116 



JOURNAL OF ECONOMIC ENTOMOLOGY 



[Vol. 3 



30° C. 



Dajs 



tion of the boll weevil into Louisiana. More recently Hunter and 

 Hooker (13) have made a practical application of the principle in 

 connection with the time of hatching of the cattle tick as related to 

 the time for pasture rotation necessary for its control. 



In his previous paper the writer has indicated that the point above 

 which temperatures are "effective" varies with the species and is by 

 no means constant at 43°F. This point was termed the "critical 

 point," but as will be further explained below the term "critical 

 point" is inapt and we believe should be abandoned. The use of 



43°F. as the starting point for accumu- 

 lating "effective temperature" seems to 

 have originated with Merriam, who at- 

 tributes it to Marie-Davy and other 

 European phenologists. That such a 

 view is wholly untenable and that the 

 point above which growth takes place 

 varies widely with species and their 

 stage of growth is readily seen by an 

 examination of botanical literature. 

 Abbe has cited this literature exhaust- 

 ively and shows that 43 °F. refers only 

 to the germination and growth of wheat, 

 and that each plant has a different 

 "minimum temperature for germina- 

 tion" and growth. He quotes the ex- 

 periments of De Candolle (4) in the 

 germination of seeds, which are shown 

 graphically in figure 6, showing that 

 growth may commence at. 0°C. for 

 Sinapsis alba up to 9°C., for Zea mays. 

 This has been fully studied by other 

 botanists, notably by Sachs (23) and 

 the literature of the subject has been 

 thoroughly digested by Davenport in 

 his Experimental Morphology, up to 

 1897, in which he shows that the same principles apply to animals of 

 all classes. That the minimum temperature aft'ecting growth is 

 variable is well shown in figures 24, 25 and 26, in which it may be 

 seen that Toxoptera and Lysiphlehus may develop at 1.65 °C. while 

 Margaropus and others will not develop under 5° to 10° C. If then 

 there is no uniform minimum above w^iich the temperature may be 

 accumulated as effective, but this varies with each species and phase 



50 



Fig. 7. Relation of tem- 

 perature to time of pupa 

 stage of Malacosoma ameri- 

 cana (original). 



