Illinois is home to a diverse population of native snakes, with approximately 40 different species found across the state’s varied landscapes. The vast majority of these reptiles are harmless to humans and play a beneficial role in the local ecosystem by controlling rodent and insect populations. Only a small number of these species possess venom, and encounters with them are uncommon for most residents.
Identifying Illinois’s Venomous Snakes
Illinois is home to four species of venomous snakes, all of which belong to the pit viper family: the Eastern Copperhead, the Northern Cottonmouth (also known as Water Moccasin), the Timber Rattlesnake, and the Eastern Massasauga Rattlesnake. They can be distinguished by several characteristics, including a broad, triangular-shaped head that is noticeably wider than the neck, and a prominent heat-sensing pit located between the eye and the nostril on each side of the face.
The pupils of all Illinois pit vipers are elliptical, unlike the round pupils found in most non-venomous snakes. Juvenile venomous snakes, with the exception of the Timber Rattlesnake, often have a bright tail tip, which they use to lure prey. The Copperhead is the most widely distributed of the four, found in the southern third of the state, and is identifiable by its distinct hourglass or “Hershey’s Kiss” pattern. The Cottonmouth is almost exclusively confined to the swamps and wet bottomlands of the extreme southern tip of Illinois, south of Route 13. Both rattlesnake species are generally concentrated in the southern and western parts of the state and are protected under state law, with the Massasauga being state endangered and the Timber Rattlesnake state threatened.
The Abundant Non-Venomous Species
Non-venomous snakes constitute the bulk of Illinois’s snake population. These species typically have more slender bodies and round pupils, contrasting with the elliptical pupils of the pit vipers. They also notably lack the facial pits and broad head shape of the venomous species.
The Common Garter Snake is the most frequently seen snake in the state, recognized by its three light-colored stripes running down a dark body and its preference for consuming insects, worms, and small amphibians. The Gray Rat Snake is a large constrictor that benefits humans by preying heavily on rodents and is often found climbing trees or rafters in barns. Its appearance changes with age, starting blotched as a juvenile and darkening to a nearly uniform black in adulthood.
Water Snakes, such as the Common Water Snake, are frequently mistaken for the venomous Cottonmouth due to their dark color and aquatic habits. These non-venomous snakes are often seen basking on logs near water and will defensively flatten their heads and bodies when threatened. The Bullsnake is a powerful rodent predator that will hiss loudly and vibrate its tail when disturbed, leading to confusion with a rattlesnake. Many non-venomous species, like Kingsnakes, are even known to prey on and are immune to the venom of pit vipers.
Key Habitats and Seasonal Activity
Snakes in Illinois occupy diverse habitats, from northern prairie wetlands to rocky bluffs and forests. Forested areas, particularly those with rock outcrops and bluffs, are the preferred habitat for Timber Rattlesnakes and Copperheads. Water-loving species like the Cottonmouth and Common Water Snake are restricted to swamps, marshes, and the edges of ponds and rivers; the Cottonmouth’s range is limited to the very southern tip.
Snake activity is heavily influenced by temperature. During the cold winter months, snakes enter a period of inactivity called brumation, seeking shelter below the frost line in rock crevices or burrows. They emerge in the spring to bask in the sun, leading to increased visibility. Snakes are most active during the day in spring and fall, but may become nocturnal during the high heat of summer to avoid overheating.
Safe Encounters and Coexistence
The risk of being bitten by a venomous snake in Illinois is very low, as all four species are nonaggressive and will only bite if they feel threatened, such as when handled or accidentally stepped on. The safest practice upon encountering any snake is to give it space and leave it alone. Never attempt to capture, handle, or kill a snake, as this is when most bites occur.
Homeowners can make their property less attractive to snakes by removing potential shelter and food sources. This includes keeping grass mowed short, clearing away piles of rocks, wood, and debris, and managing rodent populations.
In the event of a venomous snake bite, remain calm, immobilize the affected limb, and seek emergency medical attention immediately. Do not attempt to apply a tourniquet, cut the wound, or use ice, as these actions can worsen the injury.