INFLUENCE OF TEMPERATURE ON DIFFUSION. 93 



stances at a slightly lower temperature, but these instances are 

 probably too infrequent to become of economic importance. 



With the eggs in the North the case may be more important, because 

 these, deposited in dead leaves of bluegrass, and sometimes probably 

 buried under several inches of this matted grass, with the living leaves 

 covering this over, the temperature and moisture would both be 

 greater than at several feet above ground without such protection. 

 Mr. Philip Luginbill of this bureau in April, 1911, proved this to be 

 true. He placed a thermometer in just such a position as men- 

 tioned above, in a protected nook where the sun could shine directly 

 on it in the grass and no wind could reach it and found that the 

 temperature was 10° to 12° F. higher than when the thermometer 

 was several feet above the ground and in the shade. The junior 

 author has found that eggs are deposited in just such places, and that 

 hatching takes place in spring at a temperature ranging, as recorded 

 by the thermograph, from 32° to 62° F. It would appear that 

 eggs deposited in a position as mentioned above would hatch sooner 

 than those deposited in places where the temperature would not be 

 so high and the stem mothers from the former would reproduce, 

 the pest becoming more abundant in the spring and making its way 

 from grass to grain earlier and in greater numbers than they would 

 from the cooler locations. 



This leads us to a very interesting and important point in tem- 

 perature effects on the species. In the South, seemingly south of about 

 latitude 35° to 36° north, it has been impossible to find eggs of this 

 and other species of aphidids in the fields. There is in the perpetua- 

 tion of the species no apparent need of this stage, however, as it is 

 able to continue throughout the entire year reproducing viviparously. 

 In the North this is probably not possible except during very mild 

 winters. The situation is therefore about like this: Gradually as 

 we proceed southward from about latitude 38° the sexual forms and 

 eggs disappear, while to the north of about latitude 36° hibernation 

 is confined more and more to the egg stage, until this becomes ex- 

 clusively the state in which the winter is passed. 



The practical, economic importance of this is that there is con- 

 siderable doubt relative to the amount of injury the pest would 

 cause north of this belt of country if there were no Toxoptera drifting 

 in from the south. In other words, but for the countless myriads 

 developing south of this belt and sweeping over and beyond it, there 

 would be few if any destructive ravages. If this is the true state 

 of affairs, the oats crop north of this belt is to a certain degree de- 

 pendent upon the success or failure in controlling the pest in Texas, 

 Oklahoma, New Mexico, and South Carolina. 



Summarizing, then, it would appear from the information we have 

 been able to obtain, and which is given throughout this publication. 



