June, '15] 



WEBSTER: THE OYSTER SHELL SCALE 



373 



eggs, but these were so rare that no counts were made. All the remain- 

 ing samples contained no sound eggs, showing that even at this time, 

 nearly a year after the extreme low temperature, the insect had not 

 begun to regain the lost ground. 



Figure 15 shows the condition of the scale in the winter of 1912-1913, 

 together with isotherms of minimum temperatures for January, 1912. 

 All the temperature records are taken from the Iowa Climatological 

 Service of the Weather Bureau, Report for January, 1912. 



Figure 15 — Localities in Iowa where the oyster-shell scale was killed in January, 

 1912, where it survived, with isotherms of minimum temperatures for January, 1912, 



All the localities where no sound eggs were present are included 

 north of the -32° isotherm. That is, a temperature of -32° F. evi- 

 dently was too cold for the eggs to survive. That they did not survive 

 this temperature is shown by the fact that the scale did not breed in the 

 area north of the -32° isotherm, in the summer of 1912, and so no live 

 eggs were found beneath the scales that fall. Many scales remained 

 on the trees, but there was nothing living beneath them. 



On the other hand, in Marshall county, where the minimum tem- 

 perature was -31°, many scales contained live eggs in the fall of 1912. 

 From the data at hand, it would appear that eggs might safely with- 

 stand a temperature of -31°, and yet succumb at -32°. However, 

 since the temperature records and samples of scale frequently were 

 taken in slightly different localities, there is some opportunity for 



