THE CATTLE GRUBS OR OX WARBLES Ar 
stantially correct for that section in 1924. W. H. Brittain has 
sent the writers specimens collected by W. E. Whitehead at in- 
tervals during the spring of 1924 from cattle at Truro, Nova Scotia. 
In an examination of a large number of cattle by Doctor Whitehead 
on March 24 only 3 grubs were found. The largest one had been 
in the back about 10 days. Records sent the writers by C. J. Drake 
on collections of larve made by E. W. Dunnam and others at 
Ames, lowa, indicate that larve of H. lineatum first reached the 
backs in 1923 about January 8, and in 1924 about January 20. 
Hundreds of collections have been made by the writers and by 
correspondents throughout the country, and with these records as 
a basis the dates of appearance of the earliest larvee of H. lineatum 
have been estimated and the accompanying map (fig. 33) constructed. 
Unfortunately, as the number of accurate records of the time of 
first appearance is small, many discrepancies will be found, but it 
Fic. 33.—Hypoderma lineatum: Date zones of first appearance of grubs in backs of 
cattle, in the United States 
is hoped that the data available will serve a useful purpose in 
control undertakings and also stimulate others to add to these much 
needed records. 
The date when the first larvee become mature and leave the host 
is an important one from an economic point of view. This date is 
necessarily closely correlated with the date when the first grubs of 
the season reach the back. It seems extremely rare for these earliest 
grubs to complete their development in the minimum time, hence 
the first emergence of larve from the cattle should be expected to 
take place about 45 days after the grubs first reach the back. At 
Dallas the earliest larvee become mature and begin leaving the host 
between December 1 and 15 during average years. The follow- 
ing are the approximate dates when mature larve began emerging 
from cattle at Middletown, N. Y.: In 1920, March 26; in 1922, 
March 16; and in 1923, March 19. 
ib 
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