24 Control of the Greetihoase White Fbj 



two when the smaller doses with long exposures are used. The former 

 at the present price of materials would cost about £12 per acre and the 

 latter about £4. Provided that the houses are opened up at dawn there 

 appears to be no more risk to the plants with the long exposure and small 

 dose than with the short exposure and large dose, and the former should 

 always be employed. 



Toxicity of the gas. The following account of the toxicity of the gas 

 for the insect deals only with exposures of 8-11 hours' duration. 



The eggs are unaffected and the adults are rather more susceptible 

 than the scales. A large proportion of the adults fall to the ground directly 

 the gas reaches them and unless there is a sufficient concentration many 

 of these recover during the following day or two and regain the plants. 

 Whenever the mortality of the adult was total that of the scales was 

 always more than 90 per cent., generally more than 95 per cent, and often 

 100 per cent., so that if all the flies are killed the effect of the fumigation 

 is known at once to be good, if not perfect. The results obtained on 

 heavily infested plants sleeved after the fumigations will be given briefly 

 under the various dosages. The study of the life-history was carried on 

 at the same time and showed that the emergences recorded were after 

 intervals too brief for them to have been in the egg condition at the 

 temperatures ruling at the season when the particular fumigation was 

 done. 



J oz. cyanide pur 1000 eft.; 4 tests; mortality for adults always total; mortality 

 for scales in three cases total; small numbers of adults emerged on one plant beginning 

 on the 18th day after fumigation. 



J oz. cyanide per 1000 eft.; 6 tests; mortality for adults always total; mortality 

 for scales in three cases total and in three cases very small numbers emerged beginning 

 on the 13th, 16th, and 17th days respectively. 



\ oz. cyanide per 1000 eft.; 14 tests; mortality for adults always total except in 

 one case when three out of 500 recovered; mortality for scales in seven cases total; 

 in three cases a single scale survived out of hundreds, the flies emerging on the 8th, 

 14th and 15th days respectively; in four cases emergence occurred in very small 

 numbers (one to three a day) commencing once on the 10th, twice on the 14th, and 

 once on the 17th days respectively. 



^ oz. cyanide per 1000 eft.; 10 tests; mortality for adults always total except 

 in two cases, 1 and 9 surviving respectively, in each case out of many hundreds; 

 mortahty for scales in six cases total; in two cases emergence occurred in small 

 numbers after the 12th day, in one case two flies emerged on the 5th and no more to 

 the 10th day; the remaining case was an unexplained failure, only 75 per cent, of the 

 scales being killed. 



^ oz. cyanide per 1000 eft.; 9 tests; mortality for adults total in five cases, and 

 in the remainder over 95 per cent. ; mortality for scales total in three cases ; emergence 

 in the other cases in very small numbers began on the 2nd, 5th (twice), 6th, 7th and 



