Farm Ireland

Saturday 17 November 2018

Pine marten could hold key to checking rise of grey squirrel

William Merivale

The grey squirrel is a typical example of the danger of introducing a new species of wild animal into the environment. The Irish population originates from a single introduction at Castle Forbes, Co Longford in 1911, and in the ensuing century their numbers and range have exploded alarmingly.

The grey squirrel is typically twice the size of our native red, more omnivorous and with a much greater appetite. Coupled with its competitive success against the red, it transmits a disease – the squirrel pox virus – which is relatively harmless to themselves but has an 80pc mortality rate for reds that contract it.

The grey really does have the capacity to threaten the native red with extinction, and it does great harm to our trees as well.

Grey squirrels strip the bark, predominantly from broadleaf trees, and feed on the sap. The damage leads to discolouration of the timber (and a significant drop in quality and consequential financial loss).

The weakened trees are also susceptible to wind-snap, and exposure to disease and fungal attack.


Bark stripping is most prevalent from March to June, and the principal species attacked are the softer-barked broadleaves such as beech, sycamore and maple, though in the absence of these they will turn their attention to species such as oak, ash and Scots pine.

The damage can occur when the trees are between six and 40 years of age, so even well established plantations are vulnerable to attack once greys have infiltrated an area.

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Zero tolerance is the only solution to this very serious pest species. There is simply no room for them in the Irish ecosystem, and in affected areas neighbours need to work together towards their eradication.

At one time, the use of warfarin-baited hoppers was the preferred method of control but this has serious side effects in that the target species is not always the only victim – reds can also be killed – and it carries the added risk of the poison entering the food chain.

Live trapping is a more effective and species-selective option. Traps should be put out at the beginning of the growing season, baited with nuts or sweetcorn at the rate of at least one trap per hectare and checked daily.

The animals can then be killed quickly and humanely, and reds or any other species that may be inadvertently caught released unharmed.

Another, and completely natural, method of control is the pine marten. Pine martens and red squirrels have co-existed on this island for thousands of years.

The red squirrel's decline has been principally due to the rise of the grey squirrel, but the pine marten's is due to persecution by us.

However, the pine marten is now protected by law and grey squirrel numbers are declining and reds increasing where it is making a comeback.


The extent to which the pine marten actually preys on the grey squirrel remains unclear because analysis of their droppings in the areas surveyed by an ongoing research project carried out by the Mammal Ecology Group at NUI Galway was inconclusive.

However, it is quite clear that there is a direct correlation between the colonisation of an area by pine martens and a significant decline in the grey squirrel population. This is probably due to a combination of predation and stress-induced reduction in fertility, but we can't be sure yet.

Despite the fact that the pine marten is a native and that we were directly responsible for its former decline, it remains illegal to trap and release them elsewhere in order to help encourage their dispersal.

Perhaps a controlled policy such as this will be formally adopted as part of the essential war on the grey squirrel.

Irish Independent