284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



Entomologists were stationed at strategic points in the weevil- 

 infested areas to conduct demonstrations in cooperation with State 

 and Federal extension workers to teach growers the principles of 

 weevil control. The discovery of the effectiveness of calcium arsenate 

 against the weevil came shortly after World War I, when the weevil 

 had spread over most of the belt, damage was at its maximum, and 

 cotton was selling at a high price. The immediate problem was not 

 to interest growers in using calcium arsenate but in using it correctly. 



Since the boll-weevil grubs feed entirely within the squares and bolls 

 and cannot be reached by poisons, control is directed against the adults. 

 They too feed externally only slightly, and it is more difficult for them 

 to obtain a lethal dose than the foliage-feeding insects. Adults sup- 

 plied with water were found to live longer than those without water, 

 and it was assumed at first that adults were killed entirely by drinking 

 poisoned water. Later studies showed that weevils could be killed by 

 crawling over dry surfaces dusted with calcium arsenate because they 

 continually touch the tips of their snouts to the surface and sufficient 

 quantities of the tiny dust particles adhere to the moist mandibles to 

 cause death. Ordinary sprays are deposited in larger particles which 

 adhere more closely than dusts and have never given as good control. 

 The method by which weevils obtain a lethal dose of poison and the 

 fruiting habits of the cotton plant indicate that the entire plant must- 

 be kept thoroughly covered with calcium arsenate dust. As cotton 

 produces more blooms than the plant can carry to maturity, it is not 

 necessary to begin dusting until the weevils cause more shedding than 

 would normally occur. The danger point, when dusting should begin, 

 was first placed at 10 to 15 percent of the squares punctured for feed- 

 ing or oviposition but later raised to 20 to 25 percent for cotton growing 

 in fertile soil. The infestation usually varies from field to field, but 

 the punctured squares are easily seen, and counting 100 or more squares 

 per field at frequent intervals provides a reliable index of weevil 

 abundance and the need for control. From 4 to G applications of 

 5 to 8 pounds of calcium arsenate dust per acre at 5-day intervals are 

 usually sufficient to bring the weevils under control and produce a full 

 crop, though one or more later applications for boll protection may be 

 needed when infestations are heavy. At first it was thought neces- 

 sary to dust with calcium arsenate when the plants were wet with dew, 

 but later experiments showing that effective applications may be made 

 at any time of day when the air is calm have reduced the drudgery of 

 night dusting. Many other improvements and adaptations to varying 

 local conditions have been gained from continued experimentation, 

 but the basic principle of keeping the plant thoroughly covered during 

 the critical fruiting period or until the weevils are brought under 

 control remains unchanged. The improved varieties of determinate 



