Christopher Carlton, Huval, Forest, Reagan, Thomas E.
Adult females are small, cream or tan colored spiders with body lengths ranging from 1/8 to ¼ of an inch (4 to 6 mm). They possess near spherical abdomens with a distinctive pair of broad, reddish-brown, zig-zag bands extending the length of the abdomen. Additional dark areas are present on the lower side of the abdomen. These bands are variable in width and may be wide enough to give the impression of an overall darker brown abdomen with pale highlights. Their surface luster is shiny, with short, stiff brown hairs visible under high magnification. The forepart of the body (cephalothorax) is brown or reddish brown to almost black without obvious markings. The legs are paler, with darker bands. Males are smaller, 1/16 to 1/8 of an inch (2 to 4 mm) in body length with markings that are similar to but less well defined than those of females and a much narrower, oval abdomen. Webs are typical cobweb type, often covering an extensive surface area. Egg sacs are round, white puffballs of silk, 1/5 to ¼ of an inch (5 to 6 mm) in diameter.
Females and immatures build cobwebs along walls and cabinets and under boards, overhanging banks and within roots of overturned trees. The common name of the family, “cobweb spiders,” is in reference to the apparently disorganized, random construction of the web of a type familiar to most homeowners. Egg sacs are typically numerous in the loosely organized webs and may contain up to 30 eggs each. Males search for females in the females’ webs, presumably using chemical cues at a distance and tactile cues prior to mating. Triangulate cobweb spiders feed mainly on small insects and other arthropods, including ticks, spiders (including brown recluses) and pill bugs. They are capable of subduing prey many times their size thanks to a potent insecticidal venom. Prey is detected as it struggles after contacting the sticky cobweb and quickly dispatched with a single bite for leisurely consumption.
This species is thought to have originated in Europe and has been spread by human agency throughout most parts of world. It thrives in peridomestic situations and often occurs in large numbers in homes and businesses. Due to its small size and unobtrusive cobwebs, populations may go undetected for long periods of time. Houses with active stored product pest populations or entry points available for outdoor insects are ideal habitats from the perspective of the triangulate cobweb spider. Despite the potent venom possessed by this species, documented cases of human envenomations are unknown. Thus, it can be safely tolerated or ignored in domestic situations. In fact, these spiders provide some measure of pest control for moths, flies and mosquitoes that find their way indoors.
Vacuuming or sweeping the cobwebs away from places where they are not desired are safe and effective ways to deal with accumulations of spiders for periods of time. However, the only sure way to eliminate them permanently is to exclude or eliminate potential food sources. Chemical control is not recommended for this species.
Levi, H. W. 1957. The spider genera Crustulina and Steatoda in North America, Central America, and the West Indies (Araneae, Theridiidae). Bulletin of the Museum of Comparative Zoology Harvard 117: 367-424.
Adult female triangulate cobweb spider in web, with egg sac. Joseph Berger. Bugwood.org.
Adult female triangulate cobweb spider, Steatoda trangulosa. Joseph Berger. Bugwood.org.