INTERRELATIONS OF INSECTS 313 



quently stopped as though by magic by the work of insect ene- 

 mies of the species. Hubbard found, in 1880, that a minute 

 parasite, Trichogramma pretiosa, alone and unaided, almost 

 annihilated the fifth brood of the cotton worm in Florida, fully 

 ninety per cent, of the eggs of this prolific crop enemy being 

 infested by the parasite. Not longer ago than 1895, in the 

 city of Washington, more than ninety-seven per cent, of the 

 caterpillars of one of our most important shade^tree pests 

 [Orgyia, as just mentioned] were destroyed by parasitic in- 

 sects, to the complete relief of the city the following year. 

 The Hessian fly, that destructive enemy to wheat crops in the 

 United States, is practically unconsidered by the wheat grow- 

 ers of certain states, for the reason that whenever its numbers 

 begin to be injuriously great its parasites increase to such a 

 degree as to prevent appreciable damage. 



" The control of a plant-feeding insect by its insect enemies 

 in an extremely complicated matter, since, as we have already 

 hinted, the parasites of the parasites play an important part. 

 The undue multiplication of a vegetable feeder is followed by 

 the undue multiplication of parasites, and their increase is fol- 

 lowed by the increase of hyperparasites. Following the very 

 instance of the multiplication of the shade-tree caterpillar just 

 mentioned, the writer [Howard] was able to determine this 

 parasitic chain during the next season clown to quaternary 

 parasitism. Beyond this point, true internal parasitism prob- 

 ably did not exist, but even these quaternary parasites were 

 subject to bacterial or fungus disease and to the attacks of 

 predatory insects. 



"The prime cause of the abundance or scarcity of a leaf- 

 feeding species is, therefore, obscure, since it is hindered by 

 an abundance of primary parasites, favored by an abundance 

 of secondary parasites (since these will destroy the primary 

 parasites), hindered again by an abundance of tertiary para- 

 sites, and favored again by an abundance of quaternary para- 

 sites." 



Entomologists have made many attempts to import and 



