to the survival and increase of the parasite 
that produced the control. However, this is 
only one of the few possibilities that have 
been exploited among, probably, many 
thousands that have yet to be explored. 
The third approach, the liberation of 
quantities of parasites or predators of kinds 
that are already present, also is used with 
success in a few situations. Trichogrammid 
egg parasites of various pests and ladybug 
beetles that prey upon aphids, mealybugs, 
and scale insects are sold commercially for 
this purpose. In general, though, relatively 
more attention seems to be given to this ap- 
proach in the Soviet Union and China than in 
North America, perhaps because of a relative 
scarcity of chemical pesticides there com- 
bined with greater control over what is be- 
lieved to be best for the growers. 
The difficulty in using living insecticides 
of such kinds is that it is usually difficult to 
produce them at economic cost. The problem 
is that they have to be reared on living in- 
sects that themselves have to be reared, The 
widespread use of parasites and predators as 
living insecticides awaits the development of 
artificial diets on which they can be mass- 
produced at low cost in the quantities and at 
the time that they are needed, 
I now come to the fourth main way of 
manipulating parasites and predators--by 
adding new kinds imported from elsewhere. 
This is of course what most people think of 
when they hear the term "biological control," 
though in fact that term covers a much wider 
range of procedures, including, among various 
others, those that I have referred to already. 
This pest-control method has a unique ad- 
vantage, which arises from the fact that the 
control agents, the parasites or predators, 
are living organisms. This advantage is that 
the control is self-perpetuating, Parasites or 
predators not only can reduce a pest popula- 
tion but also can keep it down permanently 
without further human assistance. The result 
is that the pest ceases to beapest or becomes 
one of relatively minor significance. Obviously 
to make a pest insect a nonpest is far better 
than to have to apply temporarily effective 
controls regularly. 
Various pests in many countries have been 
brought under control in this way. To take 
Canada as an example, there are over a dozen 
67 
former pests that are now of less or no 
economic importance because they are perma- 
nently controlled by parasites or predators 
originally imported from abroad and estab- 
lished here. They include, for instance, the 
wheatstem sawfly in Ontario (though not in the 
Prairies), the woolly apple aphid, the oyster- 
shell scale, the apple mealybug in British Co- 
lumbia, and the larch casebearer in Ontario. 
The potential of this control method is by 
no means exhausted anywhere. Again I will 
take Canada as an example. Including forest 
insects, we probably have about 3,000 species 
of insects that can cause economic harm. 
Probably several hundred of them have the 
status of permanent pests that regularly cause 
sufficient harm to warrant control measures. 
Control with imported parasites or predators, 
that is, self-perpetuating, permanent control, 
has been attempted against up to 35 of them, 
or little more than 1 percent of the total and 
under 10 percent of the significant pests. So 
far as I can estimate, the corresponding pro- 
portions for the United States, Australia, and 
other countries where this method has been 
widely applied are similar. Though many pests 
are not amenable to control in this way, it 
is nevertheless apparent that many possibil- 
ities remain to be explored. 
A reason for increasing the role of para- 
sites and predators in control programs is 
that they lack those disadvantages of chemical 
pesticides that have caused so much informed 
and misinformed comment in recent years: 
(1) They are usually specific to the pests 
that they attack, at least for all practical 
purposes. This means that they are usually 
harmless to one another and to other control 
agents, and consequently, unlike chemicals, 
do not tend to intensify existing pest problems 
or to create new ones. 
(2) There is no problem whatever with toxic 
residues; and resistance by pests develops 
so rarely and slowly, as compared with re- 
sistance to chemical pesticides, as to be 
normally insignificant. 
(3) Because they are alive, they are self- 
perpetuating, self-increasing, and self- 
dispersing and can continue to produce their 
control effects indefinitely without continuing 
human assistance. Moreover, they actively 
search for the pests that they attack and 
spread into new areas. 
