20 The Bulletin. 



actually reaches the plant would be removed by natural causes in 

 the course of the year. This would leave, then, approximately 700 

 grains to each acre of tobacco sprayed — about one-seventh of a grain 

 to a plant. Assuming that 4 plants will yield approximately a pound 

 of tobacco, this would mean that there would be approximately one- 

 third of a fatal dose to every pound of tobacco harvested. Since only 

 a very small amount of tobacco is swallowed, in whatever form it is 

 used, it seems safe to say that no one person would at any one time 

 consume enough tobacco, sprayed with arsenates as indicated above, 

 to be injurious. 



It seems almost needless to say that not all of the spray mixture 

 used reaches the tobacco plant. This is very evident indeed to any 

 one who has ever done any spraying. It seems safe to say that on 

 still days, under the most favorable conditions, that at least 25 per 

 cent of the spray mixture used is necessarily wasted in this way. 



Of the amount of poison that reaches the plant, the greater per- 

 centage must be washed away by rain or blown away by the wind. 

 Experiments carried on by Professor Garman, of the Kentucky Ex- 

 periment Station, prove conclusively by chemical analysis that of the 

 amount of poison that reached the plant originally from 96 to 98 

 per cent was removed in this way in a month after the plants were 

 sprayed. 



These arguments hold true for Paris green as well as arsenate of 

 lead. Assuming that one pound of Paris green is used to an acre of 

 tobacco, and that three sprayings are given throughout the season, 

 we would have 12,600 grains of arsenic used per acre, assuming that 

 Paris green contains 60 per cent of arsenic, which is above the average 

 sample, and discarding the usual 25 per cent for spray mixture 

 wasted in making the application, and 95 per cent of the arsenic 

 which actually reaches the plant, we will have left 3-40 of a grain 

 of arsenic per plant — in other words, a fatal dose for every five 

 pounds of tobacco harvested. 



INSECTS IN GENERAL. 



In order that the farmer may successfully combat the different 

 insects which injure tobacco, it is necessary to know something re- 

 garding insects in general, their life-histories, habits, enemies, and 

 the remedies that may be used against them successfully. 



Life-histories. — Briefly, insects may be divided into two classes. 

 One develops from the egg to the adult without any resting stage. 

 Insects belonging to this class are said to have an incomplete change 

 of form. The other class has a resting stage in its life-history, and 

 insects belonging to this class are said to have a complete change of 

 form. In incomplete change of form there are three stages in the 



