179 
as injuriously abundant, argues that the conditions favorable to its 
increase are seldom met with. Its complete disappearance in the orchard 
of Mr. Brown after one year of excessive abundance is a case in 
point: and the excessive multiplication in New York State in 189L 
was followed the next year, according to Mr. Slingerland, by scarcely 
any injury in comparison: while the fact that this particular invasion 
was the first one of any importance of which we have a report further 
emphasizes the view taken. The reasons for the sudden multiplica- 
tion and quite as sudden disappearance of this pest are difficult to 
give. A succession of two or three winters favorable to hibernation 
probably leads to the unusual multiplication, and this results in a con- 
dition of plants which is probably prejudicial to the further increase of 
the insect. Mr. Slingerland has pointed out that in the later summer 
broods the condition of the leaves which have been seriously attacked 
by the earlier broods becomes such that the insect can not thrive on 
them, and it is a common experience that the insect becomes markedly 
less abundant in the later summer broods. The green succulent foliage 
of the young spring growth is especially favorable and when the leaves 
become hardened and mature, and especially dry and innutritious, from 
having been already sapped of their vitality, they are distasteful and 
unsuited to the development of the later broods. In Mr. Emory's orchard 
the eggs were being placed most numerously on the few young shoots 
and water sprouts which were manifestly not numerous enough to 
sustain even a small percentage of the coming brood. It is of course 
possible that some insect disease worked the complete extermination 
noted in the case of Mr. Brown's orchard. 
The influence of parasitic and predaceous insects should be consid- 
ered in this connection, and a very interesting experience was had in 
the case of the Maryland invasion, which will be noted later. 
LIFE HISTORY. 
As a basis for the discussion of parasitic enemies and remedial 
measures I have briefly summarized the life history of this insect, 
more particularly from the careful account given by Mr. Slingerland. 
The adults hibernate in crevices in the bark of pear trees, and emerge 
with the first warm spring days, copulate, and begin the deposition of 
eggs before the leaves have expanded, placing them singly or in rows 
or bunches in creases of the bark of the twigs, on old leaf scars, about 
terminal buds, and later, after the leaves begin to unfold, on the leaves 
themselves, as already described. The egg-laying goes on during April, 
probably later in the north than as far south as Maryland. The larva* 
hatch in from ten to seventeen days, from ten to twelve days probably 
being the ordinary summer period, station themselves on the axils of 
the leaf-petioles, on the stems of the leaves, on the fruit, and over the 
surface of the leaves. The moment they begin feeding the secretion of 
honeydew takes place, and in a very short while the bulk of liquid will 
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