Factors affecting bait uptake by the grey squirrel (Sciurus carolinensis) and the future delivery of oral contraceptives

Abstract

Context. Invasive species negatively affect natural communities and human activities. The grey squirrel is an invasive species in the UK, causing damage to forestry and the decline of the native red squirrel. Oral contraceptives have the potential to reduce numbers of grey squirrels; however, to be effective a sufficient proportion of a population must consume a bait containing contraceptives. Aims. The objective of this study was to estimate the efficacy of delivering baits via feeders to grey squirrels and to determine the factors most important to bait uptake. Methods. Bait uptake was measured using the bait marker Rhodamine B mixed with 100% hazelnut butter and delivered to grey squirrels via purpose-designed feeders. Different concentrations of Rhodamine B were first trialled on captive grey squirrels for their palatability and detectability in the hair. Bait-uptake field trials were then conducted in 10 UK woodlands by using the preferred concentration. Key results. In captive trials, it was found that a concentration of 0.18% Rhodamine B mixed in hazelnut butter was palatable to grey squirrels and that individuals needed to consume only small amounts (<5 g) for it to be detected in the flank hair by using an ultraviolet microscope. It was possible to deliver bait to the majority of grey squirrels in 6 of 10 woods within 4 days. Season, feeder density and squirrel density were important factors affecting bait uptake, with more squirrels consuming bait in summer than in winter and from three feeders per hectare rather than from one per hectare. Conclusions. This study demonstrated that baits delivered via feeders can target the majority of grey squirrels in woodland environments. Implications. Oral contraceptives could offer a cost-effective tool to reduce numbers of grey squirrels across the UK landscape and mitigate the negative impacts they have on native wildlife and tree health.

Foreword to the Special Issue on ‘Fertility control for wildlife in the 21st century’

“…As more oral contraceptives become available for wildlife applications, understanding factors that affect patterns of individual and population bait uptake is essential to maximise consumption of these agents by the target species. The study by Beatham et al (2024), featured in this Special Issue, showed that season, feeder density and grey squirrel density are important factors affecting bait uptake, with more squirrels consuming bait from three feeders per hectare than from one per hectare, and with a higher proportion of animals feeding on bait in summer than in winter.…”