19 BULLETIN 1426, U. S. DEPARTMENT OF AGRICULTURE 
FLIGHT 
When the temperature of the air rises above approximately 65° F. 
migration of the adult borers may take place by flight. Atsuch a tem- 
perature borers have been seen to climb grass stems and clover tops and 
prepare for flight. Few beetles fly, however, until an air tempera- 
ture of 70° F. or more is attained. The time of the first spring flight 
is variable, depending on whether the soil temperature has been high 
enough to induce activity within the roots for some time previous to 
the rise of temperature above the critical point for flight. In the Pa- 
cific Northwest the times of occurrence of soil and air temperatures 
sufficient to induce flight are exceedingly variable from year to year, 
and these conditions have an important bearing on the early infesta- 
tion of new clover fields, and the resultant damage. 
The earliest recorded first flights of beetles from undisturbed clover 
in the Pacific Northwest were observed April 7, 1916, and April 8, 
1921, at Forest Grove, Oreg. Late records for earliest spring flight 
were, in the case of backward seasons, May 8, 1917, and April 26, 
1920, also at Forest Grove, and May 5, 1917, at Wapato, Wash. 
According to records of the Hagerstown, Md., field station, made by 
H. L. Parker, the first spring flight probably occurred about the sec- 
ond week of April in 1915 and in 1916. Webster and Mally recorded 
April 26 as the date of first flight for the season of 1899 at Wooster, 
Ohio. Davis (9) reported the first capture in flight on May 38, 1893, 
probably near Lansing, Mich. E. J. Vosler reported the first borers 
in flight on May 18, 1912, at Murray, Utah. Schmitt (39, p. 394), of 
Mainz, Germany, reported adult root borers in greatest abundance 
above ground at the end of April and the beginning of May. Del 
Guercio (16, p. 265) stated that in Tuscany movement of borers 
begins about the middle of April in a mild year, or, in the contrary 
case, towards the end of that month. FEichhoff (10, p. 97) reported 
swarming of this species before and about the middle of June at Mil- 
hausen, Alsace. 
In Oregon favorable temperatures usually occur in the after- 
noon between 1 and 6 p. m. during May and June, and similarly at 
Murray, Utah, according to Vosler’s notes. Eichhoff (10, p. 4) noted 
that at Milhausen, Alsace, bark beetles, including clover root borers, 
were in flight in summer and fall’ only during the afternoon hours. 
The optimum temperature for flight appears to be 70° to 80° F. In 
the Pacific Northwest, because of unfavorable weather conditions, 
the normal flight period is often interrupted for several days at a 
time. The time of maximum flight, therefore, is usually several 
weeks later than the first flight of the earliest adults, and, in the 
Pacific Northwest, often occurs in May. The maximum flight at 
Forest Grove, Oreg., in 1916, occurred about May 2; in 1917, a back- 
ward season, about May 28. The maximum flight is of compara- 
tively short duration, and there is a rapid dropping off in flight rec- 
ords after the maximum is passed. Scattering adults are rarely 
taken in flight as late as July. The latest recorded date for the 
Willamette Valley is July 14, 1915 (M. M. Reeher sweeping), and 
for the Yakima Valley in 1917 it was June 18 (one observed by E. E. 
Cowin, on screen). 
The period of flight appears to be a time of great restlessness among 
the adults. The beetles do not settle upon the first plant on which 
