moles, and mice also devour many insects. A shrew, one of the rodents, 
was distributed by the Canadians in Newfoundland forests in 1958 to help 
control the larch sawfly. But these larger species are rarely used delib- 
erately to control insects. 
Advantages of Biological Control 
Use of parasites and predators for insect control presents some ad- 
vantages not possible with chemicals. Because they are living forms, 
parasites and predators can be self-perpetuating if successfully established. 
The difficulty is in getting them established. 
In those instances where parasites and predators have been established 
they become a part of the fauna to survive adverse weather, reduced food 
supply, secondary parasites and insecticides. 
Biological control agents are virtually harmless to all but their par- 
ticular hosts. Even if they parasitize or preyupon several different species, 
their diet is so restricted they feed only upon other insects. They, therefore, 
present no hazard to the health of man, animals, or plants. 
Biological control agents may be effective when used in combination 
with other control measures. It is even possible to employ insecticidal 
sprays without harming many of the beneficial parasites and predators if 
the sprays are carefully selected and applied at just the right time. The use 
of insect pathogens in combination with other biological control agents or 
even insecticides is also an especially promising field. 
The initial outlay necessary to discover, import and breed large num- 
bers of a parasite for colonization is quite high, but the savings resulting 
from the self perpetuation of an efficient biological control agent can be 
substantial. 
Limitations of Biological Control 
Biological control is no panacea for meeting the insect problems of 
agriculture despite its merits. One ofthe chief problems in biological control 
through the introduction of parasites andpredatorsisthe difficulty of getting 
them established, and in sufficient numbers to be effective. 
Another important shortcoming of biological control is that practically 
total control of some pests is necessary in order for the producer to meet 
current market standards and to safeguard the value of the crop. Biological 
control agents by themselves, however, seldom destroy allof a pest species 
even in restricted areas. 
A moderate infestation of an insect pest ona forest, grain or fiber crop 
can often be tolerated even though substantial loss may result, but the fruit 
grower of today cannot tolerate even 5 percent damage by codling moth in 
apples or just a few fruit fly larvae in a basket of cherries, because of the 
consumer demand for clean fruit and because of pure food regulations which 
are in operation in most States. Also a 25-percent survival of the citrus 
black scale, for instance, may start a serious outbreak the following year. 
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