THE CANKERWORMS 35 



until the apple blossoms have fallen, when all eggs laid below them 

 will have hatched. 



For the spring species the barriers should be in place with the first 

 thawing of the ground in February or March, and remain effective 

 until after the apple blossoms have fallen, as indicated for the fall 

 species. 



SUMMARY. 



The cankerworms have been known in this country since colonial 

 days, but not until about 50 years ago was it realized that two species 

 were present. They have since been known as the fall cankerworm 

 (now Alsophila ponietaria Harris) and the spring cankerworm (Pa- 

 leacrita vernata Peck) . 



The cankerworms prefer to feed on elm and apple, but are often 

 found on many of our common deciduous fruit, forest, and orna- 

 mental trees. 



The fall cankerworm is found in greatest abundance in the north- 

 eastern part of the United States and the southeastern part of Can- 

 ada, but has been reported from several of the States in the Missis- 

 sippi Valley, from Colorado, and from California. The spring 

 species is found in southern Canada as far west as Manitoba, in the 

 northeastern and central portions of the United States as far south 

 as Texas, and in California. 



As the females are wingless, dissemination by their own efforts is 

 slow. The larvae of both species are sometimes caught on passing ve- 

 hicles and carried to new localities. The eggs of both species, and 

 particularly those of the fall species, are likely to be carried to new 

 localities on nursery stock. 



During periods of abundance the cankerworms do an enormous 

 amount of damage, practically defoliating the trees they attack un- 

 less controlled. They are most likely to become abundant in neg- 

 lected orchards or in shade and forest trees. 



Both species have but one generation a year. 



The moths of the fall cankerworm emerge from the ground mostly 

 in late fall and early winter during warm periods after the ground 

 has been frozen. Mating and egg laying soon occur, and the species 

 winters in the egg stage, except in occasional instances in which the 

 emergence of the moths is delayed until spring. The eggs hatch in 

 the spring, during the period when the apple blossoms are in the 

 pink. The larva? feed for four to five weeks, passing through four 

 stages. When through feeding they enter the ground, construct a 

 tough cocoon, and after about a month transform to the pupa. 



The spring cankerworm moths emerge almost exclusively in the 

 spring, and lay eggs in the crevices of the bark and in similar pro- 

 tected locations. The hatching of the eggs and the larval feeding 

 period coincide rather closely with those of the fall species, but the 

 spring cankerworm passes through five larval stages. No cocoon is 

 constructed, and pupation occurs soon after the larva has entered the 

 ground. 



The two species are readily distinguished in all stages. 



The cankerworms are held in fluctuating degrees of control by an 

 extensive array of factors, among which are unfavorable weather, 

 birds, and parasitic and predacious insects. 



