Control of Ground Squirrels in California Using Anticoagulant Treated Baits

Anticoagulant treated grain baits have been used to control vertebrate pest depredations in California for over 30 years. The use of anticoagulant treated baits has increased seven (7) times in the past seven (7) years; the majority for the use of ground squirrel, Spermophilus spp., control. Since 1968-69, an average of 1,747,828 net over 5,700,919 gross acres per year has been treated for ground squirrel control. Current use patterns for ground squirrel control with anticoagulant treated baits include: (1) Repeated spot baitings, and (2) exposure of bait in bait boxes. Experimental work and many years of operational field use have proven that anticoagulant treated baits have a place in effectively suppressing ground squirrel populations in localized areas, with little hazard to nontarget animals. However, these baits have not proven to be a practical substitute for current techniques of suppressing ground squirrel populations over large areas, such as in rangeland situations. GROUND SQUIRREL CONTROL IN CALIFORNIA California is fortunate to have a unique system of county agricultural commissioners in 56 of the 58 counties in the State. The agricultural commissioner, with his staff of trained agricultural inspectors and biologists, is responsible for a variety of regulatory enforcement duties and related activities. Among these duties may be that of conducting vertebrate pest control activities, including controlling ground squirrel depredations. The county agricultural commissioners expend in excess of $3 million annually for vertebrate pest control operations (California Department of Food and Agriculture, 1977c). Ground squirrels continue to be one of the major vertebrate pests to California's agriculture. Dana (1971) reported that the estimated loss from ground squirrels in California to be $8 million annually. Records show that since 1968-69, agricultural commissioners treated an average of 1,747,828 net over 5,700,919 gross acres per year for ground squirrel control (California Department of Food and Agriculture, 1977a). Control techniques include the use of grain baits treated with Compound 1080, zinc phosphide, strychnine, or one of the anticoagulants. In addition, in excess of 100,000 pounds of the fumigants carbon bisulfide, methyl bromide, and U.S. Fish and Wildlife Service gas cartridges are used (California Department of Food and Agriculture, 1977a). Other than those baits formulated and dispensed by the county agricultural commissioners, the one bait available commercially, and used to any extent is Ramik Green, a 0.005 percent diphacinone treated pellet, manufactured by Velsicol Chemical Company. The Chempar Chemical Company also has registered a 0.005 percent Chlorophacinone treated pellet for ground squirrel control. ANTICOAGULANTS USED IN CALIFORNIA Anticoagulant treated baits have been used in California to reduce depredations caused by vertebrate pests for nearly 30 years. Field testing of Compound 42 (warfarin) for ground squirrel control began in California in 1949 (Ball, 1949). In 1950, a little over one (1) ton of anticoagulant treated bait was used for ground squirrel control (Ball, 1950). The agricultural commissioners were quick to incorporate into their vertebrate pest control programs new toxicants and techniques that would offer greater safety to nontarget animals, even though they realized the use of anticoagulants would increase costs by using more bait and requiring more labor to place and service bait boxes. Cummings (1953) reports that by early 1953 satisfactory control of ground squirrels was being obtained by county personnel in Santa Barbara and Ventura counties. By 1954, nearly 24 tons of warfarin and pival treated grain bait was used to control ground squirrel depredations (Ball, 1954). Records compiled by the California Department of Food and Agriculture (1977a) show a steady increase in the use of anticoagulants for vertebrate pest control (Table 1). In 1968-69, the agricultural commissioners reported using nearly 76 tons of anticoagulant treated grain bait. In 1975-76, the use reached a high of nearly 535 tons of anticoagulant treated grain bait. This is a seven (7) fold increase in as many years. In the past four years an annual average of about 174 tons of anticoagulant treated baits were used to control depredations of other vertebrate pests, but the majority of the total used was for ground squirrel control. The use of anticoagulant treated baits seems to be leveling off. The use in 1976-77 was down nearly 90 tons from the previous year. It is interesting to note that the use of anticoagulants in 1976-77 was 2.8 times greater than zinc phosphide treated baits, and nearly 1.8 times greater than Compound 1080 treated grain baits. 98 The annual average acreage treated for ground squirrels by agricultural commissioners has remained relatively constant. Since 1968-69, the annual average of acres treated for ground squirrels has been 1,747,828 net over 5,700,919 gross acres. Table 1. Use of anticoagulant treated baits in California. *Field testing occurred. ** Not used in computing average annual use. In California, anticoagulant treated grain baits have found a definite place in vertebrate pest control operations in spite of the increased costs for labor and materials. KINDS OF ANTICOAGULANTS USED Early use patterns have gradually shifted from Compound 42 and pival to the use of other anticoagulants. As of May 2, 1977, the Pesticide Registration Number Book (California Department of Food and Agriculture, 1977b) indicates that twenty-eight (28) of the agricultural commissioners have some type of anticoagulant treated bait registered for ground squirrel control. Some have more than one type of anticoagulant registered. The number of county registrations for the particular type of anticoagulants include: 23 diphacinone, 1 chlorophacinone, 5 pival, 1 warfarin, and 1 prolin. Not included in these tabulations are recently registered labels under the special local needs 24 (c) regulations of FIFRA. EXPERIMENTAL WORK Using caged squirrels, Spermophilus b. beecheyi, in nonchoice feeding trials Marsh (1964) concluded that diphacinone and pival treated squirrel oat groats gave the best control. This judgment was based on bait that caused death within the shortest period of time and showed the least tendency for the squirrels to recover. Prolin, warfarin, and fumarin treated baits appeared inconsistent for ground squirrel control. All baits were mixed at the 1:16 ratio with commercial concentrates. In these limited trials, death occurred in the five animals feeding on diphacinone treated bait in an average of 10.4 days (range six to 13) and each consumed an average of 144.96 gm (range 110.6 to 205.4) of bait. The five squirrels feeding on pival treated bait died in an average of 10.4 days (range five to 14) and consumed an average of 180.06 gm (range 88.5 to 271.1) of bait. Kreps and Dixon (1975) conducted a field trial comparing the effectiveness of 0.005 percent and 0.01 percent diphacinone treated grain bait for ground squirrel, Spermophilus b. beecheyi, control. The reduction of ground squirrel activity in the plot with 0.005 percent diphacinone treated bait was 91.7 percent. Reduction in activity in the plot with 0.01 percent diphacinone treated bait was 95.8 percent. In this trial grain baits were exposed in bait stations made from used automobile tires cut across the diameter and wired closed so the beads were touching. The stations were placed in a grid 200 feet apart in four (4) acre sized plots. Bait was replaced on an "as needed" basis. Ground squirrels accepted the bait within 24 hours in the (nontreated) control area and in the 0.005 percent diphacinone area. Acceptance at all stations in the 0.01 percent diphacinone plot occurred within six (6) days. The highest live squirrel counts in both treated plots was 48 (12 per acre). Total bait consumption in the 0.005 percent diphacinone plot was 300 pounds. One-hundred twenty (120) pounds of bait was consumed in the 0.01 percent diphacinone plot. Five-hundred forty (540) pounds of untreated bait were consumed in the control plot. Consumption based on maximum live squirrel counts, within each plot, was 20 pounds per squirrel in the untreated plot, 6.25 pounds per squirrel in the 0.005 percent diphacinone plot, and 2.5 pounds per squirrel in the 0.01 percent diphacinone plot.