UNEXPLAINED MORTALITY. 
17 
to separate pathogenic bacteria from material sent to the Bureau of 
Animal Industry have not thus far been successful. There is certain 
evidence that some organism is directly concerned. As a rule 
unexplained mortality is greater in heavily infested groves than in 
lightly infested groves, although it is not dependent upon this point 
to a great degree after the insects have once become well established. 
The data in Table II, illustrating ordinary conditions in groves long 
infested, are here summarized: 
Average number of forms per leaf: 
6 lowest, averaging 112.7, 61.3 per cent unexplained mortality. 
6 highest, averaging 368.3, 78.5 per cent unexplained mortality. 
It should be noted that unexplained mortality was from 2.3 to 
12.5 per cent greater in the case of record number 11, averaging 147 
forms per leaf, than in the case of either record numbers 5, 6, or 8, 
averaging 229, 434, and 597 forms per leaf, respectively. Of the 
twelve records the one showing the highest unexplained mortality 
ranks seventh in point of average number of forms per leaf. 
In newly infested groves or in groves where the white fly has been 
temporarily greatly reduced from any cause, unexplained mortality 
as a rule is comparatively low. Grove No. 1 in Table VI, that of 
Hon. J. M. Cheney, previously referred to as to its condition in 1906 
and 1907, shows a condition which may follow the reduction of the 
white fly to a negligible quantity for a season. Table IV gives the 
results of the examination of white flies in six newly infested groves, 
no fungous diseases, so far as could be detected, being present in any 
case: 
Table IV. — Conditions with regard to unexplained mortality of white flies in newly- 
infested groves. 
Grove 
No. 
When examined. 
Number 
leaves 
exam- 
ined. 
Total num- 
ber of forms 
counted. 
Average 
number 
white fly 
forms per 
leaf. 
Percentage 
of unex- 
plained 
mortality. 
1 
2 
3 
4 
5 
6 
Dec. 4, 1906 
27 
100 
10 
100 
41 
25 
5,503 
150 
2,094 
1,222 
233 
12,801 
20.4 
1.5 
20.9 
12.2 
5.7 
51.2 
12.0 
15.3 
30.5 
24.2 
12.4 
10.9 
Sept. 13, 1907 
Oct. 15, 1907 
Aug. 16, 1908 
Dec. 8, 1908 
Dec. 2, 1909 
Both species of white flies herein considered are affected by mor- 
tality from unexplained causes, but the effect on the cloudy-winged 
white fly (Aleyrodes nubifera Berger) seems to be more pronounced 
as a matter of control, since the absence of food plants other than 
citrus tends to prevent the rapid increase in infestation which results 
in the case of the citrus white fly when its useless food plants are neg- 
lected. In the foregoing records both species were present, the citrus 
white fly greatly predominating. 
21958°— Bull. 102—12 2 
