ENTOMOLOGICAL SOCIETY OF ONTARIO. 83 



but it affords another explanation of the reason why the fight with 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 coinplicited. The general laws govern- 

 ing 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 phytophag- 

 ous species will, cceteris paribus, be better qualified to keep it in check than some newly 

 introduced competitor from a foreign country, and the ppculiar 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 phytophage 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 vvhen the 

 phytophage is a foreigner and has been brought over without the enemy, which subdues 

 it in its native home. Icerya had some enemies in California, presumably American but 

 they were not equal to the task of subdumg it. Vedalia, in the Teerya'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 specie-i had excep- 

 tionally advantageous attributes. But there is very little to be hop^d from the miscell- 

 aneous introduction of predaceous or parasitic insects for the suppression of a phytopha<^'e 

 which they do not suppress in their nitive home or in the country from which they are 

 brought. 



The results of the introduction by Mr. A. D. Hopkins of Clerus fonalcarius to 

 contend with the Scolytids which were ruining the West Virginia 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 intro- 

 duced into Massachusetts without its European natural enemies, and as in some parts o? 

 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 efiective 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 correspondence with the Secretary of the Board. In 

 neither of these cases should we expect the predaceous or para.sitic forms to subdun their 

 hosts more effectually in America than they do in Europe, except in so far as they were 

 relieved, in the introduction into America, of whatever enemies they possessed in theii" 

 native home. 



There are two other laws which it is worth while to consider in this connection. 

 One is, that while a plant-feeder's natural enemies are apt to cause its excessive abundance 

 to be followed by a corresponding decrease, yet this alternation of excessive abundance 

 and excessive scarcity will often be produced irrespective of such natural checks. An 

 injurious insect which has been on the destructive march for a period of years will often 

 come to a sudden halt, and a period of relative, and sometimes complete, immunity from 

 injury will follow. This may result from climatic conditions, but more ot^ten it is a 

 consequence of disease, debility, and want of proper nutrition, which are necessary 

 corrollaries of undue multiplication. Fiequently, therefore, it may be inaccurate and 

 misleading to attribute the disappearance of a particular injurious species to some parasitic 

 or predaceous species which has been let loose upon it, and nothing but the most accurate 

 ob.servation will determine the truth in such cises. The past year furnished a very 

 graphic illustration in point. Throughout Virginia and West Virginia, where the spruce 

 pines have for some years suffered so severely from the destructive wjrk of D-'Aidroclonus 

 frontalifi, not a single living specimen of the beetle has been found during the present, 

 year. This has been ob.served by e%ery one who has investigated the subject, and particu- 

 larly by several correspondents who have written to me ; by Mr. E A. Schwarz, who 

 was commissioned to investigate the facts, and by Mr. Hopkins, who has made the study 

 of the subject a specialty. 



*Insect Life, hi., p. 3G'J, ff. 



