NATURAL CONTROL. 



123 



In hanging squares we find a somewhat different arrangement of 

 the sections: Central Texas, 24.04 per cent; southern Texas, 20.04 

 per cent; northeastern Texas, 17.39 per cent; north-central Texas, 

 17.17 per cent; Louisiana, 16.99 per cent; Arkansas, 16.43 per cent; 

 southwestern Texas, 13 per cent; Oklahoma, 9.53 per cent ; Mississippi, 

 8.21 per cent; and eastern Texas, 3.70 per cent. 



In many of the records made during 1906 it became evident that 

 certain cultural practices greatly favored the amount of control by 

 heat and dryness. The wider the rows, the greater the amount of 

 sunlight which strikes the ground. Consequently the fields with wide 

 rows or in which the stand was imperfect showed the greatest mor- 

 tality. In a similar way, fields in which were varieties with compara- 

 tively small amounts of leafage showed greater mortality due to heat 

 anol dryness. It did not become apparent, however, from the obser- 

 vations made, that the direction m which the rows ran made any 

 material difference in the mortality. 



The difference between the various sections in the mortality in 

 fallen squares is especially conspicuous. This is due undoubtedly 

 primarily to the greater precipitation in the sections with low mor- 

 tality, which, by keeping the ground more or less moist, prevents 

 such temperatures at the surface as are frequently reached in Texas. 

 The greater rainfall in Louisiana also undoubtedly has an indirect 

 effect. In that State the additional rainfall causes the cotton plants 

 to grow to a large size and to shade the ground more than is the case 

 in Texas, thus preventing the sun from reaching the squares on the 

 ground. The differences in hanging squares are not quite so con- 

 spicuous, but are probably due to some extent to atmospheric humid- 

 ity, density of foliage, and other similar factors. 



Equally interesting results were obtained in 1906 with reference to 

 the effect of heat and dryness upon the different stages of the boll 

 weevil. It was found that the mortality in the larval stage amounted 

 to 52 per cent, in the pupal stage to 18 per cent, and in the adult stage 

 to 6 per cent. Nearly 70 per cent of all the mortality caused by heat 

 and dryness occurs, therefore, during the larval stage. 



Table LIX illustrates the percentage of stages killed during the 

 warm months of the year by high temperatures and is based upon all 

 of the examinations made during the years 1906 to 1909, inclusive. 



Table LIX. — Weighted average heat control of immature stages of the boll weevil, by 

 months, Texas, Louisiana, Oklahoma, Arkansas, and Mississippi. 



Month. 



Forms 

 exam- 

 ined. 



found. 



Per cent 

 killed by 

 heat and 

 drying. 



May 



June 



July 



August 



September. 

 October. .. 



Total 



100 

 16,930 

 43.059 

 80,923 

 37,378 

 17,344 



56 



10,708 

 21,758 

 33,170 

 17,107 

 8,283 



195,734 



91,082 



7.20 

 28.33 

 25.61 

 24.62 

 22.87 

 16.59 



23.80 



Many illustrations are available to show the powerful effect of heat 

 and dryness in the reduction in the numbers of boll weevils in cotton 

 fields. The action of this agency is so powerful that it may check the 



