METHODS OF CONTROLLING TOBACCO INSECTS. 11 



that appear before the middle of July are killed by hand worming, 

 and we have already shown by means of the life-history records that 

 it will be two months from the emergence of the hibernating genera- 

 tion before the "worms/' or larvae, of the second generation will be 

 large enough to injure tobacco seriously. 



The statement has been frequently made to the writer, and personal 

 observation has convinced him of its truth, that no difficulty is experi- 

 enced in keeping tobacco free of worms by hand worming until the 

 middle of July or later. Therefore, if tobacco is not injured by 

 worms until after the middle of July, the conclusion is that until that 

 date very few large tobacco worms have escaped hand worming and 

 that the second generation from those that have escaped will be a 

 very small one. By far the greater portion of tobacco worms that appear 

 in late July and in August are the offspring of moths that have emerged 

 from hibernation; for the second generation of worms, the offspring 

 from those worms that mature after July 15 will not be of sufficient 

 size to injure tobacco seriously until about September 15. If tobacco 

 has been set early the crop will be ready for the barn by this date 

 and will, therefore, escape the second generation of worms. 



FALL AND WINTER PLOWING. 



Numerous experiments in L907, 1908, and 1909 have demonstrated 

 that as a rule only about one-fourth of the tobacco worms that hiber- 

 nate are able to survive the winter and become adult. A part of the 

 mortality is due to parasites, but a greater part of it is due to the 

 unfavorable weather. The hibernating period is, therefore, a very 

 critical period in the seasonal history of the tobacco worm, and many 

 of those that do survive this period must be greatly weakened. 

 Hence, any artificial disturbance of natural conditions will produce 

 an additional mortality. As has been stated, the hibernation period 

 is passed in the pupal stage in an oval cell (fig. 8), about 4 inches below 

 the surface of the soil. 



The most simple method of disturbing the pupa 3 during hibernation 

 is to disk or plow the land. Both methods were tried. It was found 

 that the disk would reach only from 5 to 10 per cent of the cells, and 

 that therefore little benefit could be derived from that treatment. 

 The experiments in plowing the land killed more than half tit e pu.pse 

 that would otherwise have passed the winter successfully. The ground 

 should be plowed to the same depth as it was in preparing it for 

 tobacco, for many of the tobacco worms will go down to the hard 

 soil to form the hibernating cell, that is, to the greatest depth to 

 which the soil has been broken. Plowing will throw the pupae and 

 the cells up to or near to the surface, will break the cells in nearly all 

 cases, and will place the pupa 1 in close contact with the earth, in 



