Operational Aspects of Successful Ground Squirrel Control by Aerial Application of Grain Bait

By County Ordinance the County Agricultural Commissioner has been charged w i t h rodent suppression in Fresno County, C a l i f o r n i a . For n i n e years personnel from the A g r i c u l t u r a l Commissioner's Office have conducted a county operated and funded program of ground s q u i r r e l suppression through the use of a e r i a l l y a p p l i e d zinc phosphide treated s q u i r r e l oat groats. P l a n n i n g procedures, r e l a t i v e costs and operational aspects of the program are o u t l i n e d . T h i s technique of ground s q u i r r e l suppression has been shown to have many advantages over hand a p p l i c a t i o n of rodent baits. No known hazards to non-target w i l d l i f e species have been demonstrated. By d i r e c t i o n of a Fresno County Ordinance, the A g r i c u l t u r a l Commissioner is charged w i t h conducting rodent suppression programs. In certain rangeland areas away from water courses and populated areas, the A g r i c u l t u r a l Commissioner has elected to broadcast z i n c phosphide treated oat groats over and near ground s q u i r r e l colonies by aircraft. This method was selected because of the effectiveness of the technique and low cost compared w i t h other suppression methods. It was also determined a safe and effective method of control after s t u d i e s were conducted by Rex E. Marsh, S p e c i a l i s t , Department of Animal Physiology, U n i v e r s i t y of C a l i f o r n i a , Davis, in 1968; and explored on a l i m i t e d basis by Howard, et al., in 1956. Fresno County, comprising some 6,000 square m i l e s , has led the nation's farm counties for over two decades in total gross value of a g r i c u l t u r a l output. A g r i c u l t u r e is h i g h l y d i v e r s i f i e d , producing over 200 crops in a w i d e area of the county. Cropland inventory shows 1,178,144 acres in Fresno County. The topography of the county varies widely from very f l a t lands found on the Westsi d e to the steep slopes found in some of the range areas. Almost every type of s o i l may be found in Fresno County ranging from clays through sands to coarse sandy loams. S o i l depths range from less than one foot to in excess of 50 feet. Water supplies vary widely, depending on the locality. The lower f o o t h i l l areas of Fresno County are g e n e r a l l y occupied on a year-round b a s i s w i t h the economy based on livestock operations, f i e l d crops, citrus, deciduous f r u i t s and nuts, and l i m i t e d recreational sites. Small commercial enterprises are also located throughout these areas -often at road intersections. Vast areas of undeveloped land are found in the lower f o o t h i l l s and the topography varies w i d e ly from almost flat or gently r o l l i n g slopes to steep s l o p ing h i l l s . Soils of the lower f o o t h i l l s consist of material weathered from several different k i n d s of rock. S o i l depths g e n e r a l l y range from one to three feet w i t h the texture ranging from coarse sandy loams to the various clays. Water is in very l i m i t e d supply throughout most of the area. Vegetation cover consists mostly of annual grasses and forbs w i t h trees and shrubs growing in some places up to, but not i n c l u d i n g , chaparral and woodland areas. The tree anemone, Carpenteria c a l i f o r n i c a , is a r e l a t i v e l y rare species which may be found at the upper edges of t h i s area. V a l l e y vegetation consists of annual and perennial grasses, f i e l d crops, v i n es , trees, and shrubs. W i l d l i f e is found in abundance throughout the county and i n c l ud es pheasant, mourning dove, meadow lark, woodpecker, scrub jay, r e d t a i l e d hawk, golden crown sparrow, flicker, road runner, b l u e jay, owl, towhee, turkey vulture, b l a c k b i r d , s t a r l i n g , and many other species of b i r d s . The C a l i f o r n i a condor, peregrine falcon, and southern b a l d eagle are occasionally reported in Fresno County. These are c l a s s i f i e d as endangered by the C a l i f o r n i a Department of Fish and Game. Many mammals are found in Fresno County. Species i n c l u d e cottontail, deer, bobcat, weasel, s q u i r r e l , C a l i f o r n i a jack r a b b i t , bat, pocket gopher, various species of mice, coyote, skunk, raccoon, beaver, opossum, and many others. Some mammals c l a s s i f i e d as rare or endangered i n c l u d e the spotted bat, Euderma maculatum; Fresno kangaroo rat Dipodomys n i t r a t o i d e s e x i l i s ; and San Joaquin k i t fox, Vulpes macrotus mutica.