Latrodectus mactans and L. variolus, Southern and Northern Black Widow Spiders (Araneae: Theridiidae)

Christopher Carlton, Huval, Forest, Reagan, Thomas E.

Bug Biz Pest Management and Insect Identification Series, LSU Ag Center, Research, Extension, Teaching

Description

Adult females of these two similar species are medium-to-large black spiders 0.4 to 0.5 inches (10 to 12 mm) in body length with globular abdomens that are glossy black in surface luster. The abdominal ground color, black, is consistent in both species. The forepart of the body (cephalothorax) is equally dark in color. The abdomen of female southern black widows (L. mactans) possesses a red, orange or yellowish orange marking underneath (ventral aspect) in a distinct hourglass shape. The narrow part of the marking is almost always connected, whereas in the northern black widow (L. variolus), the marking may be separated into two distinct spots and may be greatly reduced in size. The upper part of the abdomen (dorsal aspect) is variable in both species. They may be solid black from above, but many possess bold red or orange markings, sometimes with white or yellow highlights. Immature females are typically more boldly marked with red and white or yellow markings that partially disappear when they molt to mature adults. Males are small black spiders, 0.2 to 0.3 inches (4 to 8 mm) in body length, that are variable in color but often boldly marked, similar to immature females. These two species overlap broadly in distribution and can be difficult to distinguish based only on external appearances.

Females and immatures build tangled cobwebs attached at various points to structures for stability. The common name of the family in which widows are placed, Theridiidae, is “cobweb spiders” in reference to the tangled, apparently disorganized nature of the web. Egg sacs of black widow spiders are smooth and typically 0.4 to 0.5 inches (10-12 mm) in diameter.

Life Cycle

Females and immatures shelter in dark concealed corners or crevices at the rear of the web. The web itself is typically built out of direct light in wood piles, interiors of outbuildings, docks and overhanging banks. When a prey item is detected, the spider produces enough silk to prevent escape, then bites it and feeds. Males are attracted to webs of mature females and are seldom seen away from their much larger mates. Following mating, females may kill the male. However, this is not a consistent behavior, despite the common name and widespread belief that the males are doomed after mating. She then produces egg sacs containing several dozen eggs each. Spiderlings hatch within a few weeks. They then disperse or are eaten by their siblings. Female black widows may live for a year or more, while the males live less than a year even if they are lucky enough to escape their mates.

Ecological Significance and Pest Status

Both the northern and southern black widow overlap in distribution in the southern U.S., including Louisiana. Both are native species, but evidence suggests that they have been largely displaced by the introduced brown widow, at least in urban areas. Black widows are more commonly encountered in rural areas. Old farmhouses, barns, lumber piles and storage sheds are classic human-produced black widow habitats. Black widow bites pose a significant medical event that require prompt treatment. Bites are rare and typically the result of accidentally contacting females after reaching into places where undetected webs are located. In such situations the females typically retreat, but if contact is made they are capable of delivering a potent venom with both local and systemic neurotoxic effects. Serious systemic symptoms include abdominal cramps and, in severe cases, respiratory distress. In cases where a black widow bite is suspected, immediate medical treatment is required. Treatment typically involves symptom management, but an antivenom is available for severe cases. A fascinating, and perhaps ill advised, example of self-experimentation was documented in 1923 by University of Arkansas entomologist William J. Baerg (1885-1980). He forced a female southern black widow to bite his finger, received a medically significant bite, and carefully documented the subsequent symptoms. Males and small immatures are no threat to humans due to their smaller sizes.

Control

Black widows in occupied dwellings or outbuildings that are in use can be individually eliminated by using over-the-counter insecticide formulations. Be sure the chemical is labeled for spiders and follow label directions. Extensive populations around dwellings should be managed by a pest control professional. Under no circumstances should active black widow webs be probed with a hand — even if it is gloved — or a stick, as the spiders have been known to walk up the stick or glove or drop onto unprotected parts of the body in their attempts to escape.

References

Baerg W. J. 1923. The effects of the bite of Latrodectus mactans Fabr. The Journal of Parasitology 9: 161-169.

Garb, J. E., A. González, and R. G. Gillespie. 2004. The black widow spider genus Latrodectus (Araneae: Theridiidae): phylogeny, biogeography, and invasion history. Molecular Phylogenetics and Evolution 31: 1127-1142.

Vetter R. S., and G. K. Isbister. 2008. Medical aspects of spider bites. Annual Review of Entomology 53: 409-429.

LatrodectusmactansClemsonUniversityUSDACoopExtSlideSeriesjpg

Southern black widow, Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org.

Latrodectus_variolusJimJasinskiOhioStateUniversityExensionBugwoodjpg

Northern black widow, Jim Jasinski, The Ohio State University Extension, Bugwood.org.

11/10/2021 10:05:32 PM
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