Sciurus carolinensis | Eastern Grey Squirrel | Iora glas
Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted).
Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).
Regulated invasive species of Union concern under the European Regulation on the prevention and management of the introduction and spread of invasive alien species [1143/2014].
First reported in the wild
Invasive species - risk of High Impact
Introduction pathways - 1
Release in Nature
Introduction pathways subclass - 1
NAPRA Ireland risk assessed
Small arboreal mammal, larger than red squirrels, predominately grey in colour and lacking the prominent ear tufts of red squirrels.
Omnivorous, feeding mainly on seeds and plant material, though they will consume bird eggs and nestlings it is not known if this constitutes a significant proportion of their diet.
Grey squirrels range expansion has been associated with a range contraction of ~20% of red squirrels in Ireland since 1911 (Stritch et al, 2015). This is thought to be as a result of competition between the two species and the transmission of squirrel pox virus (SQPV) to immunologically native red squirrels (Lawton & Sheehy, 2014; Stritchen et al, 2015; Teangana et al., 2000). Range, tree use and foraging overlaps between the species in Italy suggest there is no niche partitioning between the species so competition will occur as grey squirrel densities increase (Wauters et al., 2002). In the presence of the SQPV, displacement of red squirrels by grey squirrels via competition is 17-25 times the rate in uninfected areas (Rushton et al., 2006), with infection rates in grey squirrels of 55% detected in Wicklow (Stritch et al., 2015).
Furthermore grey squirrels have introduced an endoparasite Strongyloides robustus and the prevalence of it and a native endo parasite of red squirrels were higher in areas with grey squirrels than red only areas (Romeo et al., 2015).
Woodland, forest and other wooded land; Constructed, industrial or other artificial habitats; Regularly or recently cultivated agricultural, horticultural or domestic habitat; Miscellaneous
Promiscuous or polygynous mating system, producing litters in either spring or summer of 1-7 pups (Gurnell et al, 2008).
Pathway and vector description
Released at Castle Forbes in Co. Longford in 1911, they have since naturally dispersed across the country at an estimated rate of 0 km/year to an 13.4 km/year, with various rivers and other geographical barriers slowing dispersal (O Teangana et al., 2000). They are seemingly restricted to the east of the country by the river Shannon, though this is likely to be the result of other factors than an inability to physically cross the river.
Mechanism of impact
Competition, Disease transmission
Eradication of this species is not possible and it is estimated that the numbers of Sciurus carolinensis and the damage caused by it in its introduced range will remain at its current levels (Bertolino et al., 2014).
Control should be carried out at the end of February and monthly until the end of May (Lawton, undated). It should be noted that the species will recolonise an isolated wood within 3 months and a non-isolated site within 1 month. The timing of the control is therefore critical to minimise damage to trees.
The use of warfarin, an anticoagulant, can only be considered in areas where native Red squirrels (Sciurus vulgaris) and Pine marten (Martes martes) are absent. 0.02% warfarin bait should be used and gloves should be worn at all times (Mayle et al., 2007). Specialised 'grey squirrel only hoppers' should be used, distributed at 1 per hectare. These help to prevent access by both smaller and larger animals (Lawton, undated). The use of warfarin is not usually considered humane by the general public.
Shooting is usually recreational and rarely removes more than a small percentage of the population (Lawton & Rochford, 2007).
Live trapping ensures only the targeted species is removed. S. carolinensis is readily trapped in the spring (Rooney and Hayden, 2002). This method is very effective but labour intensive (Lawton, undated). A period of 4 or 5 days of pre-baiting (whole yellow maize is most effective) is required to accustom the animals to being fed from the traps. Trapping should then proceed for 4-5 days. Trapped animals can then be euthanised. (Mayle et al., 2007).
This method is considered to be more humane by the general public. However, a cost-effective and species-specific product has yet to be developed
Chemical repellents, tree guards and fencing have all been tried unsuccessfully (Lawton, undated).
Predominately found in woodlands, either semi-natural or coniferous, but also found in parks and urban areas (Gurnell et al., 2008).
Established - Widespread & Common, though absent from the west of the country. Since 2007, the grey squirrel has retreated in range from several midland counties (Lawton et al., 2015). This decline is though to be as a result of a strong negative association between the grey squirrel and the recovering pine marten population (Lawton & Sheehy, 2014), which may be as a result of predation or modification of grey squirrel behaviour.
