Spider Identification Guide
Dock Spider - Dolomedes
Ontario's largest spider. Semi-aquatic and capable of running across the water's surface, dock spiders colonize waterfront structures across cottage country from May through August.
The dock spider is the undisputed largest spider in Ontario and one of the most startling wildlife encounters in cottage country. Dolomedes triton and Dolomedes tenebrosus - the two species most commonly called dock spiders or fishing spiders - are semi-aquatic predators that inhabit the edges of Ontario's lakes, rivers, and wetlands. Their leg span can exceed 75mm, making them visually impossible to miss.
Dock spiders are fishing spiders: they rest at the water's edge with their front legs on the surface, sensing vibrations that indicate prey below. They can run across the water's surface, dive below it, and catch aquatic prey including small fish, tadpoles, and aquatic insects. A dock spider resting flat on a dock board - legs spread, unmoving - is an encounter that stays with you.
Females carry their egg sac - a large, ball-shaped silk structure - attached to their spinnerets as they move. As hatching approaches, the female builds a nursery web, a tent of silk in which the spiderlings will spend their first days. A female dock spider guarding a nursery web is at her most defensive and most dramatic to encounter.
In Ontario
Dock spiders are distributed across Ontario wherever there is suitable aquatic habitat - which in cottage country means they are essentially everywhere from the waterline up. Muskoka, Haliburton, the Kawarthas, Grey-Bruce, Simcoe County, and the Georgian Bay shoreline all have dense dock spider populations throughout the summer season.
Activity peaks from late May through early August. Females with egg sacs are most visible in June and early July. After the eggs hatch and spiderlings disperse, adult populations thin out through August. By September, the season is largely over and the species overwinters in shoreline debris, bark, and structures above the waterline.
Dock spiders colonize specific structures systematically. A boathouse is ideal habitat: dark, humid, full of structural crevices, directly adjacent to water, and usually undisturbed. A neglected boathouse opened after winter may contain dozens of dock spiders at all life stages. The underside of dock boards, the corners of boathouse walls at water level, and the dock frame above the waterline are all primary harbourage sites.
Spider Squad offers dedicated dock spider and waterfront treatment for cottage properties across Ontario's cottage country regions. See our waterfront pest control page for full details on what we treat and how we treat it near water.
Frequently Asked Questions
Are dock spiders dangerous?
Dock spiders are not medically dangerous. They can bite if handled, cornered, or if you put your hand somewhere a dock spider is sheltering. A bite causes localized pain, redness, and swelling - comparable to a bee sting - that resolves within hours to a day. They are not aggressive toward people and will flee when given the opportunity. Their alarm factor far exceeds their actual danger.
How big do dock spiders get in Ontario?
Dock spiders (Dolomedes) are Ontario's largest spider species. Adult females - which are larger than males - can have a leg span exceeding 75mm (about three inches). The body alone is 15-26mm. When a dock spider is spread flat on a dock board, it is genuinely large enough to be unmistakable. Males are smaller, typically 7-13mm body length.
Can dock spiders really walk on water?
Yes. Dock spiders can run across the water's surface using surface tension. They distribute their weight across their eight long legs, each tipped with hydrophobic (water-repelling) hairs that rest on the water's surface film without breaking it. They can also dive below the surface and remain submerged for several minutes, using a bubble of air trapped by their body hairs.
How do I get rid of dock spiders on my boathouse?
Spider Squad treats above-waterline dock and boathouse surfaces with our licensed treatment product, targeting the dock boards, structural frame, boathouse walls, and swim platforms where dock spiders nest and shelter. We do not apply product to submerged surfaces or the water surface itself. Treatment in May or early June provides the best results by targeting the population before peak egg-carrying season. See our waterfront treatment page for full details.
Are dock spiders the same as fishing spiders?
Yes - dock spider is the common Ontario name for spiders of the genus Dolomedes, which are also called fishing spiders or raft spiders in different regions. The two names refer to the same species.
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