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We examined if Valley pocket gophers (Thomomys bottae) exhibited a seasonal preference for consuming steamed-rolled oat (SRO) groat bait treated with 1.6% Dupont oil blue A (fat-marking dye). Field tests were conducted in California alfalfa fields during winter, summer, and fall, and in walnut orchards during winter and summer. Five treatment units (TUs) were established each with 60 burrows for each season and habitat studied during 1997–1999. SRO groat marker bait (4 g) was placed inside each active burrow. All bait sites were re-opened about 96 h later and examined for the presence (all or some) or absence of bait. The average bait site disturbance after 4 days of baiting was 78.2%. Traps were used to capture gophers up to 5 days after baiting. Valley pocket gophers (n=744) were examined for the presence of blue dye in their cheek pouches, skin/fur, and fat. We expected that if gophers moved the bait, their cheek pouches and fur would be marked; if they consumed bait, their fat would be marked. In alfalfa, 54.2%, 46.8%, and 65.7%, of gophers were marked (trapping on days 5–9) by blue dye in one or more of their cheek pouches, skin, fur, or subcutaneous fat in the winter, summer, or fall, respectively. In orchards, 57.1% of gophers were marked in winter and 53.4% in summer. Of those that were marked, all (100%) had their fat dyed blue, followed by skin/fur (34.4% males: 43.7% females) and cheek pouches (5.7% males: 10.1% females). ANOVA results indicated no difference in seasonal marking efficacy in either alfalfa or orchards (F=3.59, P=0.0598 and F=0.12, P=0.7384, respectively). The usefulness of 1.6% Dupont oil blue A dye as a marking agent for Valley pocket gophers was not demonstrated overall or for any season in either habitat. Therefore, a better marker for this gopher is needed. Some factors that may have influenced these results were discussed including: (1) baiting methodology ( 66% of the bait sites were devoid of bait at the end of the study), (2) species specific dye properties, (3) bait acceptance (i.e. aversion to the dye), or (4) availability of alternative foods.