146 



and green pruning directly diminishes the number of the scales. Some 

 growers have thought that complete defoliation is equivalent to 

 suppressing infestation, but this cannot be the case, as infestation 

 is not confined to the foliage. 



The report of work done with insecticides occupies over three-fifths 

 of the paper and the experiments are described in detail. The study 

 of the immunising power of insecticides was undertaken on a con- 

 siderable scale in Sicily, with better success than previously obtained 

 in Liguria by Del Guercio. This attempt to discover whether the plants 

 could be rendered distasteful to the young larvae for a period sufficiently 

 long to make it impossible for the individuals of one whole generation 

 to attach themselves, showed that the addition of a colloidal adhesive 

 so increased the insecticidal power of polysulphide of calcium as to 

 make such an addition indispensable in the control of C. dictyospermi 

 on citrus plants. The polysulphide of calcium was prepared as follows : 

 6 gallons of water were put in a boiler of 69 gallons capacity and heated 

 to 113° F.; 301b. of fresh quicklime (broken just before use into 

 lumps as big as the fist) was then thrown in and allowed to disintegrate 

 without being touched or stirred up in the liquid ; 30 lb. of sulphur 

 was then added. The sulphur (which contained 2 per cent, of Cu SO4 

 and was the same as that used against vine mildew) was added by 

 degrees, being dropped through a sieve ; the presence of copper does not 

 however appear to be essential. The sieve had two long handles and 

 the workman wore protective goggles. As soon as the sulphur was 

 added, the mixture was well stirred until a homogeneous paste was 

 formed in about 15-20 minutes, 30 gallons of water being then 

 poured in and the whole boiled for about an hour and a half, when the 

 mixture w^as ready. At first it contained impurities in suspension 

 which gave it the colour of roasted cof?ee, but with a green tinge. 

 After the sediment had formed, the clear liquid w^as red-brown in 

 colour. At a temperature of 68° F. its density was about 1'21. The 

 flour paste adhesive was prepared as follows : In a boiler of the size 

 given above, 30 gallons of water were heated to ]22°-159° F. and 30 lb. 

 of wheat flour (free from bran) was dropped in through a sieve by one 

 workman while another stirred without ceasing in order to prevent 

 the formation of lumps. Stirring was continued, and after a few 

 minutes the fire was raked out to prevent an excess of froth ; a few 

 minutes later stirring was discontinued and the liquid was allowed to 

 cool. It contained about 10 per cent, of flour. The polysulphide of 

 calcium may be prepared a long time before use, but the paste solution 

 goes bad quickly and should not be made and mixed with the poly- 

 sulphide earlier than the day before that on which spraying is performed. 

 As the spray solution must contain 2 per cent, of flour and the paste 

 contains 10 per cent, of flour, it is only necessary to pour a convenient 

 quantity of paste solution into a tub, then add the proper quantity of 

 polysulphide, and finally dilute with water until the total bulk of liquid 

 is five times that of the paste solution ; the spray is then ready for 

 use. The presence of 2 per cent, wheat flour changes somewhat the 

 physical characteristics of the polysulphide solution and renders it 

 turbid. The solution of calcium polysulphide was found to be very 

 frothy if decanted with a certain violence, and this frothiness persists 

 even when the strength of the solution is reduced to 5 per cent, by 

 adding water. The solution is not hygroscopic until the paste is 

 added, when the mixture becomes so to a slight degree. 



