NATURAL CONTROL. 127 



ZONE OF ESTIVATION. 



During 1906 and 1907, in southern Texas, Mr. J. D. Mitchell ob- 

 served that many adult weevils were on the ground near the cotton 

 stalks under clods of earth and dead leaves, seeking protection from 

 the intense heat. This indicates a distinct zone of aestivation, 

 although such temperatures may exist only for a few hours at a 

 time. The exact limitations of this zone are undeterminable. 

 /Estivation is a very common habit among weevils. As throwing 

 some light on the probable action of the boll weevil under high tem- 

 peratures, it is of interest to state that Prof. C. H. T. Townsend, of 

 Piura, Peru, finds that the Peruvian cotton square borer, Antho- 

 nomus vestitus Boheman, activates during the not months in the 

 fallen squares both as pupa and adult, but remains practically 

 inactive. 



ZONE OF ACTIVITY. 



The temperatures at which most of the functions of the boll weevil 

 are exercised lie between the means of 91° F. and 56° F. It is prob- 

 able, however, that this zone approaches very close to the zone of fatal 

 temperatures. In the spring effective temperature ' begins to accu- 

 mulate at approximately 56 degrees, but the total necessary to bring 

 the weevils out of hibernation may be low if the rainfall and humidity 

 for the same period are high, and it must be correspondingly high if the 

 humidity is low. 2 When the two factors have accumulated enough 

 between them they bring about emergence. It is roughly calculated 

 that 172° of effective temperature and 5.1 inches of rain are necessary. 

 A deficiency of effective temperature must be balanced by additional 

 rainfall; a deficiency of rainfall must be balanced by additional effec- 

 tive temperature. For a fuller discussion of this subject see the 

 section on emergence from hibernation (p. 107). 



When the weevils have emerged and found food they require a cer- 

 tain number of days of feeding before oviposition can take place. 

 This preoviposition period for hibernating weevils and for the succeed- 

 ing generations is determined largely by temperature and humidity. 



As these two factors decrease the period increases. In like manner 

 we have shown on preceding pages how the egg and larval and pupal 

 stages are governed by the same laws. We have also shown that even 

 the daily rate of oviposition is accelerated by increases in temperature 

 and probably also of humidity. 



The common impression that "rain brings the weevils" has its basis 

 in the natural increase in the numbers of weevils shortly after a rainy 

 period, due somewhat to the fact that increased humidity reduces the 

 developmental period. A more important factor, however, is that 

 humidity reduces the effects of sunshine in killing the weevil stages. 



ZONE OF HIBERNATION. 



The behavior of the weevils in hibernation is fully discussed else- 

 where. In ice-box experiments at 45° F. it was found that the weevils 

 would not emerge, but Dr. W. E. Hinds found that 10 weevils which 

 had emerged from hibernation and which were confined 303 weevil 



1 That is, the temperature at which activity begins . 



' In a former publication (Bull. No. 51) we adopted the assumption made by other writers that 43° F. 

 is the general zero of effective temperature for insects. Recent experiments have shown conclusively that 

 this is an error, so far as the boll weevil is concerned. 



