508 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
On account of the fact that in California fungous and bacterial diseases 
of insects do not generally thrive, biological control in this state resolves 
itself into a matter of making use of beneficial insects alone. 
While all insects have natural control factors working against them, 
they do not all have natural enemies among the parasitic and predatory 
insects, at least not effective ones. 
Some insects are serious pests not because they have been introduced 
from a foreign country without their natural enemies, but because man 
has altered their environment in such a way as to provide a more satis¬ 
factory habitat and food. The Colorado potato beetle is a good ex¬ 
ample of this type. It is a native of the plains region at the base of the 
Rocky Mountains and its native food plant is the sand-bur, Solanum 
rostratum, a plant related to the potato. Upon the introduction of the 
potato, this beetle transferred its attention to that plant, and finding 
it much more to its liking, was able to multiply more rapidly and thus 
developed into a serious pest. A somewhat similar case is that of the 
grape Phylloxera, which is also a native insect and which became an 
important pest because of the introduction into the United States of the 
vinifera varieties of grapes from Europe, a type of grape which was a 
more favorable food plant than the native American grapes and which 
had developed no immunity to the pest. 
Neither of these insects has natural enemies of importance and there¬ 
fore the biological method is not applicable so far as at present under¬ 
stood. I cite these two cases merely to emphasize the fact that there 
are certain pests against which there is apparently no possibility of us¬ 
ing this method in a practical way. 
A general study of the problem indicates that the relative importance 
of insect enemies in the natural control of pests varies all the way from 
practically nothing, as in the cases above cited, to the most important 
of all factors making up natural control, as exemplified in the case of 
the Vedalia and the cottony cushion or fluted scale. Since, in Cali¬ 
fornia at least, the application of the biological method is practically 
confined to the use of insect enemies, the applicability of this method 
to the control of pests is dependent upon the availability of effective 
parasitic and predacious insects. It should be plain therefore that the 
degree of control which may be effected, may vary all the way from 
nothing to practically complete control, depending upon (1) whether or 
not effective natural enemies exist, and (2) whether introduced natural 
enemies find climatic and other environmental conditions satisfactory 
for their multiplication in their new habitat. This is, of course, a 
