The false black widow (Steatoda grossa) is a common spider found globally, often mistaken for the more notorious true black widow. This spider belongs to the family Theridiidae, or cobweb spiders, which includes both Steatoda and Latrodectus genera. Despite its intimidating name and resemblance to more dangerous relatives, the false black widow is less harmful to humans. Its prevalence in human environments and similar appearance contribute to frequent misidentification.
Identifying the False Black Widow
Female Steatoda grossa measure 6 to 10.5 millimeters, with a dark, round, bulbous abdomen. Their coloration ranges from purplish-brown to black, often displaying light markings. These markings vary, appearing as faint patterns, crescent-shaped stripes, or triangular spots, which may be cream-colored or purple. Males are smaller and thinner, measuring 4.1 to 10.0 millimeters, with lighter or reddish legs as they mature. Unlike some other Steatoda species, S. grossa may lose its abdominal pattern with age, appearing solid dark.
Distinguishing from True Black Widows
Distinguishing Steatoda grossa from true black widows (Latrodectus species) is important due to venom differences. The most definitive feature is the absence of the prominent red hourglass marking on the false black widow’s underside. True black widows have this distinct red hourglass, which is absent on Steatoda grossa. While both spiders have a dark, globular body, false black widow markings, if present, are usually on the top of the abdomen and are pale, not bright red.
False black widows construct irregular, tangled webs, often described as cobwebs. These messy webs differ from the stronger, more organized webs of true black widows. False black widows also exhibit less aggressive behavior, typically fleeing when disturbed, whereas female true black widows may bite defensively, especially near their egg sacs. The egg sacs of Steatoda species are distinct, appearing as fluffy white silk, unlike the papery brown or tan sacs produced by true black widows.
Habitat and Behavior
False black widows are cosmopolitan spiders, found in many parts of the world, including North America, Europe, and Australasia. They commonly inhabit human dwellings and surrounding areas, preferring dark, undisturbed locations. Indoors, these spiders are frequently found in basements, garages, sheds, and cupboards, building webs in corners or crevices. Outdoors, they are found under rocks, logs, or in wall cracks.
False black widows are nocturnal, most active at night. They build irregular, tangled webs to capture insects and other small arthropods. Steatoda grossa has poor eyesight and relies on web vibrations to detect prey or threats. They are not aggressive and typically only bite when threatened, such as when accidentally squeezed or trapped.
Bite and Symptoms
The false black widow’s bite, while venomous, is medically significant but typically has no long-lasting effects. Symptoms include localized pain, which may increase for the first hour and sometimes radiate from the bite site. Swelling, redness, and blistering at the bite location are also common. Some individuals experience systemic symptoms like nausea, headache, sweating, or malaise, which usually resolve within several days.
The discomfort of a false black widow bite is comparable to a bee or wasp sting. Severe reactions are rare, occurring in individuals with allergies or compromised immune systems. Symptoms from Steatoda bites are sometimes called “steatodism,” a less severe form of true black widow bite symptoms (latrodectism). If the bite leads to secondary bacterial infection, symptoms can become more severe.
Prevention and What to Do
To minimize encounters with false black widows, reduce clutter in and around the home, especially in basements, garages, and attics. Seal cracks and crevices in foundations, walls, and around windows and doors to prevent entry. Regular cleaning and dusting, including removing cobwebs, discourages spiders indoors. Shaking out clothing and bedding before use, particularly items stored in undisturbed areas, prevents accidental bites.
If bitten by a false black widow, wash the affected area thoroughly with soap and water to prevent infection. Apply a cold compress or ice pack to the bite for about 10 minutes to reduce swelling and pain. Over-the-counter pain relievers can be used for discomfort. Seek medical attention if symptoms worsen, if there are signs of infection like increased redness or pus, or if severe symptoms such as vomiting, radiating pain, or difficulty breathing develop.