INSECT ENEMIES OF EASTERN FORESTS 135 
die, but Friend (169, pp. 1-4) reports that on heavily infested trees 
up to 85 percent of the gall-infested twigs die. Individual trees vary 
greatly in their susceptibility to attack. In southern New England 
approximately one-third are entirely immune, and the overwintering 
females that.attempt to feed on them die. Wilford (432, pp. 30, 31) 
also reported that many trees are apparently immune to attack. 
Stands on good sites suffer less than do more slowly growing forest 
trees and ornamentals. In fully stocked stands the affected trees 
will generally be removed in thinnings. It seems highly probable that 
much of the injury now caused to ornamentals by Chermes abietis 
can be eliminated by using cuttings from immune trees in grafting 
of nursery stock. 
The Cooley spruce gall aphid (Chermes cooleyi Gill.) attacks 
Engelmann, Sitka, oriental, and Colorado spruces and Douglas-fir, 
I IGURE 28.—Chermes abietis: A, Pineapple-shaped galls on spruce; B, egg mass 
broken open to show eggs, X 10; C, cottony wax covering of females at. time 
of oviposition, X 10. (Courtesy Conn. Agr. Expt. Sta.) 
and is common on ornamentals in the Eastern States. These aphids 
overwinter on spruce as immature stem mothers. They mature early 
in the spring and lay large numbers of eggs in masses of white cottony 
wax. When the eggs hatch the young nymphs migrate to new growth, 
where they begin to feed at the bases of needles. This feeding pro- 
duces galls, which soon envelop the young aphids (fig. 29). When the 
nymphs are full grown the galls open and the aphids move to the 
spruce needles, where they cast their nymphal skins and emerge as 
winged adults, which migrate to Douglas-fir, if this tree is present. 
Eventually there is produced on Douglas-fir a winged generation 
which migrates back to spruce. Complete cycles may also continue 
on either spruce or Douglas-fir almost indefinitely. Chrystal (91, pp. 
10-18) has described the biology of this species very fully. 
