28 



BULLETIN 1238, U. S. DEPARTMENT OF AGRICULTURE 



Table 15. — Entrance into ground of larva? of the spring cankerworm, WaUing- 

 ford, Conn., 1920 and 1921. 



1920 



1921 



Date. 



Num- 

 ber of 

 larvae. 



Date. 



Num- 

 ber of 

 larvae. 



Date. 



Num- 

 ber of 

 larvae. 



Date. 



Num- 

 ber of 

 larvae. 



Date. 



Num- 

 ber of 

 larvae. 



June 10 



June 11 



June 12 



June 13 



June 14 



June 15 



June 16 



2 

 3 

 25 

 47 

 23 

 16 

 30 



June 17.... 



June 18 



June 19 



June 20.... 



Total.... 



9 

 1 

 

 2 



May 22. . . . 

 May 23.... 

 May 24.... 



May 25 



May 26.... 

 May 27.... 

 May 28.... 



3 

 2 

 1 

 5 

 3 

 3 

 10 



May 29 



May 30.... 

 May 31.... 



June 1 



June 2 



June 3 



June 4 



20 

 12 

 12 

 9 

 4 

 1 

 3 



June 5 



June 6 



June 7 



Total.... 



5 

 2 



1 



96 



158 



PREPTJPAL PERIOD. ^ 



The time elapsing between the entrance of the larva into the 

 ground and pupation has varied from 4 to 12 days, with an average 

 period of 6.68 days in 1920 and 7.03 days in 1921. 



NATURAL CONTROL. 



One outstanding feature of cankerworm history is the recurrence 

 of extremes of abundance and scarcity. In many cases the periods 

 of extreme abundance have been suddenly terminated by an almost 

 total disappearance of the worms, followed by a period of compara- 

 tive freedom from attack. In his original account Peck (2) records 

 an almost complete disappearance of the worms between 1794 and 

 1795. One of the early agricultural papers notes that the worms 

 " run out in three years." Unfortunately many of the outbreaks 

 have not " run out " at the end of 3 years, but have increased in 

 severity for as long as 8 or 10 years before natural causes have put 

 an end to them, for the time being at least. During the periods of 

 scarcity, the worms usually increase in numbers more or less stead- 

 ily, although for the most part unobserved, until they again appear 

 in great numbers. 



The records are frequently obscured by accounts of outbreaks of a 

 local nature independent of the general waves of abundance and 

 scarcity. The records of outbreaks previous to 1875 are especially 

 hard to interpret owing to the confusion of the two species, making 

 it difficult to tell whether successive outbreaks consisted of the same 

 or of different species. 



A detailed discussion of the various factors connected with the 

 natural control of the cankerworm follows. 



WEATHER CONDITIONS. 



Weather conditions have often been given the credit for the sud- 

 den disappearance of the cankerworms. Peck (*2) noted that the 

 17th of May, 1794, was so cold that ice one-third of an inch thick 

 was produced. This temperature was apparently fatal to the larvae, 

 with the result that practically no moths could be found the follow- 

 ing year. This freeze evidently occurred just after all the larvae 

 had hatched. In a communication to the New England Farmer 



