ENTOMOLOGICAL SOCIETY OF ONTARIO. 85 



that their economic value has been grossly over-estimated : in fact I am almost ready to 

 say that parasites have no real economic value to the agriculturist. This sounds like a 

 very radical statement, and perhaps I do not mean it in the fullest sense of the terms 

 that I have used, but I would not much modify the sense of the language. The " life 

 history " of an insect is incomplete until we know not only how it lives and upon what it 

 feeds, how it transforms, and the duration of its various stages, but also what species 

 prey upon ic, and to which it furnishes sustenance in one or the other of its stages. We 

 are therefore right in our studies of the " life history " of injurious insects in studying 

 also the parasites that prey upon them. We are right also in publishing the results of 

 our work, including the descriptions of the parasites. We are right in calling the atten- 

 tion of the farmer to the fact that the injurious species are very largely kept in check by 

 either parasites or by predaceous insects ; but we are wrong in leading him to suppose 

 that either para ites or predaceous insects will control the injurious species for him. Yet 

 the tendency of the language u.sed in many cases by entomologists, and more often by 

 those who are not entomologists, has suggested the possibility that injurious species may 

 be controlled by either parasites or natural enemies without very much work on the part 

 of the farmer. The impression is current that it will be possible to use natural means 

 to exterminate injurious insects, and I have been asked frequently during the past two 

 years, by farmers who may be considered as fully equal in intelligence to the best in the 

 land, those who read and usually understand, why I did not make some effort to cultivate 

 or import parasites or natural enemies of our common injurious insects. Of course these 

 questions all grow out of the remarkably successful experiment made by Dr. Riley in the 

 importation of the Australian Vedcdla cardinalis to exterminate the imported Icerya 

 purchasi, and I have decided to bring up this subject for discussion at the present meeting 

 in order that possibly a little more definite light can be obtained upon the exact place of 

 parasites and predaceous insects in economic entomology. It needs no argument on my 

 part to prove that nature never creates organisms merely to destroy others that she had 

 previously created. Parasites do not exterminate their hosts in -any instance ; their 

 mission is merely to interpose a check to undue increase, and it is natural that this should 

 be so, for were the host destroyed the parasite itself would perish, unless it were able to 

 change its food and prey upon other species. It is by no means improbable that in the 

 past certain species have been exterminated by their parasites, and, indeed, it is very 

 probable that some such cases are in progress now. Many lepidopterous larvae are rarely 

 found free from parasites, and the adults are among the rarest of our species. Here we 

 have instances where the parasite very materially lessens the nuniber of the host and 

 allows each year only a very few specimens to escape. It is only through the fecundity 

 of the species that it is enabled to maintain itself at all. These cases are exceptional. 

 Usually the relation of the parasite to its host is more moderate. Excessive increase is 

 checked, but excessive increase only. There is always a very large pi-oportion of larvaj 

 and usually a comparatively small proportion of parasites. Nature tends to preserve a 

 balance among her creatures, and a balance only. ]\Iany species which are much subject 

 to parasites are abundant each year, and remain equally abundant from year to year, 

 varying only very slightly, and these variations are rarely the result of an excessive 

 increase of parasites. Nature also works very slowly, and she adapts insects as well as 

 other animals to their environment only by means that require ages for their completion. 

 Insects that are confined to plants which, under natural conditions are not common, need 

 few parasites to keep them in check. The great difiiculty in finding food is in itself a 

 sufiicient cheek, and parasites are not necessary, indeed they could not be supported 

 under the circumstances. If, by any unnatural condition introduced by man, the supply 

 of food for this otherwise rare insect is suddenly increased, it obtains the possibility of 

 multiplying rapidly, while the number of parasites do not increase proportionally. In 

 the course of time nature may make a change and other species may attack this form 

 which has now increased abnormally ; but this is something that the farmer can not wait 

 for ; he must have some means of dealing with the insect at once, and he must leave the 

 operations of nature to benefit his descendants. The spread and increase of the 

 potato beetle, Doryphora 10-lineata, is a case in point. Here neither parasites nor 

 natural enemies assist the farmer in any noticeable way. He must depend upon his 



