NPMA Library Update: The Widows
The Widows: Spiders of the Latrodectus Genus in North America
Laura Rosenwald, BCE, Training & Resource Center Manager, NPMA
The widow spiders are some of the most recognizable spiders in North America. They belong to the genus Latrodectus in the Family Theridiidae, which are the cobweb weavers. This family is easily distinguished by their teardrop-shaped abdomens that taper to a point at the spinnerets, as well as their messy and irregular three-dimensional webs containing vertical silk lines. Latrodectus contains over 30 species that can be found around the world, but several of these species call North America home. The adult females of this genus are often recognizable by their distinct shiny black coloration with red "hourglass" patterning. However, this is not the case for all widow species, and the markings are not always visible. In addition, males and juveniles of the widow species look drastically different both in color and size. The common name "widow" comes from the observation that some adult females in this genus eat the male following mating. However, this name is a misnomer, as female widows rarely engage in this behavior.
LATROTOXIN
Members of the Latrodectus genus are medically relevant pests, as their venom contains a neurotoxin known as latrotoxin. Latrotoxin can induce pain at the bite site, muscle rigidity, vomiting and sweating. Most people bitten by widow spiders make a full recovery, but the elderly, children and immunocompromised are the most susceptible to serious illness and death. While all widows are capable of biting, only the adult females of this genus have fangs long enough to puncture human skin and often only bite in self-defense. Bites are easily treated with the use of antivenom. If bitten, seek medical attention immediately and try to bring the spider to a medical professional to ensure proper treatment. However, wearing personal protective equipment (PPE) such as gloves in areas where widow spiders are suspected to inhabit is often enough to protect yourself from bites.
NORTHERN BLACK WIDOW (L. VARIOLUS)
Despite the name, the Northern Black Widow’s range extends from Canada to Florida and across the eastern half of North America. This species features the classic shiny black with red markings. These spiders can be easily distinguished from other widow species by the "hourglass" marking underneath the abdomen, as it features a break in the red "hourglass" that is not seen in the other widow species. Northern Black Widow females may also feature a line of red dots down the back, particularly in the younger individuals. Their legs may also feature a reddish color. Adult females measure in at approximately 9-11 mm in body length, with males measuring in at about half the size of the females. Egg sacs of this species are round and have a tan paper-like appearance.
SOUTHERN BLACK WIDOW (L. MACTANS)
The Southern Black Widow is often considered to be the "original" Black Widow and is arguably one of the most recognizable spiders in North America. This species has been documented all across North America, but is most commonly found in the southeastern parts of the continent. Adult females feature the shiny black coloration across the body and the legs. Southern Black Widows can easily be distinguished from the Northern Black Widow as their red hourglass is complete and does not feature a break. The hourglass marking in this species is also often wider at the bottom compared to the top when the female reaches adulthood. This species also commonly features an additional red spot just above their spinnerets. The body length of adult females is between 8-13 mm, and the males are approximately half that size. The egg sacs of this species are tan to grey in coloration but are distinct based on their circular shape with a single point.
WESTERN BLACK WIDOW (L. HESPERUS)
The Western Black Widow is practically indistinguishable from the Southern Black Widow in terms of morphology and appearance but can often be distinguished by geographic location. Some individuals of this species can be distinguished by the hourglass being more in proportion compared to the Southern Black Widow. While there is some overlap in the range of these two species, the Western Black Widow is generally restricted to the Western half of North America, while the Southern Black Widow is much more common in the Southeast. Much like the Southern Black Widow, adult Western Black Widow females are a glossy black color with the complete red hourglass underneath the abdomen. Western Black Widows may also feature a red spot near their spinnerets. This is the largest widow species, with adult females measuring between 14-16 mm, and with the males measuring at half that size. This species also features tan to grey egg sacs that are generally circular but feature a single point.
