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 

 unex})lained mortality is greater in heavily infested groves than in 

 lightly infested groves, although it is not dependent upon this ])oint 

 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 forma 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 reconl 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 rvith regard to unexplained mortality of white flies in neu'ly- 



in/ested groves. 



Both species of white flies herein considered are afi'ected by mor- 

 tality from unexplained causes, but the efl'ect on the cloudy-winged 

 white fly {Aleyrodes nuhifera Berger) seems to be more pronounced 

 as a matter of control, since the absence of food plants other than 

 citrus tends to prevent the ra])id increase in infestation which results 

 in the case of the citrus wliite fly when its useless food plants are neg- 

 lected. In the foregoing recortls both species were present, the citrus 

 white fly greatly predominating. 

 21958°— Bull. 102—12 2 



