14 CIRCULAR 920, U. S. DEPT. OF AGRICULTURE 
and around the tree according to the formula given by Quayle (12, p. 505)? 
and without the correction for leakage dependent on the surface-volume 
1atio which is used in citrus-fumigation practice. 
A gas evacuator (fig. 9) developed from one described by Fulton and 
Nelson (7) consisted of a large blower which sucked the air from beneath 
the tent and blew it upward above the danger level for the fumigators. 
As a tule the tents were evacuated for about 1 minute for each 1,500 cubic 
feet of tree volume. This was sufficient to remove about 80 percent of the 
residual gas and to permit moving the tent on to the next tree without 
undue hazard to the crew. 
Gas samples were taken regularly by a procedure common in citrus fumi- 
gation (Fulton and coworkers 5). These samples served as a check on 
porosity of fabrics, human errors and mechanical failures, and evacuation 
efficiency. The objective was to maintain a minimum average concentra- 
tion of 6 mg. per liter; whenever a tent showed enough leakage to cause 
the average to fall below this figure, it was withdrawn from use. In the 
1949-50 season the average gas concentration in 140 trees sampled was 
8.2 mg. per liter; only 4 concentrations were less than 6 and none less than 
5 mg. per liter. 
Factors that limit fumigation are tree dormancy and weather conditions. 
Almonds are relatively tolerant of hydrocyanic acid from early in October 
until about the middle of January. Other hosts can be fumigated with little 
injury over a longer period, generally until about the end of February. 
However, the weather may get too warm to continue treatment during 
October. An air temperature of 85° F. has been set as the upper limit, 
to insure the safety of the operators—hydrocyanic acid boils at about 79°— 
and to decrease the possibility of tree injury. Tests showed that low 
temperatures did not interfere with the vaporization and distribution of 
the hydrocyanic acid. Concentrations of the gas were as high at 29° as at 
higher temperatures. However, no fumigation was done when there was 
ice on the trees, because the 1ce would prevent the gas from reaching the 
scale. 
Wind did not affect the gas concentration under gastight tents, and fumi- 
gation was continued until the wind was strong enough to raise the tent 
edges. In the presence of wind, the edges were covered with dirt or held 
down by canvas tubes filled with sand. Fortunately, with this procedure 
the wind velocity that would raise the edge of the tents would also pre- 
vent the movement of tents from tree to tree. 
About 800 trees were used as the first block on which the effectiveness 
of routine treatments was tested. This block included some of the most 
heavily infested trees. The first fumigations were made in January and 
February 1945. Mortality counts between February and May revealed no 
live scales, but late in the summer, after there had been time for build-up 
from any sutvivors, 19 trees with live scales were found. There were no 
survivors after the second or third fumigation. Search for scales on current 
growth and fruit late in the summer or early in the fall has been the standard 
method of checking results of the preceding winter's fumigation. In 
another orchard, also heavily infested, 5 trees of 189 examined were found 
infested after the first fumigation and none after the second. In all other 
cases no live scales were found after one fumigation. 
* This reference contains a typographical error. The ccrrect formula is (S—co.us))- 
T 
