10 WEEVIL-RESISTING ADAPTATIONS OF COTTON. 



SELECTIVE INFLUENCE OF THE BOLL WEEVIL. 



The boll Aveevil exerts a most prejudicial effect upon the cotton 

 croi), but, unlike most parasites, it does not cause disease or debility 

 in its host plant. The young buds and bolls are merely pruned away, 

 as it were, the purposes of the Aveevil being the better served when the 

 plants remain vigorous and continue to produce more buds and bolls, 

 in which more eggs can be laid and more larvae brought to maturity. 

 Nevertheless, if no bolls are allowed to develop no seed can be set. 

 The fate of the cotton crop in wet seasons in Texas shows that with- 

 out some form of protection the plant would have been extinct long 

 since in all localities reached by the boll weevil. 



The long contact between the boll weevil and the cotton plant in 

 Central America has given ample opportunity for the latter to profit 

 by the selection which the insect itself has provided. Ever}^ differ- 

 ence by which a cotton plant was able to resist or to avoid the weevil 

 and thus ripen more seeds than its fellows would give it a distinct 

 advantage, quite as if the selection were consciously carried on by the 

 planter or the plant breeder. The case is different from that of the 

 recent improvements of many of our cultivated plants by selection 

 for the increase of some particular quality already existing. Such 

 improvements can often be made appreciable, or even highly valu- 

 able, in comparatively few years, but under the desultory Indian 

 methods of cultivation long periods of time would be required for 

 the origination and accumulation of such characters as these pro- 

 tective adaptations. 



Climate and other local conditions must also be taken into consid- 

 eration. An adaptation which would be effective in one set of cli- 

 matic conditions may be of little use, or even a positive disadvantage, 

 in others, as, for example, the prompt shedding of the parasitized 

 buds. In a dry regi(m the falling of a bud to the superheated, 

 sun-baked earth insures the death of the weevil larva, either by the 

 heat directly or by the complete drying out of the tissues in which 

 the larva is embedded. In the moist districts of eastern Texas, how- 

 ever, this expedient is quite ineffective, the larvse often developing 

 even better when the buds fall off' and lie on moist soil than when thev 

 remain attached to the plant. 



It need not surprise us to learn also that the weevil-resisting adap- 

 tations shown by the Kekchi and other cotton varieties of Central 

 America are shared, to some extent, bv those already known in the 

 United States, since the whole Upland type of cotton appears to have 

 been, originally, a native of the C^itral American region. Varieties 

 which reached the United States from Mexico and the West Indies 

 may, however, have had little or no contact with the weevil for many 

 centuries, while in Central America the struggle for existence has 

 remained severe and continuous down to the present day. 



