INTERRELATIONS OF INSECTS 257 



The caterpillar of Hemerocampa (Orgyia) leucostigma defoliates shade 

 trees in the northeastern United States. An enormous increase of this 

 species in the city of Washington in 1895 was attended by a correspond- 

 ing increase of parasitic and predaceous species, and this unusual oppor- 

 tunity for the study of parasitism was made the most of by Dr. Howard, 

 from whose admirable paper these facts are taken. 



The primary parasites of H. leucostigma numbered 23 species 17 

 Hymenoptera and 6 Diptera; of the hyperparasites (all hymenopterous) 

 13 were secondary, 2 and probably 5 were tertiary, and one of these 

 (Asecodes albitarsis) may under certain conditions prove to be a quater- 

 nary parasite. To illustrate The ichneumon Pimpla inquisitor, an 

 important primary parasite of lepidopterous larvae, lays its eggs in cater- 

 pillars of H. leucostigma; its larvae suck the blood of their host and at 

 length spin their cocoons within the loose cocoon of the Hemerocampa. 

 These cocoons have yielded a well-known secondary parasite, the chalcid 

 Dibrachys boucheanus. Now another chalcid, Asecodes albitarsis, has 

 been seen to issue from a pupa of this Dibrachys, thus establishing tertiary 

 parasitism. Furthermore, it is quite possible that Dibrachys itself is a 

 tertiary parasite, in which event the Asecodes might become a parasite 

 of the quaternary order. 



Economic Importance of Parasitism. If a primary parasite is 

 beneficial, its own parasites are indirectly injurious, generally speaking; 

 while those of the third and the fourth order are respectively beneficial 

 and injurious. The last two kinds are so rare, however, as to be of no 

 practical importance from an economic standpoint. The first two kinds 

 are of immense economic importance, particularly the primary parasites. 

 "Outbreaks of injurious insects," says Howard, "are frequently stopped 

 as though by magic by the work of insect enemies of the species. Hub- 

 bard 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 insects, 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 growers 

 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. 



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