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 down 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 1s 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 
