420 



AN ECONOMIC ENTOMOLOGY. 



Even though in each year a certain species may be, in the long 

 run, controlled by its parasite, this does not necessarily help the 

 farmer. Let us take, for instance, the cut-worms that are so 

 abundant each year. If we gather from an infested field two or 

 three hundred specimens, we find that anywhere from fifty to 

 seventy-five per cent, of them are parasitized. This looks like a 

 huge destruction ; but when we consider that the parasites do 

 not, as a rule, affect the larvae until they have done all their 

 feeding, what benefit has the farmer derived ? What difference 

 does it make whether the cut- worm that destroys his plants is 

 parasitized or not ? At the end of the season we may note that 

 twenty-five per cent, only of the cut-worms develop into moths : 

 half of those may be males, and the remainder are females which 

 will each lay anywhere from three to five hundred and some- 

 times more than one thousand eggs ; amply sufficient to pro- 

 duce in the next brood fully as many caterpillars as existed in 

 the beginning ! Let us further take the case of the cranberry 

 Teras as an illustration. The first brood, early in the season, 

 has scarcely any parasites ; the second brood, in mid-summer, 

 is parasitized to the extent of nearly fifty per cent. ; while from 

 the third brood, in the fall, we get fully seventy-five per cent, 

 of parasites to twenty-five per cent, of moths. Yet, notwith- 

 standing this enormous increase of the parasites in the course 

 of the season, some combination of circumstances destroys so 

 many of them during the winter that in the spring following 

 the first brood is again almost free from attack ; and thus it 

 goes, year after year. The action of the parasite is only to keep 

 its host within a certain fairly well defined limit, and if that 

 limit is at a point where the host becomes injurious to the farmer, 

 he must himself take a hand in its destruction to derive any 

 benefit. I do not mean, in anything I say here, to lessen the 

 claims of parasitic and predaceous insects to our consideration. 

 They have an extremely important function in nature, and with- 

 out them there would be no possibility of an existing vegetation. 

 But, on the other hand, it is just as certain that unless there was 

 some check imposed upon the multiplication of parasites, etc., 

 they would in a short time destroy the insects that they feed 

 upon, and the destruction of their host would, logically, carry 

 with it their own destruction. Nature never works in that way : 



