ae THE CATTLE GRUBS OR OX WARBLES 113 
and have been determined and mapped with a fair degree of accu- 
yacy in several parts of the country. In general the duration of 
snfestation of the backs of cattle with ZZ. bovis is longer than with 
H. lineatum, owing to the longer developmental period required by 
H. bovis. 
Temperature, humidity, cloudiness, and drainage are important 
factors in natural control. Host resistance as acquired by age 
is also an important factor in holding down the abundance. of 
Hypoderma. No important insect enemies or diseases affecting 
Hypoderma have been found. 
The most effective time for the practice of control procedure 1s 
during the period of development of the larve in the subcutaneous 
tissues of the back. Destruction of the larve may be accomplished 
by hand extraction with or without the aid of forceps. In the case 
of certain breeds of cattle extraction of the grubs is very difficult. 
Tests of the application of washes, powders, and ointments to the 
backs of the cattle and also the injection of substances into the cysts 
containing the larve show that each of these methods of treatment 
is effective if certain materials are used. Among the most effective 
should be mentioned: Derris used as a wash, as an ointment, or as 
a powder; iodoform used as an ointment; pyrethrum applied as an 
ointment; benzol and carbon tetrachloride injected into the grub 
eysts; fine tobacco applied in powder form and nicotine dust applied 
dry. 
No adverse effects on the host have been observed in this country 
from the destruction of the grubs in the backs of cattle with suitable 
insecticides. 
The cost of extraction or treatment depends on the skill of the 
operator but in all cases it is small compared with the benefits 
derived. 
Experiments with the application to the feet and legs of cattle of 
certain insecticides and repellents by means of wading vats show 
that with the materials used control was not brought about, and indi- 
cate that attack directed against the eggs or young larve in this way 
will not be successful. | 
Results of individual effort against cattle grubs is likely to be dis- 
appointing, especially if the work is not thoroughly done and if 
untreated cattle are in close proximity to the treated herd. 
Systematic treatment of all infested cattle in a neighborhood either 
by extraction or by the use of insecticides appears to be a practical 
method of control or eradication. Although these methods are best 
adapted to dairy or farm conditions it is probable that they may be 
modified so as to be applied successfully to range cattle. 
In order successfully to combat the insects by destroying the 
larvee in backs of cattle it is necessary to treat all infested cattle at 
intervals of not to exceed 35 days. The first treatment must be given 
before the earliest larve have matured and left the host. In most 
parts of the United States it is necessary to treat the animals four 
times during a season to reach all of the larve. The time of begin- 
ning the treatments in different parts of the country is indicated in 
the discussion of seasonal history. 
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