NATURAL CONTROL OF WHITE FLIES IK FLORIDA. 39 
NATURAL EFFICACY OF FUNGOUS PARASITES. 
Under this heading are discussed subjects relating to the actual 
degree of control of which fungous parasites have shown themselves 
capable, without regard for the possibilities of increasing that degree 
of efficacy by artificial means. These two subjects are frequently 
confused, although a clear distinction is necessary for a proper under- 
standing of the economic value of the parasitic fungi. 
CREDIBILITY OF COMMON REPORTS. 
It is a well-recognized fact among economic entomologists that 
wherever predaceous insects or parasites of any kind are conspicuous 
enemies of an insect pest, popular reports are greatly exaggerated in 
regard to the efficacy of the natural enemy. The amount of control 
influence exerted by the natural enemy can not be approximated by 
casual observation, by the record of parasitism of a comparatively 
small number of specimens of the insect pests, or even by the seem- 
ingly practical results as shown by the condition of the host plant. A 
casual observation summarized by a statement that 50, 75, or 90 per 
cent of the insects are destroyed by fungous parasites is usually 
worthless and misleading. Even an experienced entomologist could 
not make a statement of value in this respect without first making 
extensive counts of specimens, recognizing the influence of unex- 
plained mortality and the effect this has upon the apparent percentage 
of parasitism. The experience of the authors in the course of the 
investigations reported herein shows that thousands of insects rather 
than hundreds, and these on leaves picked absolutely at random 
without previously making any note of their condition, can be regarded 
as the only satisfactory basis for approximate estimates of the efficacy 
of fungous parasites. Even reports based on seemingly practical 
results of fungous parasites, with the white flies greatly reduced and 
with clean leaves and fruit, should not be credited without being 
authoritatively confirmed. 
Experience has shown such reports too frequently to be incorrect 
for either of two reasons: The first is due to a misunderstanding of the 
factors influencing fluctuations in numbers of the insects; the second 
is the absolute lack of any actual foundation for the popular report of 
the character referred to. These reports are traceable to a feature of 
human nature which is found everywhere. One can not become well 
acquainted with the white-fly situation without noting instances of 
persistent and emphatic reports in regard to the complete efficacy of 
fungous parasites in certain sections or in certain groves which upon 
investigation are found to be entirely erroneous. 
It is desirable that citrus growers become acquainted with all 
important facts in regard to the white flies and the methods of their 
