139 
good. This is particularly true of those of our Coccidrc and Aphi. 
didse and those of our Lepidopterous larvae which havu numerous natu- 
ral enemies of their own class; and it not only emphasizes the impor- 
tance of preventive measures, which we are all agreed to urge for other 
cogent reasons, and which do not to the same extent destroy the para- 
sites, but it affords another explanation of the reason why the fight tvith 
insecticides must be kept up year after year, and has little cumulative 
value. 
But the problem of the wise encouragement and employment of the 
natural enemies of injurious insects in their own class is yet more com- 
plicated. The general laws governing the interaction of organisms are 
such that we can only in very exceptional cases derive benefit by 
interference with them. The indigenous enemies of an indigenous phy. 
tophagous species will, cceteris paribus, be better qualified to keep it 
in check than some newly introduced competitor from a foreign coun- 
try, and the peculiar circumstances must decide in each case the 
advisability of the introduction. The multiplication of the foreigner 
will too often involve the decrease of some indigene. If a certain phy- 
tophage is generally disastrous in one section and innocuous in another 
by virtue of some particular enemy it will be safe to transfer and 
encourage such enemy, and this is particularly true when the phy- 
tophage is a foreigner and has been brought over without the enemy 
which subdues it in its native home. Icerya had some enemies in Cali- 
fornia, presumably American, but they were not equal to the task 
of subduing it. Vedalia, in the leery a's native home, Australia, was 
equal to the task and maintained the same superiority over all others 
when brought to America. The genus was new to the country and 
the species had exceptionally advantageous attributes. But there is 
very little to be hoped from the miscellaneous introduction of preda- 
ceous or parasitic insects for the suppression of a phytophage which 
they do not suppress in their native home or in the country from which 
they are brought. 
The results of the introduction by Mr. A. D. Hopkins of Clerus formi- 
carius to contend with the Scolytids which were ruining the West Vir- 
ginia pines were doubtful, for the reason that indigenous species of the 
genus were already at work in America. Yet the experiment was safe 
and desirable, because the European Clerus is more active and more 
seemingly effective than our indigenes. The Gypsy Moth was evidently 
introduced into Massachusetts without its European natural enemies, 
and as in some parts of Europe it is often locally checked by such natural 
enemies, a great number of which are known, a proper study of them 
and the introduction of the most effective could result in no possible 
harm and might be productive of lasting good. Such a course was 
advised by me at a conference upon the subject held in the rooms of 
the State Board of Agriculture, Boston, March 4, 1891,* and in corre- 
* Insect Life, hi, p. 369, ff. 
