324 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
These counts show that there was no difference between the results 
obtained in actual field practise in Tulare County between exposures 
of 45, 50, and 55 minutes. The writer did not attempt to calculate 
the probable error and probable difference in these counts, as the 
percentage dead of the various exposures are so close that it is certain 
the probable error of the difference would be greater than the differ¬ 
ence. 
Summary 
From a series of 44 experiments (using a total of 7,485 insects) car-. 
ried on in the daytime under form “trees” covered with tents of 8 
ounce U. S. army duck, with both coccinellid beetles ( Hippodamia 
convergent) and red scale ( Chrysomphalus aurantii), it was shown that 
an exposure to hydrocyanic acid gas for 30 minutes was not sufficient 
to obtain the highest killing efficiency. It was shown, however, that 
with 45 minutes as good results were obtained as with 60 and 90 
minutes. From an examination of the results of commercial fumiga¬ 
tion against the citricola scale ( Coccus citricola ) in 125 groves in Tulare 
County during the 1917 season, it was learned that there was no prac¬ 
tical difference between the killing efficiency of the hydrocyanic acid 
gas with exposures of 45, 50 and 55 minutes. 
Conclusions 
It may be concluded from these experiments that an exposure of 45 
minutes is sufficient to kill the red scale, under the conditions as given. 
It is possible that with fumigation carried on at night, where the tem¬ 
perature is lower and the tent leakage possibly less that a longer period 
may be somewhat more efficient. However, from the data given for 
the citricola scale, it appears that under normal conditions for com¬ 
mercial fumigation a 45 minute exposure is fully as efficient as a 50 
or 55 minute exposure. 
President G. P Weldon: Is there anyone who wishes to discuss 
this paper? 
R. S’ Woglum: I am in agreement in general with the findings as 
given in the paper, but in the case of eggs of the purple scale, I found 
that a longer fumigation period resulted in a better killing. 
H. J. Quayle: It should be kept clearly in mind that these expe¬ 
riments were carried on with ordinary fumigation tenting material and 
during the daytime. Tenting material and atmospheric conditions 
are two variable factors in relation to the time of exposure. 
President G. P. Weldon: The next paper is entitled “A Native 
Food Plant of Rhagoletis fausta,” by H. H. P. Severin. 
