12 BULLETIN 875, IT. S. DEPARTMENT OF AGRICULTURE. 



injury is frequently distributed over the plants very unevenly, as the 

 weevils display quite a pronounced tendency to concentrate on the 

 upper portions of the plants, and percentage counts should be based 

 on an examination of all squares on the plants rather than on 

 squares examined at random on the upper portions. Naturally the 

 percentage of injury which can be permitted varies with the stage 

 of the cotton plant, but it has been found that usually, if 10 to 20 

 per cent of the squares are punctured early in the season, practically 

 no harm results. The number of punctured squares gradually in- 

 creases until later in the season as high as 60 per cent or more can 

 be punctured without any reduction in crop yield. To put off pois- 

 oning until there is such a high percentage is not advisable, however, 

 owing to the rapidity with which weevils multiply after they reach 

 this point, and thus exhaust the square supply and start attacking 

 the young bolls. The majority of the poisoning operations in the past 

 have been planned so as to start when about 15 to 20 per cent of the 

 squares were punctured and then to repeat often enough to prevent 

 the infestation from getting above about 25 per cent until the crop is 

 set and the bolls are safe from weevil puncturing. In some cases 

 where it is particularly desirable to confine the weevils to a certain 

 cut and prevent any chance of migration, it is well to start at a 

 somewhat lower percentage and continue even later, but where the 

 only object in view is the benefit to the particular cut poisoned, there 

 is apparently little to be gained from starting applications before at 

 least 15 per cent of the squares are punctured. 



TIME INTERVAL BETWEEN APPLICATIONS. 



The question of the time interval between applications is very im- 

 portant and one on which only conditional advice can be given, since 

 it varies under different local conditions. In the past, once the ap- 

 plications were started they were generally made once a week as 

 long as this seemed advisable or necessary. In reality, however, this 

 selection of a time interval of one week was purely arbitrary and 

 more recent results seem to indicate that in the majority of cases 

 much better results can be secured by shortening this time interval. 

 The effectiveness of a single application of calcium arsenate is de- 

 cidedly limited in its duration. Its persistence on the plants natur- 

 ally depends to a considerable extent on conditions prevailing at the 

 time of application and immediately thereafter. In fact, while a 

 very high percentage of control is secured during the first day of the 

 application, this decreases the second day, and by the fourth day 

 there is generally little or no effect. This short interval of effective- 

 ness is due to two factors: (1) The poison is either washed or blown 

 from the plants, and (2) new foliage is developed so rapidly that 



