BACTERIAL DISEASES. 19 
Because of this lack of discrimination in settling, it will be readily 
seen, death due to overcrowding is not, strictly speaking, always the 
result of overdeposition, but frequently results from the overlapping 
of larvae and pupae during growth on leaves only moderately infested. 
Since, after settling, the immature stages do not change their loca- 
tion, specimens having ample room during the early larval stages 
become so large in the pupal stage, if not before, that they may over- 
lap each other at the molting period, with disastrous results to the 
individual beneath. Partial overlapping of the posterior portion of a 
pupa does not always result in its death, but death invariably follows 
the overlapping of the anterior or head end of the body. 
EFFECT OF CURLING AND DROPPING OF LEAVES FROM DROUGHT. 
Data collected during an unusual period of drought extending 
throughout the fall and winter of 1906-7 show that curling of leaves 
as an effect of drought has little effect on the vitality of the fly at 
this season. In March, 1907, pupae of the citrus white fly were 
observed on leaves which had been curled and dry from the effects 
of droughts for more than three months. The leaves were so dry 
that they felt and tore much like paper, but they soon regained their 
normal texture after the beginning of the rains. The emergence of 
the adults on trees affected as here described was delayed for several 
weeks as compared with unaffected trees, but aside from this there 
was no apparent effect on the insects. 
Although the curling of the leaves of citrus trees as a result of 
drought has not, so far as observed, resulted in checking the white 
flies, the dropping of the leaves may be decidedly effective in this 
respect. When citrus trees suffer from the effects of drought to the 
extent of shedding a considerable part of their foliage, the resulting 
reduction in the numbers of white flies rarely proves of sufficient 
advantage to offset the injury to the trees, and the insects as a rule 
resume their normal status fully as rapidly as the trees recover. 
BACTERIAL DISEASES. 
While no bacterial disease has been recognized as such in produc- 
ing the very high rate of mortality often occurring among the larvae 
and pupae of both species of white flies, there are indications that 
bacteria play a more important role in this connection than has been 
suspected, and are at times more beneficial in holding the fly in 
check than are the fungi. The fluctuating effectiveness of the 
unexplained mortality heretofore discussed, without the visible 
appearance of any fungous parasite which might be responsible, 
seems to indicate that some parasitic organism is directly concerned. 
