SUMMABY AND CONCLUSIONS. 73 
in the line of white-fly control. It should be borne in mind that 
even though an important and successful enemy should be discov- 
ered within the next two years, it would probably require several 
years before the practical results secured would justify the abandon- 
ment of artificial methods of control. On the other hand, in planning 
for the future the proportion of successes and failures of the past in 
obtaining successful natural enemies for various pests must be con- 
sidered. Instances of such complete success as was obtained by 
the introduction of the Australian ladybird into California and later 
into Florida for the control of the cottony cushion scale are rare. 
Instances of only partial success or of entire failure are many. It is 
true that many failures have been due to the elementary condition 
of our knowledge of insect parasitology and that the failures of 
to-day and of the past may be overcome by advances in this line 
which may be made in the future. The field is almost unlimited 
in possibilities, and even the failure of the present foreign explora- 
tions should not lead to the abandonment of all hope of successful 
natural control. 
