NATURAL CONTROL OF WHITE FLIES IN FLORIDA. 39 



NATURAL EFFICACY OF FUNGOUS PARASITES. 



UndcM- this lieaclin<>; arc discussed subjects relating to the actual 

 dogivc oi" conti-ol of Avliicli funi^ous '|)ai-)isit(\s have slioAvn thciusclves 

 capable, without regard for the possibilities of increasing that degree 

 of eflicacy by artificial means. These two subjects are frequently 

 confusetl, although a cleai- distinction is necessaiy for a pi"oj)er under- 

 standing of the economic Aahie of the j)arasitic iungi. 



CREDIBILITY OF COMMON UKPORTS. 



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 eflicacy of the natural enemy. The aniount of control 

 influence exerted by the natural enemy can not be approximated by 

 casual obsenaition, by the record of parasitism of a comparatively 

 small number of specimens of the insect pests, or even by the seem- 

 ingly j)ractical results as shown by the condition of the ho&t plant. A 

 casual obsei-vation 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 niake 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 ])ercentage 

 of parasitism. The experience of the authors in the course of the 

 mvestigations I'cported herein shows that thousands of insects rather 

 than hundreds, and these on leaves picked absolutely at random 

 without previousl}^ making any note of their condition, can be regarded 

 as the only satisfactor}' basis for approximate estimates of the efhcacy 

 of fungous parasites. Even reports based on seemingly practical 

 results of fungous parasites, with tlie white fhes greatly reduced and 

 with clean leaves and fruit, should not be credited without being 

 authoritatively confirmed . 



Experience has shown such reports too frequcntlv to be incorrect 

 for either of two reasons: The first is due to a misunilerstanding 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 tlie v/hite-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 fomid to be entirely erroneous. 



It is desirable that citrus growers become acquainted with all 

 important facts in regard to the wliite fhes and the methods of their 



