ENGLISH: ILLINOIS TREES: THEIR INSECT ENEMIES 



21 



Fig. 14. — Cooley spruce gall aphid: an infestation on Douglas fir. Each 

 white splotch on the needles represents a cottony mass of wax that protects 

 an aphid. Beneath the wax the female lays her eggs. Some of the insects 

 developing from these eggs have wings; others do not. Those that have 

 wings migrate to spruce, where they produce galls at the tips of branches. 



these eggs have wings and they migrate to spruce, where they 

 produce galls. No galls develop on Douglas fir. 



Control Measure 3 (end of circular) in June. 



ELM 



Spring Cankerworm, Paleacrita vernata (Peck), fig. 15 and 

 16 — The spring cankerworm is a common pest of elms and per- 

 haps the most important leaf eater that infests the American 

 elm in Illinois. The caterpillar, or larva, is a dark green or black 

 "measuring worm" that does not wait for the buds to unfold be- 

 fore starting to feed. At first rather inconspicuous, the small 

 caterpillar eats the tissue on one side of any leaf it feeds upon, 

 but as it grows it punctures and later consumes all of the leaf 

 except the large veins. By the time the caterpillar is 1 inch long, 

 or full grown, it may be red, green, yellow, or black in color. 

 It drops to the ground by means of a thread, burrows into the 

 soil, and pupates. The adults emerge during warm spells in late 



