26 



BULLETIN 1238, U. S. DEPARTMENT OF AGRICULTURE 



IAEVAL HABITS. 



The habits of the spring cankerworm larvae are very similar to 

 those of the fall species. On hatching, the young caterpillars proceed 

 to the unfolding foliage and feed here and there, at first eating the 

 tissue on only one side of the leaf, leaving the portion opposite to 

 die and turn brown. Soon the feeding extends through the leaf in 

 the form of tiny punctures. The small worms often feed among the 

 opening blossom buds. As the larvae grow, the feeding areas increase 

 in size, but the midribs and larger veins are left untouched, and have 

 a frayed edge of ragged leaf tissue. When the infestation is light, 

 the larvae in their wandering distribute their feeding so that the 

 injury is not conspicuous. When disturbed, the larvae either spin 

 down on threads or raise the anterior portion of the body, taking on 

 the appearance of a short twig, or spur. When very small larvae spin 

 down on threads, they are often blown to an adjacent tree. When at 

 rest, the larvae of the spring cankerworm have a tendency to spend 

 their time on a twig which they closely resemble in color, while the 

 fall cankerworm larvae, which are usually green, are much more 

 likely to conceal themselves on a green shoot, or in the curl of a leaf. 



NUMBER OF INSTABS. 



All spring cankerworms under observation at Wallingford passed 

 through five larval instars. 



LABVAL FEEDING PEBIOD. 



Tables 12 and 13 give data relative to the larval feeding period for 

 96 larvae during the season 1921. Table 14 gives the records of the 

 average larval feeding period by instars for 1920 and 1921. The 

 difference in the two seasons probably explains the difference in the 

 feeding periods. The spring of 1920 was unusually late, but after 

 the eggs had finally hatched, the weather was warm, and favorable 

 to rapid larval development. On the other hand, the spring of 1921 

 was unusually early, causing an abnormally early hatching of the 

 eggs, but the period immediately following hatching failed to con- 

 tinue as warm as it had been at the start. The average daily mean 

 temperature during the larval feeding period in 1920 was 61.9° F., 

 while that during the same period in 1921 was 57.9° F., an average 

 difference of 4 degrees, which explains for the most part the longer 

 feeding period in 1921. 



Table 12. 



-Length of larval feeding period of the spring cankerworm, Walling- 

 ford, Conn., 1921. 



Date of hatching. 



Apr. 21, 



Do. 



Do. 



Do. 



Do. 

 Apr. 22. 



Do. 

 Apr. 23 



Date 

 entered 

 ground. 



May 22 

 May 23 

 May 25 

 May 26 

 June 1 

 May 30 

 May 31 

 June 6 



Num- 

 ber of 

 larvse. 



Num- 

 ber of 

 days. 



Date of hatching. 



Apr. 24 

 Do. 

 Do. 

 Do. 

 Do. 

 Do. 

 Do. 

 Do 



Date 

 entered 

 ground. 



May 24 

 May 25 

 May 26 

 May 27 

 May 28 

 May 29 

 May 30 

 May 31 



Num- 

 ber of 

 larvae. 



Num- 

 ber of 

 days. 



