42 NATURAL CONTROL OF WHITE FLIES IN FLORIDA. 



From the foregoing quotations it is seen that there is practical 

 agreement among the various writers as to the natural efficacy of the 

 fungous parasites. 



OBSERVATIONS AND RECORDS CONCERNING NATURAL EFFICACY. 



Under the suhject of unexplained mortality it has been shown that 

 even where fungous diseases are most effective mortality from tliis 

 source is secondary in importance to that from unexplained causes. 

 The recognition of this fact does not in any way detract from the 

 actual value of the fungous parasites, but should be regarded as a 

 necessary step in the proper estimate of that value. 



The theoretical efficacy of white-fly fungous parasites may be deter- 

 mined by a similar method of calculation as that employed on page 

 16, in estimating the efficacy of unexplained mortality. Instead of 

 12.4 and 15.4 per cent for the years 1908 and 1909, the efficacy would 

 become 47.6 and 53.2 per cent if there had been no unexplained mor- 

 tality. Considering the normal rate of increase of the white flies as 

 shown in a previous bulletin of this bureau, mortality among the 

 larvas and pupoe to the extent of the foregoing calculations (47.6 per 

 cent and 53.2 per cent) obviously would be of no practical advantage 

 as an average condition. The insects could continually maintain 

 themselves at the maximum of injurious abundance even if the mor- 

 tality were 25 or 30 per cent higher. Theoretically considered, there- 

 fore, the fungous diseases were entirely ineffective in either 1908 or 

 1909 for the average of the 10 groves under observation. 



There is another phase of the subject to be considered, however. 

 With unexplained mortality present in all groves it is not necessary 

 for fungous diseases or any other known cause of mortality to increase 

 to a point of independent efficacy in order to be of distinct value. 

 The most important question to be considered here, therefore, is: To 

 what extent do fungous parasites effectively supplement all other 

 causes of mortality to the direct and practical advantage of wliite- 

 fly infested citrus groves ? 



For practical purposes in tliis bulletin, fungous parasites are said to 

 have worked effectively or to have cleaned up a grove when they 

 appeared to have worked effectively on the insects not succumbing 

 to unexplained causes of mortality, bearing in mind that the same 

 rapidity of spread and multiplication and the same percentages of 

 infection do not produce similar effects in different cases. This 

 absence of standards of efficacy is plainly shown in the data pre- 

 sented in Table II and also in the ft)llowing table in which the rec- 

 ords concerning eight of the groves included in Table II are extended 

 to show the status of the white flies and their fungous diseases at the 

 end of the season of 1909. 



