Transactions of RHASS Volume 1940 - Page 030
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| Year | 1940 |
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48 THE CONTROL OF PESTS OF FARM AND GARDEN CROPS. mixed with any Spray containing such alkalies. They. are also very easily destroyed by oxidation and bright sunlight. On storage the active principles gradually undergo decompoSi- tion and lose their toxicity. The rate of decompos1tion depends upon the conditions of storage, but under the most favourable conditions the powder will lose about half its pyrethrin content in a year. The total percentage of pyrethrins in a good commercial sample varies from about 075 per cent to 12 per cent, except Kenya pyrethrum, which contains about 1'5 per cent. For horticultural purposes extracts containing the active principles dissolved in mineral 011 are generally used. When dissolved in high-grade mineral 011 and stored in opaque sealed containers the pyrethrins are fairly stable, and an extract made by a reputable firm contains a standard amount of the toxic principles, generally from 5 to 8 per cent. ‘ i _ The value of the pyrethrins lies in their great tox1City to insects and relative non-toxicity to warm-blooded animals. They act chiefly upon soft-bodied insects and have httle effect upon red Spider or mites. To increase the value of a pyrethrum spray the active principles are dissolved in a mineral oil and then emulsified. In this way we get a spray which destroys both soft-bodied inSects and red spider and mites. As the pyrethrins are broken down by sunlight a pyrethrum spray only remains active .upon foliage for a period of about three weeks. A good mineral 011 pyrethrum spray when diluted for use should contain from 1 to .2 per cent high-grade mineral oil and about 1 part pyrethrins to 40,000 parts of water. STOMACH INSECTICIDES. The stomach insecticides are poisonous substances which when sprayed upon the surface of a plant cause the death of parasites which eat the foliage and thus take the p01son into their stomachs. For this purpose the arsenical compounds are the most important, although certain fluorine compounds are also valuable. I _ . An interesting development in dry climates where irrigation is practised is the use of magnesium sulphate or Epsom salts as an insecticide. When some insects eat foliage treated with magnesium sulphate they die shortly afterwards. _How- ever, the use of this salt in the British climate is impracticable, except under greenhouse conditions, because it is readily soluble in water and is washed off by rain. The stomach insecticide most extensively used in Britain is lead arsenate, although calcium arsenate, zinc arsenate, Paris green, and other substances are also used. The latter THE CONTROL OF PESTS 0F FARM AND GARDEN CROPS. 49 arsenical compounds, however, are less stable than lead arsenate and are more apt to break down and liberate water- soluble arsenic, which is poisonous to the plant. Fluorine compounds such as sodium fluoride and sodium silico-fluoride are also used as stomach poisons, but as yet, although they are less poisonous to human beings and stock than lead arsenate, they have not been widely adopted. Load Anemia—The lead arsenate used in this country is the di-plumbic arsenate and is available in three forms— namcly, as a fine powder, as a paste, and as a colloidal suspen- sion. High—grade lead arsenate must be high in total arsenic for optimum killing efficiency, low in water-soluble arsenic in order that burning of the foliage may be reduced to a minimum, and free from impurities. Lead arsenate powder should be free from lumps and grit and contain not less than 31 per cent of arsenic calculated as arsenic pentoxide, not less than 63 per cent of lead oxide, and not more than 05 per cent of water-soluble arsenic calculated as arsenic pen- toxide.1 Lead arsenate paste should be of a uniform consistency, free from lumps and hard particles, and should readin rub down a fine cream with water. It Should contain not less :han 14 per cent of arsenic calculated as arsenic pentoxide, not less than 28'4 per cent of lead oxide, both calculated on the original paste, and not more than 05 per cent water-soluble :u'scnic expressed as arsenic pentoxide and calculated on the dry basis. Pastes are normally sold with a content of 15 pl‘l' cent or 20 per cent of arsenic pentoxide.1 Colloidal lead arsenate should conform to the same chemical standards as the paste, but the particles are much smaller in size. Because of the ease of handling and mixing, the paste and colloidal forms are generally preferred to the dry powder. Lead arsenate Spray is prepared by mixing 1 lb. of the powder or 2 lb. of the paste with 40 to 50 gallons of water. Owing to the extremely fine state of division, the colloidal lead arsenate has a much greater covering power than the other two forms, so that it is only necessary to use 1 lb. of 15 per cent colloidal lead arsenate to get the same result as 1. lb. of 30 per cent powder or 2 lb. of 15 per cent paste in a Similar amount of water. .Lead arsenate may be mixed with Bordeaux mixture, nicotine sulphate, oil emulsions, lime sulphur or any of the wettable sulphurs, but cannot be mixed with any solution containing cresylic acid. In making a combined spray mix the other ingredients in the Spray tank first, then mix the ‘ Bulletin No. 82, Ministry of Agriculture and Fisheries. ‘Specifications and Methods of Analysis for certain Insecticides and Fungicides.’ 1938. VOL. LII. D |
| Title | Transactions of RHASS Volume 1940 - Page 030 |
