When a homeowner spots an unfamiliar eight-legged guest, the first question is almost always: “Is it dangerous?” While the term poisonous spiders in Ontario is frequently searched, it is helpful to know that spiders are actually venomous (they inject toxins) rather than poisonous (which refers to being harmful if ingested).
In Southern Ontario, almost all spiders use venom to hunt insects, but only two species are of medical note to humans. Here is a guide to identifying them, where they live, and how to stay safe in your home and yard.
1. Northern Black Widow (Latrodectus variolus)
The Northern Black Widow is the only native spider in the province considered medically significant. While they are reclusive and rare, they have established populations in Southern Ontario, particularly in warmer areas like Essex County and the Chatham-Kent region.
Where to Find Them
- In the Yard: They are most commonly found in woodpiles, hollow stumps, and the undersides of outdoor furniture. They also frequent utility enclosures, such as pool pump housings or water meter covers.
- In the House: They prefer dark, undisturbed locations. You are most likely to encounter them in garages, sheds, or unfinished basements. They rarely enter active living areas like kitchens or bedrooms.
Identification & Risk
They are shiny, jet-black, and globular. Look for a red “hourglass” on the underside that is usually split into two separate triangles. While their neurotoxic venom can cause intense muscle pain and nausea, bites are rarely fatal for healthy adults and usually only occur if the spider is accidentally squeezed.
2. Yellow Sac Spider (Cheiracanthium mildei)
The Yellow Sac Spider is far more common than the Black Widow and is found throughout Southern and Eastern Ontario, from Windsor to the GTA and Ottawa.
Where to Find Them
- In the Yard: They hide during the day inside leaves that they have rolled up and lined with silk. You may also find their small, white silken “sacs” tucked into the tracks of window frames or behind shutters.
- In the House: This is a common indoor spider. They are famous for building small, white, thumb-sized silk retreats where the wall meets the ceiling. They also hide behind headboards, nightstands, or in the folds of curtains.
Identification & Risk
These spiders are small (about 1/4 to 3/8 of an inch), pale yellow or light lime-green, and slightly translucent. Because they are active hunters that roam at night, they may crawl into shoes or piles of laundry left on the floor. Their bite is often compared to a bee sting—causing localized pain and swelling—but it is not life-threatening.
3. The “Brown Recluse” in Ontario: Myth vs. Fact
One of the most common concerns regarding poisonous spiders in Ontario involves the Brown Recluse. It is important to clarify: Brown Recluse spiders are not native to Ontario.
Our climate is too cold for them to survive a typical winter outdoors. While a single spider might occasionally “hitchhike” into the province via shipping containers, there are no established populations here. Most local “recluse” sightings are actually misidentified Wolf Spiders or Nursery Web Spiders, both of which are harmless to humans.