﻿THE CATTLE GKUBS 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 

 gi'ub 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. hovis 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; Wyoming, February 15; 

 AVashington, February 8. These dates are presented to give a 

 general idea of the time of earliest appearance of larvae of H. hovis 

 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 II. hovis 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. Larvae of II. hovis 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 larvae of H. Uneatum- in the backs of cattle on March 

 29, and a few larvae of II. hovis were still present on June 10. 

 Lohmann {59) and Vaney {lOo) found the highest percentage of 

 grubby hides to occur in July in the Lyonnaise region of France. 



NATURAL CONTROL 



The combined action of nil agencies of natural control has a 

 marked effect on tlie abiiiKhuicc of both species of Hypoderma. 

 This is true even in areas wiiere the pest is at its worst. 



