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THE CATTLE GRUBS OR OX WARBLES 11 
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 lhghter 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 heavy 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. léneatum, 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 caitle 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 larve, 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 4-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. bovis is known 
to be gaining a foothold, indicates that it may really be due 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 infestation 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 matter 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 larve within the bodfes of the hosts. The first 
group, of course, varies with the abundance of the insects and also. 
with the species present. Hypoderma Yovis causes thé most excite- 
ment to stock and hence, where it occurs, produces the greatest dati 
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