Gray squirrel control: Study shows promise for effective contraceptive delivery system

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A study published in the journal Pest Management Science sheds light on the behavior of gray squirrels (Sciurus carolinensis) and explores methods for the targeted delivery of oral contraceptives to control their populations.

The research, a collaboration between APHA's National Wildlife Management Centre, Durham University, and the University of York, provides a platform for mitigating the environmental damage caused by this invasive species and protecting the UK's declining native red squirrel (Sciurus vulgaris) population.

Sarah Beatham, an ecologist at APHA, and lead researcher on the study noted, "Our main goal was to measure bait uptake in individual squirrels to understand the potential contraceptive dose rates that would be achievable, particularly in breeding females. We also wanted to find out the best time of year to deploy a contraceptive, to maximize the number of individuals targeted."

Gray squirrels were first introduced to the UK from North America in the Victorian era, where they were prized as a novelty ornamental species for the grounds of stately homes. Since then, they have spread rapidly, reaching a population of over 2.5 million and posing a serious threat to tree health and native wildlife. Traditional lethal control methods have proven insufficient for large-scale management, leading to increasing interest in non-lethal, sustainable population control measures like contraceptives.

The study focuses on understanding gray squirrel feeding behavior to develop an efficient delivery system for oral contraceptives. Researchers conducted trials using specially designed platforms for the bait (known as bait hoppers) that only gray squirrels could access.

"The only animals recorded accessing the feeder bait were gray squirrels, on hundreds of occasions, and wood mice on three occasions, meaning the feeders have a very high level of species-specificity. This is important as the contraceptive could potentially affect other mammals," explained Beatham.

Squirrels were trapped and fitted with trackers, known as PIT-tags, to monitor individual bait uptake. Trials in six woodlands across Yorkshire spanned three seasons, helping researchers understand how variables such as sex, season, and squirrel density influence bait consumption.

The bait and monitoring system developed and tested in the study demonstrated that males and females had very similar levels of bait uptake from the feeders. Beatham added, "We found that spring was the only season tested where female squirrels were more likely to visit bait feeders than males. Spring coincides with a peak in squirrel breeding and is therefore a good time to deliver a contraceptive."

One of the key challenges in developing oral contraceptives for wildlife is ensuring that the bait isn't accessible to non-target species such as pine martins. "We need to make sure that the feeders themselves are relatively inexpensive and robust enough to prevent different wildlife species trying to access the bait and to be left in the field in different weather conditions," Beatham noted.

The research findings could have broader implications beyond gray squirrels. "The most obvious pest species would be the brown rat (Rattus norvegicus), one of the most populous, widely distributed, and successful invasive mammals globally. They cause a range of issues, including damaging crops and buildings, spreading disease, and predating vulnerable species such as ground-nesting birds," noted Beatham.

"Some of the most invasive species in the world are small mammals, so there are plenty of opportunities for this type of management."

Looking ahead, the data gathered from this study could be used to design scalable, practical bait delivery systems for deploying oral contraceptives across the UK, making population control efforts more efficient and cost-effective.

Beatham added, "This study demonstrated that you could deliver a bait to most squirrels in a wood within 4 days. The effort required for contraceptive-based control should be considerably lower than lethal control as, unlike traps, feeders will not have to be visited daily and may only need to be deployed once or twice a year."

The research team is already working on the next phase of development. "We are currently finalizing the structure and manufacturing process for the contraceptive through laboratory and captive squirrel trials. Once we finalize what constitutes an effective dosing regime, we can relate this to the findings of this study to design an effective field delivery method," Beatham said.

More information: Sarah E. Beatham et al, An assessment of seasonal bait uptake by individual grey squirrels to develop a delivery system for oral contraceptives, Pest Management Science (2024). DOI: 10.1002/ps.8379

Journal information: Pest Management Science

Provided by Society of Chemical Industry