38 NATURAL CONTROL OF THE COTTON BOLL WEEVIL. 



Ants (Solenopsis geminata) accomplish a greater destruction of 

 weevil stages than heat and parasites combined. Parasites are decid- 

 edly the least important of the three factors. 



So many conditions influence the effect of sunshine especially that 

 it is difficult to determine whether there exists anything like a con- 

 stant relationship between the maximum temperatures at a locality 

 and the mortality from heat or drying. In some cases a relationship 

 is evident which can not be regarded as merely accidental. The 

 localities in which a comparatively small series of examinations was 

 made should not be considered in this connection. 



The highest proportion of rainy days occurred at Calvert during July 

 and August, being twenty-five in sixty-one days. The influence of each 

 factor of natural control seems to have been greatly reduced by the 

 excessive amount of rain (10.22 inches) distributed over so many 

 days. The total mortality was but 15.1 per cent, distributed among 

 the factors as follows : Heat 3.9 per cent, ants 9.3 per cent, parasites 2. 1 

 per cent. The high maximum temperatures occurring were evidently 

 prevented from exerting their usual influence by the almost continu- 

 ously wet condition of the ground. This produced a high percen- 

 tage of relative humidity which would naturally favor the development 

 of the weevils. 



Fortunately for this portion of our study an automatic self-regis- 

 tering combined thermograph and hydrograph had been located at 

 Calvert early in the season of 1906, and the records from this machine 

 furnish valuable data regarding humidity conditions which could not 

 otherwise be obtained. These records may be used in connection 

 with Weather Bureau reports for the locality of Calvert and some 

 interesting facts will appear. 



Around Calvert very light rains fell early in June, but from that 

 time until June 25 there was no rainfall. The period from June 5 to 

 June 25, 1906, shows an average relative humidity amounting to 



a Relative humidity is the term used to denote the proportion of atmospheric mois- 

 ture which actually exists as compared with the amount of moisture which it would 

 be possible for the air to contain at any given temperature. If the air contains all 

 the moisture possible, it is said to be "saturated" and this is the condition during 

 fogs and sometimes, though not always, during rains. It is the condition of the air 

 at a given point when dew is deposited. The relative humidity under ibis saturated 

 condition is 100 per cent. The condition opposite to this extreme is a perfectly dry 

 air in which the relative humidity is 0. The amount of moisture, or water vapor, 

 which it is possible for a given amount of air to contain is much greater if the air be 

 warm than if it be cold. Thus, taking a certain quantity of air containing a definite 

 quantity of water vapor, the percentage of relative humidity will increase as the tem- 

 perature is lowered and decrease as the temperature is raised. A low percentage of 

 relative humidity naturally promotes evaporation owing to absorption of the moisture 

 by the drier air, while evaporation ceases, even from a water surface, when the air 

 becomes saturated. During the summer in central Texas the lelative humidity 

 increases at a few feet above the ground during the nights until it reaches, as a rule, 

 a maximum of between 90 and 95 per cent. During the day it decreases until in 

 the warmest part of the afternoon it reaches a minimum of between 30 and 50 per cent. 



