THE MONTHLY BULLETIN. 291 



THE MEXICAN COTTON BOLL WEEVIL. 



By W. D. Pierce, Entomologist Southern Field Crop Insect Investigations, U. S. 

 Department of Agriculture. 



The most destructive pest of cotton is the Mexican cotton boll weevil (Anthono- 

 nuts grandii Boheman), which now extends its depredations on cotton from Costa 

 Rica, through Central America and Mexico into Texas, Oklahoma, Louisiana, 

 Arkansas. Tennessee, Mississippi, Alabama, Florida, Georgia, and to the very border 

 of South Carolina. A native race of the weevil which breeds on a nearly related 

 mallow, Thurbiria thespesioides, evidently has extended its range northward through 

 the mountain valleys of western Mexico and into the southern part of Arizona. This 

 weevil occurs on its wild host in canyons around Tucsou, within a few miles of 

 cultivated cotton, and has demonstrated its ability to attack cotton in that locality. 

 It is therefore only a matter of time until it is established on Arizona cotton. This 

 brings home to California the necessity of extreme watchfulness against the intro- 

 duction of cotton or Thurberia seed into the Imperial Valley. 



The boll weevil lays its eggs singly in the cotton squares, sealing the punctures 

 with a liquid secretion. The location of the puncture is soon marked by a little 



Fig. 90. Cotton boll on left being punctured by weevils. Bloom on the right has 

 been injured by the attack of larvae. (Original.) 



wart-like prominence. The egg is tiny, white and oval. It rests among the anthers 

 and is easily detected if the bracts and petals are carefully removed. The larva is 

 a white grub, curved like most weevil larvae, and with a light brown head. It feeds 

 among the stamens until about half an inch long and then makes an oval cell from 

 trass and excretions and pupates. During the midsummer the entire developmental 

 period occupies only about fifteen days, but in cooler weather it is much longer, 

 depending of course upon temperature and humidity variations. 



When a cotton square has been injured it usually flares its bracts, thus giving a 

 very good sign of infestation. Most varieties of cotton form a transverse absciss- 

 layer on the petiole of injured squares, which causes them to fall off in a few days. 

 Some varieties, however, have a diagonal absciss-layer which runs down the stem 

 and does not permit the square to completely sever its connection with the plant. 

 The result is that on such varieties the square hangs and dries on the plant. It has 

 been found that these hanging dry squares permit of a much higher percentage of 

 parasitism by Hymenopterous parasites than the fallen squares, due to the sunlight- 

 loving propensity of these parasites. 



If the fallen squares lie in the shade of the plant or on moist or rough plowed 

 soil, they offer the best opportunity for the rapid development of the weevil. If, 

 however, the square falls where the heat of the sun will strike it for several hours, 

 death is very likely to occur. 



