﻿THE CATTLE GRUBS OR OX WARBLES 



77 



[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. Kecords sent the writers by C. J. Drake 

 on collections of larvte made by E. W. Dunnam and others at 

 Ames, Iowa, indicate that larvse of H. Uneatum 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 larvse of H. Uneatum 

 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 



Fig. 



-Hypoderma Uneatum: Date zones of first appearance of grubs in backs of 

 cattle, in the United States 



is hoped that tlie 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 larvae become matui-e 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 gi-ubs of 

 the season reach the back. It seems extremely rare for these eai-liest 

 grubs to complete their development in the minimum time, hence 

 the first emei'gence of larvaj from the cattle should be expected to 

 take place about 45 days after the grubs first reach (he back. At 

 Dallas the earliest larvtc become mature and begin leaving the host 

 between December 1 and IT) (hiring average years. The follow- 

 ing are the appi-oxiinate dates when mature larviu licgan emerging 

 from cattle at Middletown, N.Y.: In 1920, March 20; in 1922, 

 March 10; and in 1923, March 19. 



