December, ’23] 
smith: biological control 
507 
This parasite thrived wonderfully under California conditions and gave 
promise for a time of doing to the black scale what the Vedalia did to 
the cottony cushion scale. As time passed, however, and the black 
scale failed to disappear from the orchards in spite of comparatively 
heavy parasitism, this second wave of enthusiasm for the biological 
method of pest control began to lose its force. From then until 1918 
was a period during which there was no more than a very general interest 
in this method. In 1918 the work of the California State Department 
of Agriculture on the biological control of the citrus mealybug began to 
make itself felt and interest in this subject began to reawaken. It was 
not, however, until the successful introduction and establishment by the 
Department of Aphycus lounsburyi Howard as an aid in the control of 
black scale, that interest in biological control again reached its height. 
History repeated itself, and there was a tendency again to expect greater 
results than the facts in the case warranted. However, this wave of 
what might be termed “over-enthusiasm” has receded, and there is now, 
I believe, a pretty general understanding on the part of the growers of 
just what may reasonably be expected in the way of control of the black 
scale by Aphycus. 
My purpose in calling attention to the effect on the interest of the fruit 
grower of these various efforts to control pests by the biological method 
is to bring about a better understanding as to just what part natural 
enemies should play in our general scheme of pest control. Over¬ 
enthusiasm generally has a reaction which is not beneficial. The greatest 
good will come from biological control only when the majority of our 
agriculturists have a better understanding of the principles involved 
and the limitations to which the work is subject. 
In appraising the possible value of the biological method as a pest 
control measure, there are certain more or less fundamental biological 
principles which must be taken into consideration. 
In the first' place, just what is meant by “biological control?” It 
means the suppression of insect pests by the use or encouragement of 
those organisms which in nature tend to reduce their numbers. All 
forms of life are subject to the action of factors which limit their in¬ 
crease. The most important of these are meteorological conditions, 
which include heat, rain, cold and drought; limited food supply; 
fungous and bacterial diseases, which destroy numbers of insects, 
especially in the more humid climates; and predatory and parasitic 
enermes. These various limiting factors taken together form what is 
known as “natural control.” 
