DURATION OF EXPOSURE. 45 



present system, whereas an hour is supposed to be the length of 

 exposure throughout. Thus if fifty minutes is found to give as 

 satisfactory results as an hour, it would be poor policy to reduce the 

 general exposure to this basis, inasmuch as with a general exposure 

 of one hour some trees are already receiving but little more than fifty 

 minutes. 



As a rule, very little gas remains under the tent at the expiration of 

 one hour. The amount is usually so small that the mortality among 

 the scale-insects could be but slightly increased by greatly lengthen- 

 ing the exposure. Various authorities have recommended two hours 

 or more as the duration of exposure, and it is possible that these long 

 exposures would produce slightly better results than an exposure of 

 one hour. 



From the standpomt of the fruit grower, who requires the best 

 results at the least possible expense, the item of time is highly impor- 

 tant. The question which must be considered is whether it is more 

 advantageous to sacrifice time or cyanid. No doubt it is cheaper to 

 sacrifice time up to a certain point, but beyond this it is cheaper to 

 sacrifice cyanid. As previously stated, the mortality among scale- 

 insects, when a two-hour exposure is employed, might be slightly 

 greater than at one hour. Before advising a two-hour exposure, 

 however, we must determine whether or not it would be more eco- 

 nomical to employ an exposure of one hour and use sufficient cyanid to 

 accomplish the same results secured by the longer time. Fumigators 

 are usually paid by the hour. Where tents are left on the trees two 

 hours, with the same number of tents the cost for labor is exactly 

 twice that for one hour. From 4 to 6 men, at an average wage of 35 

 cents per hour, are used on an outfit (infrequently 3), making the 

 hourly cost for labor from $1.40 to $2.10. This would purchase from 

 5 to 7 pounds of cyanid.'* Under these circumstances, if we can 

 obtain as good results in an hour by using 5 to 7 pounds more of 

 cyanid — or a smaller amount, according to the number of men in the 

 outfit — it would be more economical in the end to use the additional 

 cyanid and expose for the shorter time. The writer's own field 

 experience leads him to believe that as good results can be accom- 

 plished in one hour as in two hours by using an amount of cyanid 

 costing far less than would the extra hour's labor. 



It will be seen that the question before the fumigator is not simply 

 one of using that length of exposure which will produce the best 

 results, but that which will at the same time be most economical. 

 From field experience and other considerations the writer is led to 

 believe that this will be between fifty minutes and one and one-half 

 hours. 



a Cyanid is here considered as including acid, both costing about 28 cents per pound. 



