Tigrosa

Physical description and identification

Tigrosa is a genus of five large wolf spiders, all confined to North America. It is young as genera go: the American arachnologist Allen R. Brady erected it in 2012 for a group of Nearctic species that had been sitting uncomfortably inside Hogna, and before that inside the catch-all genus Lycosa. One popular claim needs correcting because it reverses the history: species were not moved out of Tigrosa into Hogna before 1990. The genus did not exist until 2012, and every one of its members came out of Hogna, not the other way round.

Adults

• Size: these are big spiders. Females of most species run 18–21 mm in body length, and Tigrosa aspersa can exceed 25 mm, which makes it one of the largest wolf spiders in the eastern United States. Males are clearly smaller at roughly 10–18 mm, with longer legs relative to the body.
• Colour: brown, grey-brown or nearly black, usually with a pale median band down the carapace and darker bands beside it. The legs are often warmer, orange-brown or tawny, and several species have a boldly marked underside to the abdomen.
• Eyes: the standard lycosid arrangement of eight eyes in three rows — four small ones in a low front row, two very large forward-facing eyes above them, and two set wide on top of the head. The big pair carries a reflective tapetum, so a headlamp held at eye level picks out a green-white spark from several metres away. Our guide to spider eyes covers how the arrangement differs between families.
• Distinctive features: a heavy, thickset body and strongly spined legs. Species-level identification needs an adult and a microscope; colour alone is not reliable within the genus.

Eggs and egg sac

The female builds a rounded, pale silk sac and fastens it to her spinnerets, dragging it behind her while she hunts. A large Tigrosa sac can hold a few hundred eggs. She will fight to retrieve it if it is knocked loose, and has been recorded carrying a substitute object of similar size — the drive is triggered by shape, not by recognising her own brood.

Spiderlings

When the young are ready the mother opens the sac and they swarm up onto her abdomen, hooking into specially knobbed hairs. A common error is to say they leave her "upon maturation": they disperse after roughly one to two weeks as tiny juveniles, long before maturity. Northern populations overwinter part-grown and mature the following summer.

Hunting instead of a web

No Tigrosa spins a capture web. They walk their territory after dark, locating prey by vibration and by the large forward eyes, then closing the last few centimetres in a rush. By day they shelter under logs, boards, stones and matted vegetation, sometimes in a shallow scrape, but they do not dig the deep permanent shafts of the true burrowing wolf spiders.

Where Tigrosa spiders live

The genus is entirely Nearctic. Its centre of gravity is the eastern and central United States — from Texas and the Gulf states north through the Midwest to New England — and Tigrosa helluo reaches southern Canada at the northern edge of the range. Claims that the genus is widespread through Mexico overstate things; it is a North American group with only marginal records at its southern limit.

Habitat is the other correction worth making. Tigrosa are not house spiders and do not live in "dark corners of the home". They are animals of damp ground: wet meadows, marsh margins, stream banks, ditches, the grassy edges of crop fields and deciduous leaf litter. T. helluo is strongly tied to moist, vegetated field edges, which is why it turns up so often in agricultural research. One is found indoors only when a mature male wanders in at floor level in late summer or autumn while searching for females.

Are Tigrosa spiders venomous?

Yes, in the same sense that nearly every spider is: Tigrosa species use venom to subdue insects. It is not medically significant to people or pets. Wolf spiders as a family contain no species dangerous to humans anywhere in the world, and Tigrosa has none of the tissue-destroying enzymes of the brown recluse or the neurotoxins of the widow spiders. There is no antivenom because none has ever been needed. A large female can deliver a painful bite simply because her fangs are big, but that is a puncture wound, not envenomation of consequence.

Can Tigrosa spiders bite?

They can, and they will if handled roughly or trapped against skin, but bites are uncommon. A cornered Tigrosa normally runs; if it cannot, it may rear and raise its front legs as a display before it commits to biting. The usual result is sharp local pain like a bee sting, a small red mark, and perhaps an hour or two of tenderness and mild swelling. Wash the area with soap and water, apply a cold pack, and take a painkiller if needed. Seek medical advice if swelling spreads well beyond the bite, if there is difficulty breathing, or if the wound looks infected days later — those point to allergy or infection, not to the venom.

Ecological importance and behavior of Tigrosa spiders

Tigrosa are generalist nocturnal predators and, because they are large and abundant along field margins, they matter economically. Tigrosa helluo is one of the most intensively studied spiders in North American agricultural ecology, used in field and enclosure experiments on the biological control of crop pests in soybean and alfalfa systems. A single large female takes crickets, grasshoppers, caterpillars, beetles, cockroaches and other spiders, and she hunts every warm night she is not moulting.

Natural predators: ground-foraging birds, shrews, toads, lizards and snakes take adults, and spider wasps of the family Pompilidae paralyse them to provision their nests. Wolf spiders also eat one another — intraguild predation between large Tigrosa and smaller lycosids such as Pardosa is a well-documented feature of the same habitats. Our guide to what eats spiders covers the wider picture.

Prey–predator dynamics: because Tigrosa hunts on the ground at night, it intercepts a different slice of the insect community from the daytime hunters and the web builders above it. Where numbers are high the effect on ground-active pest insects is measurable, which is the reason the genus features so often in pest-management trials.

Seasonal behaviour: juveniles overwinter under litter and debris and mature through late spring and summer. Females carry egg sacs from midsummer onward, and mature males wander in late summer and autumn — the season when one occasionally ends up indoors. Growth proceeds through a series of molts, and most individuals live about a year, though large females in the south may survive into a second season.

Relationship with humans: beneficial and low-risk. A Tigrosa in a garden or field edge is free pest control. One indoors is best cupped and carried outside; sealing gaps under doors and reducing outdoor lighting near the house prevents repeats better than any spray.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus name comes from the tiger-like banding on the carapace and legs of several species, and T. aspersa is still sometimes called the tiger wolf spider.
• Tigrosa helluo has appeared in hundreds of ecological papers on predator behaviour, cannibalism and biological pest control, making it one of the best-known spiders in North American field research.
• The eyeshine trick works: sweep a headlamp across a damp meadow at night and the scattered green sparks at ground level are wolf spider eyes, often these.
• Because the genus is only a little over a decade old, most older field guides still list these spiders as Hogna helluo or Lycosa helluo.

In summary, Tigrosa is a genus of five large, harmless, hard-working American wolf spiders. They build no webs, run down their prey in the dark, carry their families with them, and remove a great many ground-dwelling pests along the way. The one worth remembering is T. helluo — big, brown, and almost certainly the wolf spider in your field margin.