April, '12J HEADLEE: HESSIAN FLV 101 



Although ahiiost every writer on Hessian fly has recommended the 

 employment of late sowing as on(> of the means of control, no important 

 attempt was made to understand tlie underlying cause for the ob- 

 served fact until the work of Webster in Ohio and Indiana. Through 

 the medium of widely scattered seodings made throughout the normal 

 period of wheat sowing, Webster was able to show a distinct ratio 

 between the dates of the disappearance of fall brood and the latitude, 

 and this ratio was determined as about one day for each one fourth of 

 a degree — the fall brood disappearing one day earlier for each one 

 fourth of a degree north and one day later for each one fourth of a 

 degree south of a given point. One year later, Hopkins'* confirmed 

 in AVest Mrginia the latitude ratio obtained by Webster in Ohio and 

 showed that wherever altitude was sufficiently variable to bring about 

 difference in climate, there existed a ratio between the disappearance 

 of the fall brood and the height above the sea. He showed that a 

 difference of 100 feet in altitude made a difference of one day in the 

 time of disappearance of the fall brood — one day earlier if 100 feet 

 higher and one day later if 100 feet lower than a given point. Although 

 Hopkins did not attempt to state these ratios in the form of a law, he 

 set them forth for the first time with sufficient clearness to merit 

 such a designation. In substance he said, under normal climatic 

 conditions the date of the disappearance of the fall brood of Hessian 

 fly and consequently the date of safe sowing of wheat varies with 

 latitude and altitude, being one day earher if north one fourth of a 

 degree or higher by 100 feet, or one day later if south one fourth of a 

 degree or lower b}- 100 feet than a given point. This ratio may well 

 become known in Hessian fly annals as "Hopkins Law of Latitude 

 and Altitude." 



The universality of this law may be tested by applying it to deter- 

 mine the date of safe sowing over the fly-infested parts of the f nited 

 States generally. The writer purposes to test it in the light of studies 

 made in Kansas. During each of the last four years, a series of stations 

 has extended from the north to the south boundarj' of the state through 

 the eastern edge of the great central wheat belt. During each of the 

 last three years, a parallel series has extended through the western 

 edge of that portion of the wheat belt infested with fly. 



Tlie individual stations of the eastern series are located from north 

 to south at Marysville, ^Manhattan, Marion, Sedgwick, Wellington, 

 and Caldwell. The individual stations of the western series are 

 located from north to south at Norton, Smith Center, Wilson, Great 

 Bend, Pratt and Sawyer. During the first two years, representatives 

 of the Bureau of Entomology participated in securing and handling 



* Hopkins, .\. D., Bui. 67, W. Va. Expt. Sta., 1900. 



