tuft at the rear end. Heavy infestations occasionally develop on forest-grown trees. 
The woolly alder aphid, Prociphilus tessellatus (Fitch), occurs throughout the 
Eastern United States and is often abundant on alder and silver maple. P. fraxinifolii 
(Riley) feeds on ash and P. corrugatans (Sirrine) on serviceberry. P. longianus 
Smith is associated with root galls caused by cancer-root on black oak. 
Certain species of aphids produce galls by their feeding. The elm cockscombgall 
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. In early summer, it feeds on 
leaves, causing the formation 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 1s spent in the 
egg stage on elm, and by midsummer the aphids have left for a secondary host 
thought to be a grass. 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 become unattractive. Another species, 7etraneura ulmi (L.), produces 
pedunculated, bladderlike galls up to 25 mm long on the upper surfaces of leaves 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. 24) up to 13 cm in diameter. These galls may 
occur singly or in clusters of three to five each. The winter 1s usually spent in the 
egg stage in old galls or occasionally in nearby bark crevices (623). Other gall- 
producing aphids include Hormaphis hamamelidis (Fitch), which forms conical 
galls on the upper surfaces of witch-hazel leaves: Hamaelistes spinosus Shimer, the 
witch-hazel gall aphid, which causes galls to form on the stem buds of witch- 
hazel: Kaltenbachiella ulmifusa (Walsh & Riley), which forms spindle-shaped, 
saclike galls on the upper surfaces of leaves of slippery elm; and the poplar petiole 
gall aphid, Pemphigus populitransversus Riley, which occurs on various species of 
poplar where it overwinters and produces oval galls on the leaf petiole. 
F-506747 
Figure 24.—Galls of the poplar vagabond aphid, 
Mordwilkoja vagabunda, on poplar. 
80 
