THE CATTLE GRUBS OR OX WARBLES 79 
February 20 in 1923. The last date on which a third-stage larva 
was observed to reach the region of the back was June 6, 1922, at 
Middletown, N. Y. The record of earliest dropping of a mature 
erub of this species at Middletown was April 11, in 1923. Emer- 
gence from the host probably begins rather later than this on the 
average. In New York the maximum number of grubs of this species 
in hosts occurs about May 1. : 
Judging by collections and observations made in other States 
the seasonal occurrence of H. bovis tallies rather closely with that 
noted in New York. One record of earlier appearance has been 
made in western Pennsylvania, where in 1916 the grubs reached 
the back about January 25. That year grubs seemed to have been 
exceptionally early, especially in the Northeastern States. The dates 
of earliest appearance in the backs of cattle as calculated from ob- 
servations and collections in some of the infested territory are as 
follows: Maine, February 15; New Hampshire, February 8; Ver- 
mont, February 20; New Jersey, February 10; Ohio, February 8; 
Indiana, February 3; Illinois, February 5; Michigan, February 25; 
Wisconsin, March 2; Iowa, February 15; Wyoniing, February 15; 
Washington, February 8. These dates are presented to give a 
- general idea of the time of earhest appearance of larvee of H/. bovis 
in the subdermal tissues. It is recognized that they are not based 
on continuous accurate observations and that they will vary in 
different parts of a State and during different seasons. 
The date when all the grubs leave the host has been found to 
vary widely and in general to be much later than has been sup- 
posed. In New York in both Orange and Herkimer Counties most 
of the grubs are out of the backs of cattle by August 1, but 
healthy specimens have been taken as late as September 2 in 1922 
at Middletown and August 30 during the same year at Herkimer. 
Collections of H. bovis made by W. E. Dove at Aberdeen, S. Dak., on 
April 21, 1924, indicate that the cattle would be free from grubs in 
that locality about May 15. | 
It is worthy of note that the development of grubs in cattle in- 
fested in the North and shipped into the South agrees closely with 
that which occurs in the native habitat of the species. For instance, 
infested cattle shipped to Miami, Fla., from Syracuse, N. Y., during 
the winter showed a good infestation of well matured grubs on 
May 25. Larve of H. bovis in cows shipped from Michigan to 
Dallas, Tex., continued to develop and emerge normally up to June 
3, when the last one dropped. | 
In Ireland, Carpenter, Phibbs, and Slattery (19) record the 
presence of larvee of H: ineatum in the backs of cattle on March 
29, and a few larve of H. bovis were still present on June 10. 
Lehmann (49) and Vaney (105) found the highest percentage of 
grubby hides to occur in July in the Lyonnaise region of France. 
NATURAL CONTROL 
The combined action of all agencies of natural control has a 
marked effect on the abundance of both species of Hypoderma. 
This is true even in areas where the pest is at its worst. 
