INSECTS AFFECTING PARK AND WOODLAND TREES 635 



of the diseased tissues. Fresh excavations are bright green, showing that 

 the dentate mandibles are very effective tools. The gall is sometimes 

 inhabited by an uncletermined curculionid larva. 



This gall insect is preyed on by a parasite, determined through the 

 courtesy of Dr Howard as Urogaster forbesi Ashm. This gall- 

 making fly has been reared by Professor Webster from larvae burrowing in 

 the roots of clover and tunneling the pith of the common garden sun- 

 flower, and Mr Pergande has bred it from stems of ambrosia. 



Vagabond gall 



Pemphigus vagabundus Walsh. 

 A peculiar folded convolute mass of foliage some two inches in diameter, frequently 

 occurs on the tips of poplar twigs. 



This species appears to be rather common in the vicinity of Albany, 

 and the peculiar convolute, greenish galls or the dry brown remains of the 

 same may often be met with on poplar shoots [pi. 51, fig. 1 j. 



This gall is sometimes very abundant on the tips of certain cotton- 

 woods and poplars, and according to Walsh is much more numerous some 

 years than others. The old blackened galls hang on the twigs for several 

 seasons, thus giving them a characteristic appearance, particularly in winter. 

 The winged plant lice make their appearance in September, and the green, 

 shining, hollow gall appears the following summer. The skin of the latter 

 is quite thin and contains a single wingless plant louse, which is the parent 

 of the colony subsequently inhabiting this peculiar shelter. All become 

 winged in September and desert the gall. 



Poplar leafstem gall 



Pemphigus populi-transversus Riley 



Oval, somewhat elongated galls, with transverse openings, develop near the middle 

 of the leaf petioles of Cottonwood during the latter part of the summer. 



The galls are nearly ]/ 2 inch long, about 3/g inch in breadth, and of the 



same color as the leaf petiole. The mouthlike orifice, when the plant lice 



are mature, gapes, is nearly transverse and may extend two thirds the way 