RED WIDOW (L. BISHOPI)
The Red Widow is the rarest widow that you may encounter in North America, as it is generally restricted to the sand pine scrub habitats of south and central Florida. However, this species may take residence in sheds, garages, and firewood. They are large spiders, with the females measuring in around 12 mm in body length. The males are much smaller than the females. The cephalothorax and legs of this species are red orange, with a black abdomen. The abdomen will feature several red, orange or black spots that are surrounded by a white outline. The distinctive hourglass is much reduced in the Red Widow to two small red marks. Egg sacs of this species are smooth and creamy white in color.
BROWN WIDOW (L. GEOMETRICUS)
While the Brown Widow does not share the same distinctive coloration as its cousins, it still features the infamous "hourglass." This species is light brown in color and features an orange to red orange hourglass under the abdomen. Brown Widows will also often feature distinctive black, white and brown geometric patterning on the sides of the abdomen, which is where their species name originates. The coloration on the legs darkens at the joints. Females are around 12-16 mm in body length, with the males measuring in between 6-8 mm with longer legs. Brown widow egg sacs are round, pale tan, and covered in spikes. This species is cosmopolitan and is a frequent stowaway in human trade. It was first introduced in Florida and has quickly spread across most of the southern part of North America. This is likely due to its high reproductive potential, as females can have up to 5,000 young a year. Brown Widows additionally seem to be edging out other widow species in the same areas, suggesting that they may be outcompeting Black Widows for the same resources.
MANAGEMENT
The management of widow spiders is much like the management of any other pest spider, but extra care should be taken regarding medically relevant species such as widows. Spiders often congregate where there is easy access to shelter and food. Often, the presence of spiders can indicate that there is another pest issue persisting in the same area, so inspection should be a key part of your management strategy. Not only should you be looking for potential food sources for the spiders but keep an eye out for signs of spider activity, including webs, egg sacs and the spiders themselves. Widow webs are distinctive by their messy three-dimensional appearance with vertical or nearly-vertical anchor lines running through them. Widows generally choose areas that are sheltered and rarely disturbed, which can include both natural and manmade structures. Most spider management efforts will involve non-chemical methods, such as implementation of a regular cleaning schedule, eliminating potential harborages and changing exterior lighting. In some cases, the use of insecticides may be necessary. Follow and adhere to all pesticide label instructions when applying and focus on the areas of activity as well as their harborages. The combination of both non-chemical methods along with chemical methods will be your best bet for management of spider infestations.
REFERENCES
Johnson, J. Chadwick, Trubl, Patricia, Blackmore, Valerie, Miles, Lindsay. 2011. Male black widows court well-fed females more than starved females: silken cues indicate sexual cannibalism risk. Animal Behavior. Vol. 82, Issue 2. Pg. 383-390. https://doi.org/10.1016/j.anbehav.2011.05.018
Ushkaryov, Yuri A., Rohou, Alexis, Sugita, Shuzo. 2008. α-Latrotoxin and Its Receptors. Handbook of Experimental Pharmacology. 184, pg. 171-206. https://doi.org/10.1007/978-3-540-74805-2_7
Vetter, Richard S. and Ibister, Geoffrey K., 2008. Medical Aspects of Spider Bites. Annual Review of Entomology. Vol. 53. 409-429. https://doi.org/10.1146/annurev.ento.53.103106.093503
Bradley, Richard A. 2013. "Common Spiders of North America". University of California Press.
Sadir, Melissa, Marske, Katherine A. 2021. Urban Environments Aid Invasion of Brown Widows (Theridiidae: Latrodectus geometricus) in North America, Constraining Regions of Overlap and Mitigating Potential Impact on Native Widows. Frontiers in Ecology and Evolution. Volume 9. https://doi.org/10.3389/fevo.2021.757902
Rust, Michael K., Vetter, Richard S., Reierson, Donald A. 2010. Cobweb Management and Low-risk Sprays as IPM Strategies to Control the Marbled Cellar Spider, Holocnemus pluchei (Araneae: Pholcidae). NPMA Research Reports. https://www.npmapestworld.org/content-pages/technical-resources/pmf-research-reports/research-report-cobweb-management-and-low-risk-sprays-as-ipm-strategies-to-control-the-marbled-cellar-spider-holocnemus-pluchei-araneae-pholcidae/
