NATURAL CONTROL. 125 



GENERAL DISCUSSION OF THE RELATIONS OF TEMPERATURE TO THE 



BOLL WEEVIL. 



Our studies of the boll weevil lead us to the conclusion that there 

 is a certain degree of temperature above which, under any condition 

 of humidity, no individuals can exist even for a limited time. This 

 point is known as the maximum fatal temperature. Below this there 

 is a zone of varying width of temperatures which may be fatal in 

 cases of long exposure or under certain conditions of humidity or 

 insect vitality. This may be known as the upper zone of fatal tem- 

 peratures. Below the zone of fatal temperatures is a zone of tem- 

 peratures which, when continuing for any length of time, force the 

 insects to shelter. This zone may therefore be fitly called the zone 

 of aestivation, and it must be understood that the relative humidity 

 will have a strong influence in moving this zone upward or downward, 

 according to regional conditions. This zone is limited by the point 

 at which effective temperature ceases. Below this point is the zone 

 of activity. In this zone will be found the degree of effective tem- 

 perature, a long continuance of which is necessary to draw the insects 

 out of hibernation. This is not an absolutely fixed point, for it 

 varies with humidity. 1 Possibly the relation could be stated in a 

 definite formula if a sufficient amount of data was available. 



The temperature which causes activity is usually known as the 

 zero of effective temperature. It is assumed that active metabolism 

 begins at this point and that a certain amount of effective tempera- 

 tures accumulated in daily units is necessary to bring about a given 

 transformation or function. This sum is known as the total effective 

 temperature for any given function. It will vary in accordance with 

 the humidity. Below the zero of effective temperature there will 

 be no necessity of feeding, and locomotion rapidly becomes impossi- 

 ble. On the approach of the zero of effective temperature the 

 insects will display considerable activity in finding winter quarters. 

 We therefore designate the zone below this zero as the zone of hiber- 

 nation. The lower limit of this zone is the highest temperature 

 which may be fatal under certain conditions of humidity or rapid 

 alternation of temperatures. Below this point occurs a more or less 

 restricted lower zone of fatal temperatures. The lowest point at which 

 life can exist is known as the minimum fatal temperature. 



i Of course the manifestation of the absolute temperature which draws the weevils out of hibernation is 

 dependent upon the density of shelter. Certain forms of shelter prevent the weevils from being affected 

 until long after the outside air has been sufficiently warm to cause activity. 



