with a white, cottony substance that strings out in fairly long 
threads and forms a tuft at the rear end. Heavy infestations oc- 
casionally develop on forest-grown trees. The woolly alder aphid, 
P. tessellatus (Fitch) is often abundant on alder and silver maple. 
P. fraxinifolu Riley feeds on ash and P. corrugatans (Sirrine) on 
shadbush. 
Certain species of.aphids produce galls by their feeding. The 
elm coxcomb-gall aphid, Colopha ulmicola (Fitch), is probably the 
most important tree-infesting species. It occurs throughout, most 
of the United States and Canada, wherever its hosts, American, 
rock, and slippery elms, grow. It feeds on leaves, causing the for- 
mation of galls up to 25 mm. long and 6 mm. in height. The gall 
is irregular in shape and resembles a rooster’s comb. The winter 
is spent in the egg stage on elm, and there are six generations per 
year. Damage is not severe, but when large numbers of galls occur 
on the leaves of young trees and ornamentals the trees may be- 
come unattractive. The related species, C. ulmisaccult (Patch) 
produces pedunculated, bladder-like galls up to 25 mm. long on the 
upper surfaces of elm in New England. 
The poplar vagabond aphid, Mordwilkoja vagabunda (Walsh) 
feeds at the tips of twigs of cottonwoods and occurs from New 
England to Utah, causing the formation of convoluted galls (fig. 
18) up to 5 inches in diameter. These galls may occur singly or in 
clusters of three to five galls each. The winter is usually spent in 
the egg stage in old galls or in nearby bark crevices (390). Other 
gall producing aphids and their hosts include Hormaphis hama- 
melidis (Fitch)—conical galls on the upper surfaces of witch- 
hazel leaves; Hamamelistes spinosus (Shiner)—causes galls to 
form on the stem buds of witch-hazel; and Gobaishia ulmifusa 
(Walsh and Riley)—spindle-shaped, saclike galls on the leaves of 
slippery elm. 
F-506747 
FIGURE 18.—Galls of the poplar vagabond aphid, Mordwilkoja vagabunda, 
on poplar. 
85 