Native to Canada and the United States.
Date of first record category
Fifty year date category
Records submitted to Data Centre in 2023
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How can you help
Report any sightings to the National Biodiversity Data Centre.
Invasive Species Specialist Group (ISSG) of the SSC- Species Survival Commission of the IUCN -International Union for Conservation Nature 100 Worst Invaders globally. Delivering Alien Invasive Species In Europe (DAISIE) project list this as one of the 100 Worst Invaders in Europe. The collective name for a group of squirrels is a scurry, though as they are predominately solitary animals scurries rarely occur.
Bertolino, S., Martinoli, A., Wauters, L.; reviewed by Gurnell, J. and Lurz, P. (2014) Risk Assessment for Sciurus carolinensis (Grey Squirrel) Invasive alien species – framework for the identification of invasive alien species of EU concern (ENV.B.2/ETU/2013/0026) https://circabc.europa.eu/sd/a/0db88111-71f0-4d41-aeb3-cb3b8d8b42ea/Sciurus%20carolinensis%20-%20GBNNRA%20based.pdf Site accessed 27 September 2017.
Gurnell, J., Kenward, R.E., Pepper, H. & Lurz, P.W.W. (2008). Grey Squirrel Sciurus carolinensis In: Mammals of the British Isles : Handbook, 4th edition. Harris, S. & Yalden, D.W. (eds) The Mammal Society, UK.
Lawton, C (undated) Controlling Grey Squirrel Damage in Irish Broadleaved Woodlands. Silviculture/Management No. 7. COFORD. Dublin. http://www.coford.ie/media/coford/content/publications/projectreports/cofordconnects/GreySquirrel.pdf Site accessed 27 September 2017.
Lawton, C., Flaherty, M., Goldstein, E.A, Sheehy, E. and Carey, M. (2015) Irish Squirrel Survey 2012. Irish Wildlife Manuals, No. 89. National Parks and Wildlife Service, Department of Arts, Heritage and the Gaeltacht, Ireland.
Lawton C., Rochford J. 2007. The recovery of grey squirrel Sciurus carolinensis populations after intensive control programmes. Biology and Environment: Proceedings Royal Irish Academy 07B: 19-29. https://www.researchgate.net/publication/237522821 Site accessed 27 September 2017.
Mayle, B., Pepper, H. and Ferryman, M. (2007). Controlling grey squirrel damage to woodlands. UK Forestry Commission Practice Note, Edinburgh. https://www.forestry.gov.uk/fr/infd-69fey8 Site accessed 27 September 2017.
Romeo, C., Ferrari, N., Lanfranchi, P., Saino, N., Santicchia, F., Martinoli, A., & Wauters, L. A. (2015). Biodiversity threats from outside to inside: effects of alien grey squirrel (Sciurus carolinensis) on helminth community of native red squirrel (Sciurus vulgaris). Parasitology Research, 1-8.
Rooney, S. and Hayden, T. (2002). Forest mammals - management and control. COFORD, Dublin. http://www.coford.ie/media/coford/content/publications/projectreports/mammals.pdf Site accessed 27 September 2017.
Rushton, S. P., Lurz, P. W. W., Gurnell, J., Nettleton, P., Bruemmer, C., Shirley, M. D. F., & Sainsbury, A. W. (2006). Disease threats posed by alien species: the role of a poxvirus in the decline of the native red squirrel in Britain. Epidemiology and Infection, 134(03), 521-533.
Sheehy, E., & Lawton, C. (2014). Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland. Biodiversity and Conservation, 23(3), 753-774.
Teangana, D. O., Reilly, S., Montgomery, W. I., & Rochford, J. (2000). Distribution and status of the red squirrel (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis) in Ireland. Mammal Review, 30(1):45-56.
Watt, H.B. (1923) The American Grey squirrel in Ireland. Irish Naturalist Journal, 32:95–353.
Wauters, L. A., Gurnell, J., Martinoli, A., & Tosi, G. (2002). Interspecific competition between native Eurasian red squirrels and alien grey squirrels: does resource partitioning occur?. Behavioral Ecology and Sociobiology, 52(4), 332-341.
IUCN Red List of Threatened Species
Global Invasive Species Database