558 THE ECONOMIC STATUS OF INSECTS AS A CLASS. 



selves affected by other living causes in an almost endless chain, some- 

 times, to all appearance, dwarf even temperature as a controlling factor. 



There is not a species of insect that has not its natural enemies in 

 the guise of other insects; there is not one of these other insects which 

 has not its own insect foes. From a single species of Bombycid moth, 

 the larva? of which frequently damage forests in Europe to an alarm- 

 ing extent, there have been reared no less than 60 species of hymen- 

 opterous parasites. From a single caterpillar of Plusia brassicce have 

 been reared 2,528 individuals of a little hymenopterous parasite, Copi- 

 dosonia truncatellum. 1 



Outbreaks of injurious insects are frequently stopped as though by 

 magic by the work of insect enemies 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 90 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 97 per cent of the caterpillars of one of our 

 most important shade-tree pests 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 practi- 

 cally 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. 



The control of a plant-feeding insect by its insect enemies is an 

 extremely complicated matter, since, as we have already hinted, the 

 parasites of the parasites play an important part. The undue multi- 

 plication of a vegetable feeder is followed by the undue multiplication 

 of parasites, and their increase is followed by the increase of hyper- 

 parasites. Following the very instance of the multiplication of the 

 shade-tree caterpillar just mentioned, the writer was able to determine 

 this parasitic chain during the next season down to quaternary para- 

 sitism. Beyond this point true internal parasitism probably 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 parasites, and favored again by an abundance of quaternary 

 parasites. 



The subject of practical handling of insect enemies of insects has 

 come into great prominence during the past ten years. The sugges- 



^his observation, which for some years "held the record," as the expression is, 

 was made by Mr. Pergande, of the United States Department of Agriculture. 

 Recently, however, Prof. A. Giard, of Paris, has more than 3,000 specimens of the 

 same parasite reared from a Plusia caterpillar. 



