or if chemical decomposition takes place, then its efficiency is reduced. Care must be exercised, therefore, to see that the materials made are suitable for the purpose in view. There have been many cases where costly spray materials have been wasted and where the protection of the crop has been inefficient through neglect of this precaution. Many water solutions fail to cover the surface of a plant because the solution does not wet it, and when applied merely runs off again more or less completely. Other solutions, although they wet the surface, collect in large droplets upon the leaf surface, and, on drying, we get an uneven deposit of the poison which may harm the leaf at those parts where it is concentrated. Many of the early sprays suffered from these defects, and the discovery of means of improving the wetting power of Sprays has been among the most valuable of recent improvements in the means of making sprays efieetive. To overcome wetting difficulties, substances known as wetters and spreaders are incorporated in the sprays. A wetter makes the Spray wet the foliage, and a spreader makes it spread in a uniform film over the leaf. Many substances act as both wetters and spreaders. Where protection is desired to last for some time substances which make the THE CONTROL or rnsrs OF FARM AND GARDEN CROPS. 41 spray adhere to the leaf are mixed in the spray, and these substances are known as stickers. Wetters.—The first wetter and spreader used was soap, by Robertson in 1821, who used it for a sulphur spray. Soft soap has been widely used, but it has the great disadvantage that it is not suitable for use with hard water. Also it cannot be used in sprays which contain lime. Where lime is present casein or skim milk has been found to act as a good wetter. Recently a Substance named sodium hexa-meta-phosphate has become available in commercial quantities, and this material at the rate of 3} lb. to 100 gallons of water allows ordinary soaps to be used with ordinary hard waters. During the last few years a large number of organic soaps have been found to possess high wetting powers. Most of these sub- stances are proprietary articles sold by the makers under trade names such, for example, as Agral I. and Agral N., and Aresket and Areskap. The use of such substances is well worth while, for their wetting power may be as much as ten times greater than that of ordinary soap. Although the incorporation of a little wetter and spreader may increase the efficiency of a spray, one has to be careful not to add too much. If this is done it may increase the spreading properties so much that it will cause most of the spray to run off. For greatest efficiency a correct balance must be made to allow the surface to be properly wet, and at the same time a sufficient amount of spray to be retained upon the surface of the plant. When the poison to be applied is in the form of a solid which is not soluble in water, then a suspension must be made. Sometimes a substance, such as sulphur, will not mix readily with water. In Such a case mixing may easily be accomplished by including a little of a suitable wetter with the solid. For example, when only 1% ounces of butyl-phenyl-phenol-sodium- sulphonate—a proprietary article known as Aresket—is mixed with 100 lb. of precipitated sulphur, the sulphur mixes with water at once. If the spray contains an oil which does not mix with water, then it must be emulsified. It was difficult at one time to make a good emulsion, but now there are a number of sub- stances upon the market which when dissolved in the oil emulsify it immediately on mixing with water. A great deal of attention has been paid to emulsions recently, and some of the most efiicient pest control substances are what are termed quick-breaking emulsions. A quick-breaking emulsion Is one which breaks up on being sprayed upon the plant and deposms a film of oil containing the poison on the surface. This film of oil is not readily washed off by rain, and so pro- longed protection is obtained. Muted Sprays.—Sometimes it is advisable to mix two or |
| Title | Transactions of RHASS Volume 1940 - Page 026 |
