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JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
sorts of conditions, third, the fact that it is a most efficient host of 
larvae and nymphs. The degree to which these mice are infested de¬ 
pends largely on the combination of conditions present in any locality. 
Every variation may be found from the locality where an infested 
mouse is a rarity to that in which all are infested. For example, in 
the Musselshell operations, 54 mice taken on a certain homestead at 
various times during the season were all uninfested, whereas 9 mice 
taken in a badly infested coulee on August 5 averaged 19.88 ticks 
each, and 5 taken in a similar location on August 23 averaged 19.40. 
As many as 125 ticks have been taken from a single mouse, 39 of them 
fully engorged. The average infestation for 307 mice taken under 
prairie conditions was .19 (in a poor tick season), that for 343 mice 
under hill conditions 1.25. In prairie country these mice were found 
in every conceivable sort of place, while under the hill conditions stud¬ 
ied they were abundant only on the rocky slopes along the edges of 
the valleys. 
Field Mice .—The possible importance of field mice was suggested 
by the work on the prairie where the average infestation was found 
the same as for deer mice. Under hill conditions more definite data 
were secured. For 26 mice examined during 1917 the average of 
larvae and nymphs was 5.15. The average for 9 taken from a badly 
infested coulee on August 5 was 10.73. The high seasonal average in 
1917 does not mean that they are in any way comparable to deer mice 
as hosts. They are not. It is only where very favorable habitat 
conditions occur, and this is not frequent, that they are of importance. 
Where this is the case, however, they are a factor to be reckoned 
with. 
One may easily be deceived as to the importance of mice unless 
great care is used. Suspecting that such was the case observations 
were made which showed that the great majority of deer mice get 
into traps before 11 o’clock at night and most field mice much earlier. 
The ticks are therefore afforded ample time to leave the host even 
though the traps are collected early in the morning. The precaution 
of examining the ground around each trap has not only greatly in¬ 
creased the value of our records, but has also made it evident that the 
figures given are too low. 
As to domestic animals as hosts of adult ticks there is little of definite 
value to add to the facts learned in the Bitter Root. There is some 
difference in habits as regards the points of attachment to the host. 
It was discovered, however, that pigs running at large will pick up 
large numbers of ticks, a fact of some interest. Tabulation of data for 
1917 shows that horses, cattle and pigs are efficient as tick hosts in 
the order named. The records show that of the ticks taken from 
