DANA'S LETTER ON CANKER-WORMS 



closes up the breathing holes arrayed along its sides 

 (called in science spiracles). 



[< But to make the method of prevention sure the oil 

 must be kept in the troughs throughout the season of the 

 ascent of the females. It is hence to be noted : 



" i. That the oil may be blown out by the winds. 



" 2. That the rains may fill the troughs with water so 

 that the oil (which always floats on water) may thus be 

 floated out. 



" 3. The troughs as put up are often not horizontal, so 

 that all the oil goes to one side and flows out at the first 

 rain-storm. 



" 4. The troughs are often too shallow; and the cover 

 of lead above is too narrow to serve as any protection 

 against the rain. 



' 5. The oil, when not altogether neglected, is gener- 

 ally not put in early enough in the autumn and spring, 

 nor continued long enough. 



6. The insects sometimes fill up the troughs by 

 their dead bodies before the season of ascent is passed, 

 and thus form a bridge for aftercomers. 



' The following, then, are the rules to be regarded : 



" i. Have the lead troughs well put up and of good 

 size. 



"2. In the autumn, put in oil as early as October, and 

 keep it in until the ground is frozen solid or covered with 

 snow. 



" 3. In the winter, fill up again when the frost is out 

 of the ground, even for a few inches. 



"4. In the spring, put in oil, whenever the frost 

 begins to leave the ground, and keep it in until the 

 canker-worms appear. As the females lay their eggs on 

 the trunk of the tree below the lead trough as well as 

 above on the branches, if the oil is not kept in until after 

 the hatching in May, the young which then appear may 

 crawl up to their feeding place. 



' 5. Examine the troughs once a fortnight after the oil 

 has been put in for the season and fill up whenever 

 needed, clearing out the dead moths that have accumu- 

 lated. 



" 6. Examine the troughs after every heavy storm. 



" 7. Fill the troughs each time from one-third to one- 

 half their depth ; more oil is a waste as it is so liable to 



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