179 



as injuriously abundant, argues that the conditions favorable to its 

 increase are seldom met with. Itscouipletedisapiiearauce in the orchard 

 of Mr. Brown after one year of excessive abundance is a case in 

 point; and the excessive nuiltiplication in New York State in 1891 

 was followed the next year, according to Vr. Slingerland, by scarcely 

 any injury in comparison; while the fact that this particular invasion 

 was the first one of any imi)ortance of whicli 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 diificult 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 i)rejudicial 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 

 tliem, 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 i.s especially favorable and when the leaves 

 become hardened and mature, and especially dry and innutritions, from 

 having been already sapped of their vitality, they are distasteful and 

 unsuitedtothedevelopmentof 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 Avorked 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, wliich will be noted later. 



LIFE HISTORY. 



As a basis for the discussion of parasitic enemies and reniedial 

 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 exj)anded, 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. Thelarvte 

 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 

 5210—^0. L* 9 



