INSECT ENEMIES OF EASTERN FORESTS 59 
host and parasite. Unless an absolute measure of host abundance 
is given, therefore, this percentage means very little. If the host popu- 
lation is suddenly reduced by lack of food or by some climatic factor, 
the percentage of parasitization will show a marked increase because 
of a concentration of parasites among the remaining host population. 
Thus, over a series of years the host may increase or decrease in actual 
abundance and the percentage of parasitization may show a coincident 
decrease or increase without any actual change in the parasite popu- 
lation. Parasites are therefore often erroneously credited with check- 
ing outbreaks, when, as a matter of fact, some other factor was 
primarily responsible. 
In some instances parasites may prolong outbreaks. In connection 
with an outbreak of the forest tent caterpillar in the Chippewa and 
Superior National Forests of Minnesota from 1935 to 1937, L. W. 
Orr found that had it not been for the retarding effects of parasites 
this outbreak might have died out quickly because of starvation of 
the larvae, before serious damage was done to the host trees by repeated 
annual defoliation. It should not, however, be inferred that over 
a long period of years the parasites of the forest tent caterpillar are 
detrimental, for they may play a major role in holding the host in 
check during long periods between outbreaks. 
The Governments of the United States and Canada have carried out 
a combined attack on the European spruce sawfly by the troduction 
of its natural enemies from Europe. This sawfly threatened the de- 
struction of large areas of spruce in eastern Canada and the north- 
eastern part of the United States. 
‘Some parasites attack a great variety of hosts. <A striking example 
of this is Compsilura concinnata Meig., a parasitic fly mtroduced 
originally into New England from Europe as a parasite of the gypsy 
moth. Since its establishment some 40 years ago it has been found 
to attack over 140 species of native caterpillars. A species of such 
polyphagous habits offers excellent possibilities for utilization as a 
natural enemy of forest insects in other regions of the United States. 
On several occasions during the past 30 years C. concinnata has 
been reared by the Division of Forest Insect Investigations from 
the gypsy moth in New England and shipped to other parts of the 
United States for release against injurious insects. It is known to 
be established in the State of Washington as an enemy of the satin 
moth, and subsequent rearings may reveal its presence elsewhere. 
Parasites of two species of western sawflies have been reared at the 
New Haven, Conn., laboratory of this Division and have been tried out 
as possible parasites of the European spruce sawfly. A native sawfly 
attacking red pine in New England was also subjected to attack by 
the western parasites. These species of western parasites attacked 
the red pine sawfly and developed readily on it and one attacked and 
developed on the European spruce sawfly. The red pine sawfly was 
definitely preferred to the European spruce sawfly probably because 
it was more closely related generically to the western host species from 
which the parasites were obtained. These cases illustrate the possi- 
bilities of biological control within continental North America through 
transfer of parasites from one region to another. Certain species of 
spruce budworm parasites are now being transferred from the Rocky 
Mountain region to budworm infestations in the Northeast. 
