18 



NATURAL. CONTROL OF WHITE FLIES IN FLORIDA. 



DROPPING FROM LEAVES. 



Daily observations made on marked larvae from date of settling 

 to emergence of adults, in connection with life-history studies, proved 

 that a small proportion of larvae loses hold upon the leaves and drops, 

 especially at moltmg periods. Of 231 marked larvae, 20 (or 8,6 

 per cent) dropped before reaching maturity. This droppuig occurred 

 in nearly every case after the larvae had passed several days in the 

 plump condition preceding molting and were in no way pressed for 

 room. While dropping is largely restricted to the earlier instars, 

 one pupa has been kno^vn to drop after having shown developed 

 eye-spots for nine days. Where infestation is excessive, dropping 

 is more frequent than noted above, but is then due more directly 

 to overcrowding, as shown under the following heading. 



MORTALITY DUE TO OVERCROWDING. 



The excessive overcrowdmg of leaves with eggs always results 

 in the death of practically all the larvae that hatch, as it either becomes 

 a physical impossibility for them to find suitable places for attach- 

 ment, or, because of the closeness of the eggs, such spaces as they do 

 find are far too limited to permit development to the pupal stage. 



Table V. — Effect of overcrowding upon development of the citrus white fly. 



The data in Table V illustrate the inevitable outcome of over- 

 deposition. The leaves on which these data are based were heavily 

 infested with eggs, No. 3 being a very small leaf. Unfortunately 

 this wholesale mortality is not so important a factor m the develop- 

 ment and spread of the citrus white fly as in the case of the cloudy- 

 winged white fly, smce the habit of the female leads her to scatter her 

 eggs over the older as well as over the more tender growth. With 

 the former species on more than one occasion effective control has 

 been observed to follow certain favorable conditions as to the rela- 

 tive abundance of the adult insects and new citrus growth. It has 

 been computed that the larvae hatching from the 13,882 eggs deposited 

 on Leaf No. 1, would require about 25 times the surface of that leaf 

 in order to reach the pupal stage should they settle with the view 

 of utihzing the least possible space. Since the larvae do not show 

 such discrimination in locating themselves, an even larger amount of 

 leaf surface would be required. 



