﻿THE CATTLE GRUBS OR OX WARBLES H 



areas of unusual abundance or scarcity; but it is believed that the 

 effect of drainage, soil conditions, and the presence of certain types 

 of vegetation are factors. In general, the lighter soils and good 

 drainage, especially in the areas of heavy rainfall, appear to be 

 favorable to the development of grubs. The presence of timber or 

 moderately heavj^ vegetation also seems to favor them. There ap- 

 pears also to be an inhibiting influence existent along the coast, 

 especially on the Atlantic. 



It has not been possible to determine any special correlation be- 

 tween altitude and the abundance of grubs. It has been noted, how- 

 ever, that they seem to thrive admirably in fairly high altitudes. 

 This is especially true with H. lin-eatum., which is known to be 

 abundant at elevations above 7,000 feet. Some reports received 

 from hide dealers indicate that they think grubs relatively fewer in 

 numbers in the mountain ranges, but the writers' observations do 

 not bear this out. 



The evidence seems clear that in general where cattle are kept 

 on the range, especially during the spring months, the grubs are 

 more abundant. This condition is associated with the care the 

 cattle receive, the absence of protection from the flies by barns and 

 sheds, and the lack of cultivation of large areas. It is also probable 

 that the poor and weak condition of stock on the range during early 

 spring gives the flies a better opportunity of successfully depositing 

 their eggs on them. Cultivation is apparently destructive to many 

 of the larvae, although the reduction in the number of grubs may 

 be brought about by the combination of conditions which usually ac- 

 company intensive cultivation of large areas. 



There is considerable variation in abundance of grubs from year 

 to year in any given region, but in general this variation does not 

 prevail throughout the entire country. Statistics on the percentage 

 of grubby hides during the years 1921 to 1923, inclusive, furnished 

 by some of the leading meat packers as recorded at their principal 

 plants, show a distinct increase (average of 10 per cent) in grubbi- 

 ness during this -l-year period. At first thought this might be at- 

 tributed to seasonal conditions; but the fact that this increase was 

 greatest and most consistent in the North, where H. hovis is known 

 to be gaining a foothold, indicates that it may really be clue in part 

 at least to the spread of that species. If this is true, a very con- 

 siderable further increase in percentage of grubby hides and degree 

 of infe.station may be expected. 



ECONOMIC IMPORTANCE 



That grubs are highly injurious to cattle is generally conceded. 

 Opinions vary greatly, however, as to the exact extent of the in- 

 jury, and it is a difficult matt-er to determine with any degree of 

 accuracy the ill effects caused through the activity of the various 

 stages of these insects. Injury is produced in a number of different 

 ways. These may be classed in two general groups — (1) annoyance 

 caused by the flies during the deposition of eggs, and, (2) irritation 

 produced by the larvae within the bodies of the hosts. The first 

 group, of course, varies with tiie abundance of the insects and also 

 with the species present. Ilypoderma hovu causes the most excite- 

 ment to stock and hence, where it occurs, produces the greatest dam- 



