Scientists design contraceptives to limit grey squirrels


a red squirrel eating some corn in a forest

The UK’s native red squirrel – driven close to extinction by invasive grey squirrels

A plan to use oral contraceptives to control grey squirrel populations in the UK is making good progress and could soon be put to the test in field trials, say government scientists.

The mass birth control plan involves luring grey squirrels into feeding boxes only they can access, using pots containing hazelnut spread. These will be spiked with contraceptives.

The project could help eradicate the grey squirrel in the UK without killing them, says environment minister Lord Benyon.

It should reduce the “untold damage” grey squirrels do to woodland ecosystems and native red squirrel populations, he says.

The government scientists leading the research say the contraceptive, which makes both male and female grey squirrels infertile, should be ready to deploy in the wild within two years.

Grey squirrels, first introduced from North America in the late 19th century, damage UK woodlands by stripping bark from trees to get at the nutritious sap beneath.

The species has flourished in the UK. There are now reckoned to be 2.7 million grey squirrels here.

The animals target young trees, typically 10-50 years old, and favour broadleaf species including oak, beech, sweet chestnut, and sycamore.

They can kill or maim trees, leaving scarring that allows an entry point for other tree pests and diseases which can stunt their growth.

The damage they can do threatens the effectiveness of government efforts to tackle climate change by planting tens of thousands of hectares of new woodlands, environment minister Lord Goldsmith has warned.

Grey squirrel up closeGrey squirrel up close

The grey squirrel – “a widespread and extremely numerous invasive animal” according to broadcaster and naturalist Chris Packham

Grey squirrels have also driven the UK’s native red squirrel to the verge of extinction across much of the country.

There are thought to be just 160,000 red squirrels left in the UK, with only 15,000 remaining in England.

Grey squirrels are significantly larger and stronger than reds and carry a squirrel pox virus that is deadly to reds but to which they are immune.

The traditional way of managing the grey squirrel population is by culling them. But grey squirrels breed rapidly and populations can recover quickly. A century of culling programmes has failed to reduce the population.

Contraceptives for squirrels

Dosing the animals with a contraceptive drug is a more humane alternative and will ultimately be more effective, says the lead scientist on the project, Dr Giovanna Massei, from the government’s Animal and Plant Health Agency (APHA).

She says her team have developed a vaccine that prompts the immune system to restrict the production of sex hormones, which leaves both male and female squirrels infertile.

The drug is not permanent, and further tests are being carried out to find a dose that has a long-lasting effect and is safe to use in the wild.

The team have also developed a special feeding hopper with a weighted door designed to keep out species other than squirrels.

Trials in Yorkshire and Wales showed 70 per cent of squirrels in each wood visited the bait boxes over a four day period – the only other animals that got in were two particularly enterprising mice.

The plan is to bait the hoppers with pots of a hazelnut paste greys find “irresistible” and which will be laced with the new contraceptive.

The scientists are also exploring special feeders to be used in areas where there are both red and grey squirrels. These will be triggered by a plate which weighs the animals and only lets the heavier greys get access to the bait.

picture of a trappicture of a trap

Special feeding hoppers are being designed to administer the contraceptives

Dr Massei says computer modelling shows the contraceptive method could bring grey squirrel populations to the brink of extinction in some places.

“It could even eradicate them from some areas, provided you can do a coordinated control over an area, so they don’t start to re-immigrate”, Dr Massei says.

If the work with grey squirrels is successful Dr Massei believes similar techniques could be used to help control the population of other invasive mammals including rats, mice, deer, and wild boar.

The research is being funded by the UK Squirrel Accord (UKSA), a partnership of forestry and conservation organisations. It has raised just over £1m to cover the research and development of the project.

The contraceptive scheme is an important additional non-lethal tool for managing grey squirrels, says Kay Haw, the director of the UKSA.

“Red squirrels now only survive in island ecosystems where there aren’t any grey squirrels or where a red squirrel community group are working hard to keep back the grey squirrels”, she says.

A grey squirrel sitting by a tree in a forestA grey squirrel sitting by a tree in a forest

Contraceptives – a non-lethal way of controlling grey squirrel populations

The financial cost of the damage grey squirrels do is estimated at £37m a year in England and Wales alone. The cost to the UK’s biodiversity has not been calculated.

A mature oak tree can support up to 2,000 other species.

The initiative has wide support, including from the Prince of Wales, who was instrumental in the setting up of the UKSA.

The animal rights pressure group People for the Ethical Treatment of Animals says if population control has to be implemented, it backs non-lethal options.

But it cautions: “We mustn’t forget that grey squirrels and other species deemed “invasive” are where they are through no fault of their own and entirely due to human carelessness, and they deserve to be left in peace.”

The broadcaster and naturalist Chris Packham believes a species-specific oral contraceptive could be a “dream ticket” if it proves to be economically viable and practical.

He describes the project as “non-violent direct action” to control “a widespread and extremely numerous invasive animal” and suggests an effort should also be made to encourage pine martens back into their previous haunts across the country.

These predatory mammals – a native UK species that has suffered historic persecution – have been shown to reverse the spread of invasive grey squirrels in Scotland and Northern Ireland.

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