NATURAL CONTROL OP WHITE FLIES IN FLORIDA. 11 



UNEXPLAINED MORTALITY. 



Early in tJie present investigations it became evident tliat mortality 

 among larvaj and pupvQ 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 ai)i)lied 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 d3ang from unexplained causes ordinarily have the same 

 characteristics as insects that have been killed by finnigation 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 s])ring be properly classed 

 as such, instead of being considered as due to climatic conditions. 



In a previous publication ^ 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 degi-ee. 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 liave 

 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 (piite generally conversant con- 

 cerning the grove conditions. 



I BuL 76, Bur. Ent., U. S. Dept. Agr., pp. 64-65, 1908. 



