5S MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
IMPORTATION OF PARASITES AND PREDATORS 
In 1892 A. D. Hopkins introduced Thanasimus formicarius (M.) 
from Germany into West Virginia, in an attempt to control the 
southern pine beetle (Dendr octonus frontalis Zimm.). This marked 
the beginning of the importing of natural enemies of forest insects 
into this country. Gypsy moth parasites and predators were 1m- 
ported from foreign countries from 1905 to 1930. This project was 
perhaps the most ‘extensive of its kind im history. Information on 
methods and technique of rearing and shipping parasites obtaimed 
from this extensive experiment has been largely the basis for work 
with the parasites of a great many insects In later years. This pioneer 
undertaking has demonstrated in many ways the obstacles, as well as 
some of the achievements, of this type of endeavor. 
Parasites of eggs, larvae, and pupae of the gypsy moth were intro- 
duced into the United States from Europe, Africa, and the Orient. 
Some of the predators of larvae and pupae of this moth also were 
imported from Europe. A number of natural enemies have become 
established, and several are now present throughout the gypsy moth 
area in this country. Their aggregate effectiveness In certain areas 
approaches that obtained in central “Europe. The gypsy moth repre- 
sents a type of forest defoliator that will probably never be completely 
controlled by natural enemies. Even in Europe, where it is appar- 
ently an indigenous species, outbreaks occur from time to time when 
conditions for increase are favorable. There are, however, certain 
areas in Europe, and in this country as well, where outbreaks of the 
e@ypsy moth are rare, and some of the parasitic species may be respon- 
sible for such conditions, 
Parasite introduction is more or less a trial-and-error project. This 
makes it necessary to import and liberate a large number of species 
in the hope that some may find conditions favorable for increase. Of 
course this does not mean that an analysis of the factors of environ- 
ment is unimportant, for sometimes there is some very obvious reason 
why a given species could not become an effective enemy of its host 
when introduced into a new region, such as the absence of an alter- 
nate host, the lack of some climatic requirements, or the lack of syn- 
chronism with the seasonal development of the host. 
Some natural enemies of nearly all our important introduced forest 
insects have been imported and released. Usually several years are 
required to obtain and colonize successfully a complete series of natural 
enemies which attack the different stages of the host. When a para- 
site has become firmly established in a given area its further dispersal 
can often be aided by recolonization in other areas of infestation of 
the same host. 
Because of the effect of the various factors of an environment on a 
given organism, it is difficult to place a quantitative value on the effec- 
tiveness of a parasite or group of parasites in controlling a given 
host. Obviously, in order to evaluate properly, the effect of any one 
factor of control it is necessary to know the effect of the other con- 
trolling agents. 
The conventional method of expressing parasite abundance is in 
percentages of hosts attacked by the parasites. The percentage of 
parasitization, however, simply indicates the relative abundance of 
