Insects of Shade Trees and Their Control. 89 
Rarely is a tree harmed by these galls sufficiently to menace its 
life or even good health. Likewise are they seldom numerous many 
years in succession, but while they last they do mar the appearance 
of otherwise healthy and beautiful trees, and their spectacular aspect 
tends to alarm the less informed tree owners and wardens. 
Because of their evanescence and comparative harmlessness, treat- 
ment for gall insects is rarely imperative. Another deterrent factor 
in the undertaking of control measures against this sort of injury is 
the necessity of rather expensive spraying machinery for the work 
in many cases, and the cost of purchase or hire of this, as well as 
of its operation, added to the cost of the requisite insecticides, is 
apt to be rather excessive. It can only be justified where cost is no 
consideration, and even then, where the injury is not likely to receive 
general treatment in a neighborhood, the results of the operation are 
likely to be unsatisfactory on account of reinfestation from untreated 
near-by trees. 
LEAF-STEM AND OTHER POPLAR GALL INSECTS.” 
How injurious.—The vitality of trees infested with these aphids is 
never seriously affected. When the insects are abundant, the galls 
they make and their feeding frequently cause a heavy dropping of 
the foliage during the growing season, which, like the misshapen 
galls produced by some of them, is objectionable because it tem- 
porarily disfigures the tree and litters the ground. 
How recognized.—The presence of these aphids is shown by heavy 
premature falling of the leaves, which bear rather prominent swell- 
ings on the midrib or leaf stem, the licelike insects lining the insides 
of them. 
Habits—Of the several species of these aphids known to make 
galls on poplar leaves, leaf stems, and twigs, the seasonal history of 
only the species here noted © has been worked out. This is somewhat 
complicated but rather interesting and is, briefly, as follows: By the 
time the galls are full grown in the fall, winged forms are developed 
in them and these migrate to the roots of cruciferous plants like 
cabbage, turnip, etc., where they breed until the following spring. 
At that time a generation of winged forms is again produced and 
these fly back to poplar where they give birth to wingless males and 
females on the bark. After mating the female lays one egg which 
hatches into the form known as stem mother. The young stem 
mother appears about the time the leaves begin to develop and mi- 
grates to and settles to feed on a stem of a leaf, which begins to 
swell and grow around her until she is completely enveloped by the 
8 Pemphigus populi-transversus Riley et spp. 
