IjL. Lloyd 23 



f.ion about temperature as, in the case of tomato plants, very severe 

 damage with the same amount of cyanide has occurred at a temperature 

 of 57° with unprepared plants and none at all at 69° with prepared plants 

 of a similar soft growth. The best advice that can be given to the grower 

 is that he should have his houses cool for the fumigation and carefully 

 prepare the plants beforehand as described below. 



The toxicity of the gas for the insect was not found to vary within 

 the mean temperature limits of 47° and 66°, total mortalities being ob- 

 tained with both at the correct dosage. 



Thne of day and duration. Every writer on the fumigation of green- 

 houses with this gas mentions the importance of not commencing the 

 operation till dusk, but growers in many cases persist in starting several 

 hours before sunset. Damage which means practically the death of the 

 plant results. All tissue on which the sunhght falls is seared as though 

 with flame, the growing points are killed and the buds and flowers cut 

 off. The materials are always blamed for this and the operation dis- 

 credited. Moreover the gas is less toxic in sunlight as the following cases 

 show. A fumigation with | oz. cyanide per 1000 eft. commenced four 

 hours before sunset and lasted 13| hours gave a mortality of less than 

 50 per cent, for adults and negligible for the scales, as contrasted with 

 a usual mortality of 90 per cent, for adults and 75 per cent, for scales 

 in fumigations with this quantity commenced at dusk and lasting S-1 1 

 hours. A fumigation with I oz. cyanide per 1000 eft. commenced three 

 hours before dusk and lasted 12 hours gave an almost total mortahty 

 for adults and 70 per cent, for scales, as contrasted with a usual total 

 mortality for adults and total, or almost total, mortality for scales with 

 the same quantity commenced at dusk and lasting 8-11 hours. 



It has generally been the custom to recommend relatively large doses 

 of cyanide with short exposures rather than small doses with long expo- 

 sures. Sasscer and Borden (6) using i-f oz. cyanide per 1000 eft., exposure 

 one hour, obtained with this pest a mortality which was total except for 

 eggs and late pupae. In the course of this work | oz. cyanide, 1000 eft., 

 exposure one hour, gave 95 per cent, mortality for adults, had practically 

 no effect on late pupae, and destroyed about half the younger scales. The 

 same amount, 1-| hours' exposure, gave 100 per cent, mortality for adults 

 and about 90 per cent, for scales. \ oz. cyanide, 1000 eft., exposure 

 three hours, gave 100 per cent, mortality for adults and a poor result 

 for scales. In the two last cases tomato plants were damaged, the 

 temperature being 57° and 62° respectively. To control the infestation 

 by these means at least three fumigations would be necessary as against 



