48 FUMIGATION INVESTIGATIONS IN CALIFORNIA, 



first tree, 20 by 12 feet, there would be 0.86 of a square foot of tent 

 surface for each cubic foot of gas to escape through; whereas in the 

 last tree, 70 by 54, there would be only 0.22 of a square foot of tent 

 surface for each cubic foot to escape through. This would mean 

 that there w^ould be about four times as great an opportunity for 

 leakage, or that the leakage would be approximately four times as 

 rapid in the smaller tent as in the larger one. 



There can be little doubt that the leakage of gas in tents covering 

 different-sized trees is nearly in accordance with these figures. Hence 

 it can be readily seen that, in order to secure uniformity of results, 

 this leakage must be taken into consideration, and small trees must 

 receive more cyanid per 100 cubic feet than do the larger trees. 



The correctness of the foregoing deduction has been frequently 

 demonstrated in the field. In using on a smaller tree a certain dosage 

 strength with which on large trees we were able to secure splendid 

 results against the purple scale, we were always much less successful. 

 In other words, if we used 1 ounce of cyanid per 100 cubic feet on 

 the 70 by 54 foot tree, we would get far better results than had we 

 used the same dosage rate on the 20 by 12 foot tree. A very forcible 

 exemplification of this condition has been .'],iven in experiment 

 No. 3, in fumigating for the purple scale. In this particular experi- 

 ment much less satisfactory results were secured on the small trees 

 when using a one and one-half hour exposure than on the large 

 trees of experiment No. 2, with a one-hour exposure. 



TIME OF THE YEAR FOR FUMIGATION. 



Although fumigation is carried on in California at all times of the 

 year, there are certain periods in which the operations are more 

 general. There are two main factors to be taken into consideration 

 in fumigating, i. e., the species of scale-insect and the condition of 

 the tree. As to the latter, it may be said that at certain periods of 

 the year trees are in such a tender condition that they can not 

 withstand a heavy dosage without injury, especially to the fruit. 



The bulk of fumigation in California at the present time is carried 

 on between the latter part of August and December. Probably the 

 principal reason for fumigating during this period is that at this 

 time the black scale is most successfully reached. The eggs of the 

 black scale, and the insects themselves when full growTi or nearly so 

 (commonly spoken of as in the "rubber" stage), require very heavy 

 dosages. On the other hand, the young of the black scale, or those 

 which have not reached the so-called "rubber" stage, can be 

 destroyed with a moderate dosage. Although the life history of 

 the black scale has never been thoroughly worked out for the region 

 with which we have to do, it is generally understood that the majority 



