322 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. XI 
The results of the fumigation work in Tulare County were obtained 
from reports by Commissioner C. F. Collins of Tulare County. Every 
year he has had an inspection made of each grove, and leaves collected 
from many of the trees, about a month or a month and a half after 
fumigation. The insects on these leaves are counted, the number 
present and the number alive being noted. As about 1,600 insects 
are examined from each ten acre grove, a fair estimate of the percentage 
killed is obtained. The figures in Table III were obtained from the 
1917 report by Commissioner Collins. 
Table I —Hippodamia convergens, November 19-23, 1917 
Time of ex¬ 
Number 
Number 
Percentage killed 1 
Percentage 
posure 
of lots 
of insects 
Mean 
Standard 
deviation 
difference 
60 
10 
632 
97.47± .82 
2.99zb .58 
11.00=b2.53 
30 
10 
702 
86.47zb2.39 
8.67=bl.69 
60 
6 
374 
95.19dh2.72 
8.69zbl.31 
4.23=b3.75 
45 
6 
177 
90.96zb2.58 
8.25=bl .24 
These few experiments show a decided difference in favor of the 
60 minute exposure over the 30 minute exposure. In ten lots, totaling 
1,332 insects, the difference is 11.00±2.5 3per cent, which is fairly 
definite inasmuch as the difference (11.00 per cent) is more than four 
times as great as its probable error (2.53 per cent). However, in six 
lots, totaling 551 insects, the difference in favor of 60 minutes over 45 
is but 4.23±3.75 per cent. This means that there is no real and con¬ 
stant difference, inasmuch as the probable error of the difference (3.75 
per cent) is almost as large as the difference (4.23 per cent). 2 
After obtaining these results it was decided to repeat the experiments 
using lemons infested with red scale (Chrysomphalus aurantii) instead 
of the coccinellid beetles. Table II shows these results. 
1 The mean percentage killed is obtained by dividing the total number of dead in¬ 
sects by the total number used; ^ =M. The standard deviation (<r) is an index 
of variability and is obtained by the formula a- 
=ViF- K2 - 
The probable 
error of the mean is an index of the reliability of the mean. It is obtained from 
the standard deviation and the number of experiments. The formula is 
<T 
Em — ±0.67449 - 7 = (when N =the number of experiments). 
VN 
2 The probable difference between two means (Ai and A 2 ) is: 4 i — 4 2 ±V E^-{-E 2 2 
(Davenport, C. B. Statistical Methods, p. 15, 1914). In the case of the 45 and 60 
minute exposures it is therefore: 95.19%—90.96%dbv / 2.72 2 +2.59 2 = 4.23±3.75%. 
