In cities, dogs and cats take their toll, although adult squirrels are usually too agile to be caught. In some areas the greatest enemy of the grey squirrel is probably people, who regard it as an important small game species. People have also cleared great tracts of hardwood forest, severely reducing the natural habitat of the squirrel and thus its population. Large numbers of squirrels are killed every year by cars. The failure of a vital food source can lead to either starvation or a general weakening of the squirrel population, making it vulnerable to diseases and pests.
Parasites have a detrimental effect on the health and productivity of the grey squirrel.
The worst insect pest is the botfly larva. Scabies, or mange, leaves squirrels with patches of bare skin. In the resulting weakened condition, they often succumb to cold or infections, or become easy prey for predators. Other pests that parasitize grey squirrels are ticks, fleas, lice, and round and tape worms. The economic importance of grey squirrels is limited.