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 would 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 given 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 grown or nearly so 
(commonly spoken of as in the u 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 
