DIXON — RODENTS IN IMPERIAL VALLEY 
139 
land were thus forced to seek protection in the sheltering mesquite 
thickets of the adjoining sand dunes. Periodic flooding of the fields 
during irrigation destroyed the burrows of such rodents as returned 
to their former homes in the cultivated fields. This concentration of 
the rodent population meant increased competition for food. At the 
same time the food producing area was greatly reduced especially where 
cotton was raised. As a result, considerable damage was done, partic- 
ularly by the little squirrels (Citellus), to grain fields that were enclosed 
or bordered by mesquite thickets. Some difficulty also resulted 
from the burrowing of the rodents, chiefly squirrels, into sandy irri- 
gation ditch banks between irrigations. Some ditches or levees that 
remained unused for a few months were thus rendered useless. The 
abundance of squirrels at this locality was indicated by four being 
caught in as many days in one trap set at the mouth of one burrow. 
Poisoning with barley coated with strychnine afforded temporary 
relief, and the gradual settlement of the land resulting in the complete 
removal of the sand dunes, which are being leveled into fields, means 
that this first class of rodents will soon be controlled and in many 
instances exterminated over large irrigated areas. In other words, 
once their food supply, breeding dens and temporary refuges, the 
essentials of a successful existence, are all destroyed, the rodents will 
cease to exist. 
In considering the rodents that have increased through reclamation, 
we find that the construction and use of irrigation canals has been the 
fundamental cause of their successful battle for existence. The irri- 
gation ditches have simply served as highways for the spread of aquatic 
or stream-side animals. Thus, harvest mice and cotton rats have 
followed down along the canal banks through Lower California from 
the Colorado and Alamo Rivers until they are now to be found in 
considerable numbers in American territory. On March 17, 1921, 
the writer found many well-defined runways and trapped an adult 
breeding male cotton rat one half mile south of the International 
Boundary near Allison Heading. Six harvest mice were also captured 
in the cotton rat runways. Extensively used cotton rat runways were 
also found in Imperial County on the American side of the line, so 
that this unwelcome rodent is now definitely known to be established 
in the cotton districts of Imperial Valley. It is unlikely that the 
harvest mouse will do any considerable damage, but the cotton rat is 
dangerous. In Texas and certain other gulf states cotton rats have 
on occasion proved very destructive to cotton and corn crops. These 
