INSECT ENEMIES OF EASTERN FORESTS 133 
be possibly a form in the life cycle of WV. attenuatus O. & S., a species 
infesting Smilax sp. 
The poplar vagabond aphid (Mordwilkoja vagabundus (Walsh) ) 
infests various species of poplars from New England to Montana and 
southwest to California, causing peculiar convoluted galls at the tips 
of twigs. The complete life cycle of this species has hot been deter- 
mined, Only part of the year is spent on poplar, for in the summer 
these aphids migrate to some as yet unknown host. In the fall they 
return to poplar : and reenter the galls from which they migrated in the 
spring. This habit accounts for the fact that certain trees are infested 
year after year, while nearby trees o the same species remain unin- 
fested (Gillette and Palmer, 785, pp. 220-221). 
In the genus Pemphigus the thr ee species considered are named for 
the galls they produce. P. populicaulis Fitch infests various species 
of poplar and produces semiglobular galls normally located at the 
base of the leaf where the blade 
and petiole unite. These galls = 
may be one-half inch or more in i 
diameter (Maxson and Knowlton, 
DOSE DanZoS). le. populi- -globuli 
Fitch infests various species of 
poplar and forms semiglobular 
galls, sometimes more or less cone- 
“shaped at the bases of leaves, on 
the under side. According to 
Maxson and Knowlton (293, p. 
2063), the galls are composed of a 
much enlarged and thickened por- 
tion of the leaf blade. The third 
species considered in this genus, 
P. populi-transversus Riley is 
known as the poplar stem gall 
aphid and may be found on various 
poplars, where it overwinters. _In- 
jury consists of galls produced on 
leaf petioles (fie. 20) ; 
The balsam twig aphid (Min- FIGURE 27.—Gall caused by Pemphigus 
Reeds = s populi-transversus on petiole of 
darus abietinus Koch) attacks both Ola 
balsam fir and spruce. Injury 
consists of a curling and roughening of balsam and spruce twigs. On 
balsam fir a great deal of flocculence and honeydew are sometimes pro- 
duced. Winged migrants may be encountered on pine. This species 
is distributed from New England to the Pacific coast. 
SuBFAMILY HORMAPHIDINAE 
Only two species in this subfamily are considered here. Both spe- 
cies, the witch-hazel leaf gall aphid (/ormaphis hamamelidis 
Fitch) and the spiny witch-hazel gall aphid (Hamamelistes spin- 
osus Shimer), infest witch-hazel and birch. The former species is 
found from New England to North Carolina on the south and to 
Illinois on the west. Z. s pinosus covers the same range, plus an ex- 
