A STUDY IN INSECT PARASITISM. 
By L. O. Howard. 
INTRODUCTION. 
The white marked tussock moth (Orgyia leucostigma Smith & Abbott) 
is an insect which prior to the early seventies was known mainly from 
the occasional defoliation of the leaves of orchard trees by its larvre. 
Harris referred to it as an enemy of apple trees and rose bushes and 
sometimes of "other trees and shrubs." Fitch mentioned it as an 
enemy of apple trees and rose bushes; while Riley, in his First Mis- 
souri Report (18G9), referred to it as an apple insect, but stated that it 
feeds upon differeut kinds of trees, such as elm, maple, horse-chestnut, 
and oak, seeming, however, to prefer the apple, plum, rose, and pear. 
Since a somewhat indefinite date, which we can fix no closer than 
the early seventies, this insect has become more or less prominent as 
an enemy to shade trees in the cities of the northeastern United States, 
and correspondingly has become less prominent as an enemy to 
orchards. The beginning of its rapid increase in our cities is nearly 
coincident with the beginning of the remarkable multiplication of the 
English sparrows after their introduction into this country, and there 
seems little doubt that this coincidence is really a matter of cause and 
effect. One of the early results of the introduction of the English 
sparrow was the x>ractical extermination by this bird of the canker- 
worms, which at that time were the principal insect enemies of our city 
shade trees. The removal of the cankerworms afforded room for the 
multiplication of the tussock moth, which, from the fact that its larva 
is hairy, was not eaten by the sparrows, and consequently multiplied 
with rapidity. 1 Furthermore, the tussock moth must be considered as 
one of those species which are becoming attached to cities — which are 
slowly altering their habits and accommodating themselves to city 
environment. 
For many years the shade trees in more northern cities, and notably 
in Boston, New York, Brooklyn, and Philadelphia, have suffered 
severely from the attacks of this hairy caterpillar. Until recently, how- 
ever, the species has not been excessively abundant in the city of 
'This supplanting of the one species by the other was also undoubtedly due in 
part to the driving away by the sparrows of the native birds which previously had 
fed upon the tussock-moth caterpillars. Le Conte has shown (Proc. Am. Assoc. Adv. 
Sci., vol. 23, p. 44, 1874) that the larva of Unnomos subsignaria, a measuring worm 
which had been very injurious to the shade trees of Philadelphia, was replaced by 
the tussock-moth caterpillars through the sparrows eating the former and avoiding 
the latter^. 
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