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^be Iflee of paraeitic & ipre&aceoue Sneecte.* 



By Clarence B. Weed. 



THERE has recently been much discussion concerning the 

 ing utilisation of parasites and predaceous insects in destroy 

 injurious species. A knowledge of the conditions under 

 which such insects act would render it evident that we cannot hope 

 to exterminate any species of noxious insect by means of its para- 

 sites alone; and many too sanguine expectations have been aroused- 

 But, on the whole, parasitic and predaceous insects are of immense 

 service to man. Without them many plant-feeding species would 

 multiply to such an extent that the production of certain crops 

 would require vastly more effort than it does now. To say, as has 

 been said, that parasitic and predaceous insects have no economic 

 value, is to put the case too strongly. Take, for example, two crop 

 pests of the first class — the army worm and the hessian fly. The 

 history of a century shows that these insects fluctuate in numbers ; 

 that there are periods of immunity from their attacks, followed by 

 seasons when they are overwhelmingly abundant. It is universally 

 acknowledged that in the case of the hessian fly, this periodicity is 

 due almost entirely to the attacks of parasites, and in the case of 

 the army worm to the attacks of parasites, predaceous enemies, and 

 infectious diseases. Remove these checks and what would be the 

 result? The pests would keep up to the limits of their food supply 

 and would necessitate the abandonment of the culture of the crops 

 on which they feed. Take another case : — Professor J. B. Smith 

 has argued that " under ordinary conditions neither parasites nor 

 predaceous insects advantage the farmer in the least," and to prove 

 it he cites this instance : — " Fifty per cent, of the cutworms found 

 in a field early in the season may prove to be infected with para- 

 sites, and none of the specimens so infested will ever change to 

 moths that will reproduce their kind. Half of the entire brood has 

 been practically destroyed, and sometimes even a much larger pro- 

 portion; but — and the ' but ' deserves to be spelled with capitals — 

 these cutworms will not be destroyed until they have reached their 

 full growth, and have done all the damage to the farmer that they 

 could have done had they not been parasitised at all. In other 



* American Naturalist. 



