Climate editor byJustin Rowlatt.

a red squirrel eating some corn in a forestImage source, BBC/Tony Jolliffe
Image caption, 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 be put to the test in field trials.

Grey squirrels are lured into feeding boxes only they can access with pots containing hazelnut spread. They will be spiked with contraceptives.

Lord Benyon is the environment minister.

He says it should reduce the damage grey squirrels do to the environment.

The contraceptive that makes both male and female grey squirrels infertile should be available in the wild within two years.

The grey squirrel was first introduced from North America in the 19th century.

In the UK, the species has grown. It is thought that there are over 2 million grey squirrels here.

The broadleaf species favored by the animals include oak, beech, and sweet chestnut.

They can kill trees and leave scars that allow other tree pests and diseases to enter.

Lord Goldsmith warned that the damage they can do threatens the effectiveness of government efforts to tackle climate change by planting tens of thousands of hectares of new woods.

Grey squirrel up closeImage source, BBC/Tony Jolliffe
Image caption, The grey squirrel - "a widespread and extremely numerous invasive animal" according to broadcaster and naturalist Chris Packham

The UK's native red squirrel is in danger of extinction due to the actions of grey squirrels.

The red squirrel is thought to be extinct in the UK and England.

There is a squirrel pox virus that is deadly to reds but to which grey squirrels are immune.

The grey squirrel population is culled. Populations of grey squirrel can recover quickly. The population has not been reduced by the programs.

The lead scientist on the project says that dosing the animals with a contraceptive drug is a more humane alternative.

Both male and female squirrels are infertile because of a vaccine that causes the immune system to restrict the production of sex hormones.

Further testing is being done to find a dose that is safe to use in the wild and that has a long- lasting effect.

A weighted door has been developed by the team to keep out other animals.

In trials in Yorkshire and Wales, 70 percent of the squirrels in each wood visited the bait boxes.

The plan is to bait the hoppers with pots of hazelnut paste greys that are resistant to contraceptives.

Special feeders are being explored to be used in areas where there are both red and grey squirrel. The heavier greys will only be allowed to access the bait if they are weighed by a plate.

picture of a trapImage source, BBC/Tony Jolliffe
Image caption, Special feeding hoppers are being designed to administer the contraceptives

The contraceptive method could bring grey squirrel populations to the brink of extinction, according to Dr. Massei.

Dr Massei says that it could eradicate them from some areas if you can control the area.

Similar techniques could be used to control the population of other mammals if the grey squirrel work is a success.

Funded by the UK Squirrel Accord is the research. The research and development of the project has been funded by just over one million dollars.

The contraceptive scheme is used to manage grey squirrels, according to the director of the UKSA.

She says that red squirrels only survive in island environments where there aren't any grey squirrels or where a red squirrel community group is trying to keep the grey squirrels away.

A grey squirrel sitting by a tree in a forestImage source, BBC/Tony Jolliffe
Image caption, Contraceptives - a non-lethal way of controlling grey squirrel populations

In England and Wales, the financial cost of damage done to grey squirrels is estimated to be around $37 million a year. It is not known how much the UK's biodiversity costs.

Up to 2000 other species can be supported by an oak tree.

The Prince of Wales was involved in the setting up of the UKSA.

People for the Ethical Treatment of Animals supports non-lethal options if population control is needed.

"We mustn't forget that grey squirrels and other species are where they are because of human carelessness, and they deserve to be left in peace."

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 animal invaders and suggests an effort should be made to encourage pine martens to return to their previous homes.