76 



INJURIOUS AND BENEFICIAL INSECTS OF CALIFORNIA. 



latter of which later fly back to the cottonwood trees to produce sexual 

 forms which mate and the females lay the overwintering eggs. 



Distribution. — This species is widely distributed throughout the 

 State. 



Food Plants. — This plant louse often infests roots in such a way 

 as to ruin portions of a crop. Sugar beets are the only economic 

 plants attacked to any injurious degree. Cottonwood, dock, fox- 

 tail grass, knotweed, lambsquarters and yarrow may also become 

 infested. 



THE POPLAR-STEM GALL-LOUSE 



Pemphigus populicaulis Fitch 48 



(Fig. 59) 



Description.— The stem-mother is slightly green or yellow, covered 

 with distinct rows of small cottony patches, £ to \ of an inch long 

 and nearly as wide. The winged female is much smaller, dark in 

 color and covered with fine wax and long white flocculence. 



Fig. 59. — The poplar-stem gall-louse, Pemphigus populicaulis Fitch. Winged female 

 and stem-mother. Greatly enlarged. (Author's illustration, P. C. Jr. Ent. ) 



Life History. — The stem-mother starts the peculiar gall on the 

 stem at the base of the leaf, and produces young until the large gall 

 is completely filled. So tightly closed is the entrance of the gall 

 that few, if any, parasites can gain entrance. When the inhabitants 

 are ready to migrate in the summer a distinct opening is made, 

 through which they escape readily. These feed upon other plants 

 and late in the fall migrants fly to the cottonwood trees where the 

 sexual forms are born and the females of which lay the overwintering' 

 eggs on the bark. These hatch into the stem-mothers referred to 

 above. 



Distribution.— This louse occurs throughout the State, wherever 

 the host plants are found. 



Food Plants. — Common cottonwood (Populus trichocarpa) , P. moni- 

 Ufera and aspen are attacked. The Lombardy poplar appears to be 

 immune according to Davidson. 



Natural Enemies.— This species is preyed upon by internal para- 

 sites in late summer, but is more often attacked by a small bug belonging 

 to the family Anthocoridcp. . 



"The galls of the transverse poplar-stem gall-louse. Pemphigus populitransversus 

 Riley, resembles those of this species, but may be distinguished by the transverse 

 slit in the side. 



