Not just a matter of taste: palatability of bait markers is influenced by the need to search for alternative food

Abstract

Context Bait palatability is a key issue influencing the uptake of toxic baits or non-toxic bait markers. Animals often reject baits with high concentrations of the active compound (whether it is a toxin, vaccine or marker) because of poor palatability, thus reducing the efficacy of baiting. Foraging theory predicts that palatability will be affected not only by the taste of active ingredients in bait but also by an animal’s ability to access alternative foods. Yet few studies of bait palatability are measured in the context of an animal’s need to search and forage for other food types. Aims The present study examined whether the palatability of Rhodamine B (RB) baits for black rats (Rattus rattus) was affected when foraging constraints were placed on access to alternative food compared with when alternative food was freely accessible. Rhodamine B is a bait marker and was used as a surrogate for other active ingredients likely to be used in pest control management. Methods Each day, RB bait at one of four concentrations was provided to an individual rat along with an alternative food that was either freely available (spatially clumped with foraging constraints absent) or hidden within a matrix of tubes (spatially scattered, thus with foraging constraints present). Key results Black rats exhibited a gradient in how palatable they found RB and preferred baits that contained the lowest concentrations of RB. Importantly, RB baits were more palatable when access to alternative food was made more difficult by applying a foraging constraint. In particular, a 0.2% RB concentration appeared to represent a threshold in palatability where intake at or above this concentration was significantly affected by a rat’s ability to freely access alternative foods. The ingestion of RB dye (mg kg–1) was highest in rats that consumed the highest concentrations, even though food intake was reduced. Conclusions The consumption of baits at high RB concentrations was greatly affected by the ease of access to other foods. We suggest the willingness of the animal to consume the bait can be influenced by the effort needed to find alternative foods. Implications A higher incidence of marking in the whiskers or hair of target individuals in the field will only be achieved with the use of the most palatable concentrations of RB and environments providing low alternative food access and abundance. A trade-off between reliable marking and palatability of RB at varying concentrations must be achieved if actual bait uptake in the field is to be more accurately represented. A re-evaluation of palatability experiments may be required as access to alternative foods can have profound impacts on bait uptake.