NATURAL CONTROL OF WHITE PLIES IN FLORIDA. 11 
UNEXPLAINED MORTALITY. 
Early in the present investigations it became evident that mortality 
among larvae and pupae resulting from unrecognized causes was the 
most important element of natural control affecting the species of 
white flies herein considered. To this the authors have applied the 
term "unexplained mortality." This mortality has never been 
taken into consideration in previous publications, and in the past no 
little confusion has existed owing to the failure to distinguish between 
the benefits derived from it and those from fungous parasites, and 
attempts at artificial control by ineffective insecticides or other futile 
means. 
White flies dying from unexplained causes ordinarily have the same 
characteristics as insects that have been killed by fumigation or by 
some sprays. After dying the insects turn light brown and in the 
course of several weeks generally become more or less whitened. 
The dead insect sometimes becomes infected with the saprophytic 
" white-fringe" fungus. There is very little unexplained mortality 
among overwintering pupae, unless the mortality occurring during 
the period of emergence of the adults in the spring be properly classed 
as such, instead of being considered as due to climatic conditions. 
In a previous publication 1 the senior author has recorded an 
instance where entire credit for the temporary freedom from white-fly 
injury was commonly given to fungous parasites when mortality from 
other sources was concerned to an unknown degree. The instance 
referred to was that in Manatee County, Fla., in the summer of 1906. 
Observations during the past two years have led the authors to con- 
sider that the unusual reduction in the numbers of the citrus white 
fly in Manatee County in 1906 was accomplished by the combination 
of two important factors — fungous diseases and unexplained mortality ; 
also, that the former probably by themselves could not have produced 
the conditions which existed, while the latter unaided might have 
done so. 
These two conclusions are based on the general studies of the effect- 
iveness of fungous parasites, in the first case, and on a knowledge of 
the possible effectiveness of unexplained mortality in the second. 
The effectiveness of fungous parasites is discussed elsewhere. The 
possible effectiveness of unexplained mortality is shown by one of the 
most remarkable instances on record of the temporary checking of the 
citrus white fly and the cloudy-winged white fly by this natural- 
control influence, which occurred during the summer and fall of 1906. 
The effect was most noticeable in Orange County near Orlando, in a 
district where the senior author was quite generally conversant con- 
cerning the grove conditions. 
i Bui. 76, Bur. Ent., U. S. Dept. Agr., pp. 64-65, 1908. 
