UNEXPLAINED MORTALITY. 



15 



An examination of the tiata in (he hist three cohimns of the above 

 table shows a strikinj^j rehilionsliii) between the unexphiinod mortahty 

 juul the insects which survived. In the case of the funf:;oiis]){irasites 

 however, there seems to be no strikinjjj rehitionship of this kind. In 

 order to make this point clear the six records (Nos. 1 to 6, inclu- 

 sive) with the lowest ])ercentages of unexplained mortality and the 

 six records (Nos. 7 to 12, inclusive) with the highest percentages of 

 unex{)lained niortality are here summarized and compared with a 

 similar summary with regard to fungous parasitism rearranged from 

 the same data: 



Unexplained mortality: 



() lowetit percentages averaging 58.2 per cent, 24.9 per cent survived. 



G highest percentages averaging 81.7 per cent, 5.4 per cent survived. 

 Fungous parasitLsm: 



6 lowest percentages averaging 8.5 per cent, 15.1 per cent survived. 



6 highest percentages averaging 21.1 per cent, 15.2 per cent survived. 



It appears from the above summary that a difference of about 24 

 l)er cent in unexplained mortahty in two groups of groves was asso- 

 ciated with a difference of about 20 per cent in the insects which 

 survived. On the other hand, a difference of about 13 per cent in 

 the deaths due to fungous parasites was associated with no appreciable 

 difference in the proportion of insects wliich survived. 



In December, 1910, Mr. S. S. CVossman, at the suggestion of the 

 junior author, made a series of records to correspond with 10 of the 

 12 included in Table IV. A summary of the 10 records for the two 

 years is given in Table III. 



Table III. — Status of white flies in 10 groves at ends of seasons 1908 and 1909. 



In five groves a larger percentage of surviving insects was found in 

 1909 than in 1908, in four groves a smaller percentage of surviving 

 was found in 1909 than in 1908, and In one grove there was no 

 appreciable difference in this percentage, as shown by the two exami- 

 nations. Unexplained mortality ranged from 23.6 to 91.7 per cent 

 in 1908 and from 61.8 to 78.8 per cent in 1909. The following is a 

 summary for 1910 based on arrangements of the data to show rela- 

 tion between unexplained mortality and fungous diseases to the 

 number of insects surviving;. 



