Transactions of RHASS Volume 1940 - Page 027
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| Year | 1940 |
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I . '4. misfit? it}? i l try 42 THE CONTROL OF PESTS OF FARM AND GARDEN CROPS. more different substances intended to attack difierent pests in one spraying solution in order to save labour. This cannot be done indiscriminately, because some substances when mixed together in one solution interact and destroy the active principles. The mixing should be done by following a definite procedure. Compatibility tables showing what substances may be mixed are given at the end of this paper. Spraying Machiiics.—Spraying solutions should be applied by a good type of sprayer. Cheap equipment almost invariably proves the most uneconomic. The Spraying equipment should be made of non-corrosive metal. Rubber tubing, especially if oil emulsions are to be used, should be the petrol-resisting kind, which is now made by many rubber manufacturers. The selection of the correct type of spraying jet is very important. No definite rules can be laid down, because the type of jet depends upon the make of machine and conditions of spraying. Jets should be, where possible, of the self-clearing type. The actual spray should be fine so that the droplets do not fall readily to the ground before hitting the plant. On the other hand, it should not be so fine that it is easily floated away when there is almost no wind. When possible, Spraying should be carried out when there is little or no wind. Dusting.—Dusting is of increasing importance, but much more research work is required before it becomes so well established as spraying. It has decided advantages over spraying in that no water is required, which lessens labour by making transport of materials easier and doing away with the preparation of solutions. However, it is doubtful if it is as efficient as spraying. The powder used must be very fine and light. This allows it to drift better than a spray, but dusting can be used only where there is little or no wind. The best time to apply dusts to secure adhesion is when the foliage is wet with dew or rain. WINTER OR DORMANT SPRAYS. Winter sprays are used to kill the eggs and hibernating stages of insects, and to destroy lichens, mosses, &c., which afford them protection. TheSe sprays being applied during the dormant period may be made of substances which are not of a high degree of purity and may be used at comparatively high concentrations without injuring the plant. There are three types of sprays used as winter washes—namely, alkaline sprays, tar oil sprays, and mineral oil sprays. The alkaline washes consist of preparations of caustic lime and caustic soda, and have long been used. Probably the earliest practice of this kind was the lime-washing of the trunks and main branches of fruit trees. Then came the use THE CONTROL OF PESTS 0F FARM AND GARDEN CROPS. 43 of dilute solutions of caustic soda. These practices are well known and are dealt with in many text-books, so we do not propOSe to deal further with them here. The tar oils have been used for some time, and in the early years of their use it was hoped to obtain through them complete control of certain insect pests. The continued use of tar oil washes, however, has not fulfilled this expectation, as it was found that they do not destroy effectively certain important pests. It is found, for instance, that aphis and apple-sucker are controlled by tar oil distillate, while sawfly and red spider are not. Since the advent of tar oil sprays red Spider infesta- tion has increased in some places, possibly due to the destruc- tion of useful insects which preyed upon their eggs. Subsequent work showed that mineral oil preparations are fairly efficient in controlling both the sawfly and the red spider, although they do not kill the eggs of aphis and apple-sucker. Combined sprays of tar oil and mineral oil have given good control of all these pests, but some operators prefer to use a tar oil spray in mid-winter, followed by a mineral oil spray in the late winter. Tar Oil Washes.——It is stated by Dr Hubert Martin 1 that the tar oil for a high-grade winter wash should conform to the following Specifications: contain 88 per cent of neutral oil and not more than 6 per cent phenols ; the boiling range should be such that 90 per cent by volume should boil above 370° 0., 50 per cent above 325° C., and 20 per cent above 365° C.; and the specific gravity should be 1-09 to 111 at 60° F. The tar oil may be made miscible with water by dissolving 25 parts of sodium resinate soap in 75 parts of the oil. When mixed with water this mixture gives a milky-white emulsion. Mineral Oil Washes—The mineral oil for a winter wash should have, according to Dr Martin,2 the following specifica— tions : a specific gravity of 0'86 to 0'92 at 60° F., a viscosity of not less than 125 sees. and not greater than 500 secs, Redwood, at 70° F., and an unsulphonated residue of not less than 60 per cent by volume. It should be neutral and free from bases, and at least 90 per cent by volume should (listil above 315° C., 50 per cent above 350° C., and 20 per cent above 380° C. The usual grade of mineral oil used in winter washes is what is known as a medium Spindle oil. Mineral oils may be rendered miscible with water as follows : mix together 2 lb. of soft soap and 1 quart of cresylie acid, and stir, warming if necessary, until the soap is dissolved. Add this mixture to about 1 gallon of the mineral oil, stir until it is dissolved, and then mix with other 4 gallons of the mineral oil. This forms a miscible oil which emulsifies on mixing with water. 1 Ann. of Appd. Biology. Vol. XXII., pp. 381-82., 1935. = 1bicI., p. 355. |
| Title | Transactions of RHASS Volume 1940 - Page 027 |
