December, ’23] 
smith: biological control 
509 
statement of perfectly obvious facts, but there are many complications 
which enter into the situation and determine the degree of control which 
may be brought about. 
Assuming that natural enemies of apparent value are located and 
successfully shipped to destination, that their life-history and habits 
are definitely determined and that a sufficient breeding stock is on hand, 
upon what conditions does practical success depend? Among known 
factors which bear on this question the following are important: 
There must be the proper co-relation between the life-history of the parasite 
and that of the pest. No better demonstration of this principle could be 
asked than that exhibited by the black scale and the Aphycus. Aphycus 
thrives in the coastal areas of the state, where the life-history of the black 
scale is such that the stages of development upon which Aphycus is able 
to breed exist at all times of the year. On the other hand, it does not 
thrive in the interior, where for several months at a time these stages 
are not present and the parasite is therefore unable to maintain itself in 
abundance for lack of the proper hosts. This principle is fundamental. 
It is conceivable however that we may find parasites whose life-histories 
more nearly correspond to that of the black scale under interior con¬ 
ditions. The same is probably true of the citricola scale, which has only 
one generation a year. On the other hand the red and purple scales 
have several generations per year and from this standpoint are more 
favorable subjects for biological control. Because most scale parasites 
are short-lived and must pass through several generations a year to 
maintain themselves in abundance, it follows that scale insects such as 
the red and purple scales, the black scale under coastal conditions and 
the mealybugs are more favorable subjects for biological control than 
are such pests as the citricola scale or the black scale under interior 
conditions. There are however two possibilities by which these latter 
pests might be brought under control by the biological method. We 
may succeed in finding parasites which have life-histories corresponding 
to that of these scales or predators which feed on all stages, or we may 
develop methods of propagation of the natural enemies in such a way 
that we can afford to stock the orchards artificially every year with the 
parasites. The latter procedure may seem far-fetched, but the possibili¬ 
ties of this kind have not been exhausted. The control of these pests 
costs the citrus growers as much as $50 per acre per year. A very 
considerable sum could be expended in the production of natural 
enemies for orchard use, and still leave a margin of profit over fumigation 
costs, provided, of course,- it could be done with equal effectiveness. 
