362 



THE AGRICULTURAL NEWS. 



November 4, 1916. 



INSECT NOTES. 



A SCALE INSECT PEST OF CITRUS 



AND ITS CONTROL. 



The following is a eontinuatibn of the article in 

 the last issue of the Agricultural Netvs, on Ghrysom- 

 plialus dictyos^Jermi, reproduced froiii the Review of 

 Applied Entoiriolugy. The first part dealt princi- 

 pally with the characteristics and habits of the pest. 

 This part concerns the important subject of control: — 



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 in.'iecticides 

 was undertaken on a considerable scale in Sicily, with better 

 success than previously obtained in LigJiria 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 11.3° F., 30 Bb. of f^'esh -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; 300). of sulfihur was then added. The sulphur 

 (which contained 2 per cent, of Cu 1SO4 and was the same as 

 that used against vine mildew) was a^ded by degrees, being 

 drepped through a sieve; the presence of coppsr does not 

 however appear to be esseiitial. The sieve had two long 

 handles, and the workman woru protective goggles. As .soon 

 as the sulphur was added, the mixture was well stirred until 

 a homogeneous paste was fnimed in about fifteen to twenty 

 minutes, 30 gallons of water being then poured in and the 

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

 was ready. At first it contained impurities in suspension 

 which gave it the colour of roasted coffee, but with a green 

 tinge. After the sediment had formed, the clear liquid was 

 red-brown in colour. At a temperature of 68° F. its density 

 ■was about 1 '21. The tiour paste adhesive was prepared as 

 follow.'?: in a boiler of the siz(> given above 30 gallons of 

 water were heated to 122-159° F., and 30Ib. of wheat flour 

 (free from bran) dropj)ed in through a sieve by one workman, 

 while another stirred without ceasing in order to prevent form- 

 ation »f 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 

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

 polysulphide of calcium may be i)re[xir6d a long time before 

 use, but the iniste solution goes bad quickly, and should not 

 be made and mixed with the polysulphide earlier than the 

 day before that on which spraying is j)erf()rmed. 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 p;i.ste solution into a tub, then add 

 the proper quantity of polysulphide, and finally dilute with 

 water until the total bulk of li(juid isfiyo times t^iat 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. 



The potassium poly.sulphide used in the experiments was 

 in the form of tiles weighing about 2 fc. each. These are 

 very hygroscopic, and if broken up small, they dissolved 

 quickly even in cold water. In the experiments a .'iSJ per 

 cent, solution was produced by dissolving 10 ft), of potassium 

 polysulphide in 2 gallons of water and then diluting up to 

 3 gallons of liquid. This stock solution was kept in tightly 

 closed glass bottles; at a temperature of 58° F. its density 

 was 1-23. Both the 2 per cent, and 8 per cent, working 

 strengths were less frothy than those of polysulphide of 

 calcium, even with the addition of the boiled paste. Strong 

 solutions, such as 5 per cent, and 7 per cent, were very 

 vi.scid to the touch owing to their alkalinity, and they 

 somewhat affected the skin. They never damaged the 

 rubber hose, whether cold or hot. In cases where the calcium 

 and potassium polysulphides were mixed, flocculent tufts 

 were formed — even when the two solutions were clear before 

 mixing. Though the mixture w:us somewhat viscid, it did 

 not stick to the citrus leaves, and therefore required the 

 addition of the paste. With such an addition, it yielded oh 

 drying a coating which was highly re.^istant to atmospheric 

 agents, and this property is the mcx-e marked when the 

 polysulphide of calcium is predominant. For instance, a spray 

 containing 2 per cent, of polysulphide of potassium, 4 per cent, 

 of the concentrated solution of polysulphide of calcium, 

 and 2 per cent, of flour, was very adherent to the foliage and 

 resisted well the action of rain. The leaves of eitrusplants 

 are able to resist chemical action to a considerable extent, 

 and in December calcium polysulphide was safely applied 

 at a strength of 14 percent.; in summer 5 percent, was 

 found to scorch the young fruit if applied in the hot hours 

 of the day, but not at other times. The potassium polysul- 

 phide was variable in action according to the danqine-ss of 

 the atmosphere, the temperature being a less important 

 factor; in July a 7 per cent, .solution destroyed the Coccids 

 and did little damage to leaves or fruit. In autumn 

 however, the same dose injured many leaves and fruit, 

 and a dose of 2 per cent, of the solution containir.g both 

 polysulphides also proved injurious. In no ease should tha 

 polysulphides be applied when the citrus plants are io 

 blossom. Even at doses of 3 or 4 per cent, polysulphide of 

 potassium had a very powerful and comparatively prompt 

 action on the Coccids. The males were even more sensitive 

 to it than the females, and the nyuqjhs succumbed very 

 •luickly; tf) mobile larvae a dose of 01 per cent, proved fataL 

 The immunising action of polysulphide of potassium was 

 also very .strong and lasting, unless there was rain. Poly, 

 sulphide of calcium behaved in a very different manner. 

 Its action on the Coccids, though etUciont, was mueh slower. 

 In the early nynqthal forms the mortally was ascertainable 

 after eight to ten days, but with the adults a 5 per cent, 

 dose required a fortnight or more to reveal its efficiency. 

 Spraying should be done from Juno to September and after 

 harvesting the crt)p, up to early in March. In the experi- 

 ments good results were obtained both in summer and winter, 

 but in Si<'ily the sumnu\r is preferable as rains are then less 

 frequent, while wint(^r treatment may mean several applica- 

 tions and a waste of material. In summer, with maximum 

 temperature of 104° V., two a|iplications of 5 per cent, 

 polysulphide of calcium removed all infestation. If the 

 applications were put otf until the crop had been gathered, 

 a stronger dose of 7 or S per cent, was meeessary. If th& 



