MONITORING BAIT ACCEPTANCE IN BRUSH-TAILED POSSUM POPULATIONS : DEVELOPMENT OF A TRACER TECHNIQUE

Rhodamine B dye, Lissamine green dye, and demethylchlorotetracycline were evaluated as tracers for monitoring bait acceptance by brush-tailed possum (Trichosurus vulpecula Kerr) populations. Demethylchlorotetracycline left no detectable trace in possums, even at high dosages. Lissamine green was not persistent enough externally and did not stain the gut tissue. Rhodamine B marked animals for 7 days when used at 0.5% concentration, surface-coated on baits, and can be used at up to 1% concentration without reducing the palatability of the bait. Rhodamine B was therefore selected as a suitable tracer. INTRODUCTION This work was conducted as a prerequisite to another study (Morgan in prep.) in which was estimated the proportion of possums in various populations that would accept different types of bait. Tracers have been used in several baiting studies (Lindsey et al 1971; Nass et al. 1971; Nelson & Linder 1972; Sullins & Verts 1978). They have also frequently been used to study the movements of animals. Comprehensive reviews of the literature on this subject have been given by Taylor & Quy (1973) and Evans & Griffith (1973). A successful tracer for field use in possum bait-acceptance studies had to: (1) Be effective as a marker for a minimum of 7 days, thus allowing time for capture of a sample of possums after baiting; (2) Not affect the palatability of the carrier bait; (3) Not affect the behaviour of the possum, particularly in relation to feeding; (4) Be inexpensive in large quantities since the proposed field use would eventually involve addition of tracer to several tonnes of bait concurrently. New Zealand Jou rna l of Fores t ry Science 11(3): 271-7 (1981) 272 New Zealand Journal of Forestry Science 11(3) SELECTION OF MATERIALS Whilst radioisotopes offered the potential of a quantifiable technique (Spragg & Fox 1974) they were considered to be too hazardous and, like many of the otherwise apparently suitable chemical dyes, too costly for bulk usage. Demethylchlorotetracycline (DMCT) has been a useful tracer in a variety of vertebrates. Single doses of lOmg/kg body weight administered orally to> coyotes (Canis latrans) produced fluorescence under ultraviolet light that was noticeable in the mandible and which lasted for at least 5 months (Linhart & Kennelly 1967). A single oral dose of 50mg/kg body weight produced fluorescence in the jaw of laboratory white rats in less than 1 day and the fluorescence remained visible under ultraviolet light for up to 6 months (Crier 1970). Other tetracyclines have been used to label teeth in fur seals (Yagi et al. 1963) and bones and scales of fish (Weber & Ridgway 1962). Hence it was expected that DMCT might prove to be a successful marker in the possum. Rhodamine B, a red dye that fluoresces orange under ultraviolet light, was selected for evaluation since it has been used successfully in bait acceptance studies. Evans & Griffith (1973) fed 100 g of carrot treated with a 0 . 1% solution of Rhodamine B to black-tailed jack rabbits (Lepus californicus) and cottontail rabbits (Sylvilagus nuttallii). The black-tailed jack rabbits remained marked around the mouth, paws, belly, anus, and urogenital openings for 7 days and the cottontail rabbits were marked around the anus and urogenital openings for 15 days. Sullins & Verts (1978) found marking of internal organs in Beldings ground squirrels (Spermophilus beldingi) that fed on oat groats treated with 2.5 g Rhodamine B/kg. Lissamine green dye is used in poisoning operations against rabbits and possums in New Zealand as a means of deterring birds from eating the carrot bait (Caithness & Williams 1971). Even at the concentration of 0.01% used operationally, contaminant stains appear to be persisent. Since it is also inexpensive it was considered worthwhile evaluating this dye for use as a tracer.