28 THE MEXICAN COTTON-BOLL WEEVIL. 



1904 has been 27,000 square miles a year, with 402,000 square miles 

 of cotton-producing area ye't free. It is therefore reasonable to 

 estimate that it will take at least 15 years before the entire cotton 

 region of this country can become infested. 



It is evident, however, that the weevil will find certain definite 

 checks in the cotton-growing regions of this country. Among the 

 more important of these checks are (1) dryness, (2) low winter 

 temperatures, (3) altitude, and (4) such combinations of these factors 

 as tend to form definite life zones. The possible effects of these 

 factors will be discussed separately. 



Dryness is the most important check the boll weevil experiences. 

 The insect has repeatedly advanced into western Texas, but has 

 invariably been prevented from gaining a foothold by the dry 

 climate of that region. Occasional wet seasons have resulted in 

 apparent gains in that quarter, but they have been nullified by the 

 recurrence of normal years. The extremely dry conditions in 

 Texas, Oklahoma, and Arkansas during the summers of 1909 and 

 1910 had a remarkable effect upon the weevil. Combined with the 

 very severe winters, these dry summers practically excluded the 

 weevils from the western half of the infested region of Texas and 

 most of the infested region in Oklahoma. The damage done in 

 these regions for two seasons has occurred only after the breaking 

 of the intense summer heat. The occasional occurrence of a dry 

 summer, however, does not give any promise of future immunity 

 from the boll weevil, because its tendency to disperse in the fall in 

 all directions enables it to regain any ground which it might lose 

 during such a season. It is also important to note that the practice 

 of irrigation in dry regions may counteract the effects of the lack 

 of precipitation and enable the weevil not only to maintain itself, 

 but to cause considerable damage. 



The low temperatures of the winters of 1907, 1908, and 1909 had a 

 very pronounced effect upon the numbers of the weevil in the follow- 

 ing years. An analysis of the minimum temperatures reached in 

 the regions where the weevils were most affected indicates that such 

 control was the result of a temperature of 12° above zero. In some 

 places where this temperature was reached there were earlier low 

 temperatures which may have forced the weevils into considerably 

 heavier shelter than they would have selected normally. This 

 apparently enabled the weevils to survive even a temperature of 5° 

 above zero. Although the information at hand is rather incomplete, 

 we can nevertheless hold out some hope that regions having a mini- 

 mum temperature of from 5° to 10° above zero will have little trouble 

 from the boll weevil. In later sections of this bulletin we will show 

 how even higher minimum temperatures can greatly reduce the weevil 

 damage of the following year. The weakness of predictions of this 

 kind is that they do not take into account the fact that the weevil is 

 rapidly adjusting itself to changed conditions and that eventually the 

 result of natural selection will be a class of weevils which can with- 

 stand greater vicissitudes than those of the present. 



The extremely slow progress into western Texas might be explained 

 on the basis of altitudes. So far, the weevil has not established itself 

 at an altitude above 2,000 feet. It may be possible that this altitude 

 is its extreme limit. Again, there is danger in this assumption, 



