Western black widow spider, Latrodectus hesperus
The western black widow spider is the most common of the Latrodectus spp. inhabiting Colorado. Females are usually black or dark brown and rarely grow larger than 6.4-8.5 mm (1/4-1/3 inch). The female western black widow spider has the characteristic red or orange hourglass shape on the underside of the abdomen. However, the shape varies and can appear as unconnected dots, a rectangle with no hourglass shape, or can be severely faded which may result in mistaking this spider for a different species. Another distinguishable feature of females is the spherical shape of their abdomen.
Male western black widow spiders are generally much smaller than females at 3.2-6.4 mm (1/10-1/4 inch) and are usually light brown or gray with banded legs. Males have an elongated body with large palps extending from their first segment. Males also have the distinctive hourglass shape underneath their abdomen, but it is typically orange or yellow instead of red.
Immatures look drastically different than adults, with a gray or light brown body and banding patterns, and the hourglass shape on their abdomen usually darkens and becomes more defined as they mature. They can also have stripes and spots on the top of their abdomen, usually red, orange, or yellow. Immature female western black widow spiders are often mistaken for mature males given their similar appearance.
- Adult female western black widow spiders are shiny and usually black or dark brown. The distinct red or orange hourglass shape underneath the abdomen can consist of two spots, a rectangle, or is barely visible.
- These spiders produce sticky mesh-like webs in undisturbed and dark areas, such as the corners of rooms, garages, or sheds. They can also live in areas where a ‘tunnel’ is already present such as pipes, drains, outdoor tables, etc.
- Young children and older adults are highly susceptible to bites. Bites can cause chest pain or tightness, stomach cramping, nausea, swelling of the eyelids, and burning at the bottom of the feet.
- Effective management involves regularly scouting potential nesting spots while wearing protective clothing such as heavy leather gloves. When discovered, spiders and egg sacs should be destroyed by crushing or vacuuming while wearing protective clothing. It is also recommended that all members of the household be educated on the appearance and behavior of western black widow spiders.
Female western black widow spider and its irregular sticky web. While bites are infrequent, they are most likely to occur near nests with egg sacs. Western black widow spiders feed on other arthropods and are occasionally cannibalistic. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Immature male western black widow spider feeding on a grasshopper. Note the palps (white arrows) and presence of red, orange, and yellow spots and stripes on the abdomen, which is a distinguishing feature of immatures. Image credit: Joseph Berger, Bugwood.org
There is variability in the shape of the hourglass on western black widow spiders. Note the two disconnected red spots on this adult female. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
A mature western black widow spider in an ideal dwelling area with an egg sac. Image credit: Josh Shoemaker, Bugwood.org
Side-by-side comparison of the size difference between mature male (left) and female (right) western black widow spiders. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Mature females build characteristic cobwebs, which are sticky mesh-like structures made of silk. Each egg sac may contain up to 200 eggs that typically hatch within two weeks of being laid. Egg sacs are usually light yellow but can also be brown. After hatching, the immature spiders remain in the egg sac for a few days where they molt. Western black widow spiders will then ‘balloon’ to spread after leaving the egg sac, which involves releasing a strand of silk for dispersal via wind currents. Western black widow spiders molt several times throughout their life span and females usually become mature in four to six months. Males go through fewer molts than females and mature more quickly as a result.
Females will occasionally feed on males when food is scarce, but this is uncommon. Males take part in a specific mating ritual which involves a species-specific vibration, transmitted through the female’s web. If the female is responsive, she will send her own vibration back. Sperm is stored in the palps of the male and is inserted into the female genitalia. Sometimes males will remain around the female’s web and live off the food that she captures, which can extend the male’s lifespan.
Mature females can live over a year while males usually live only a few weeks. Like all arthropods, development and life span are influenced greatly by temperature and food availability. Western black widow spiders can overwinter indoors as adults or as immatures. Hardly any will be found outside during the winter.
Although extremely rare, bites can be fatal to young children and the elderly. The venom of western black widow spiders contains a neurotoxin that results in muscle and chest pain or tightness, abdominal pain, stomach cramping, nausea, restlessness, anxiety, sweating, a burning sensation on the soles of the feet, and difficulties with breathing and speech. The severity of symptoms generally increases during the first day of being bitten. In healthy adults, symptoms typically decline after two or three days after being bitten but may persist up to a month after the bite.
Bites can go unnoticed, though there is usually a sharp and immediate painful pin-prick sensation. Bites are almost always inflicted by females. This is because females are more likely to be encountered than males and the fangs of males are smaller and less likely to penetrate human skin.
Educating members of the household on the appearance and behavior of western black widow spiders will help prevent encounters. Western black widow spiders like to nest near the ground in undisturbed and dark areas such as underneath furniture, in corners, garages, crawl spaces, wood piles, and low shrubs. Periodically inspect such areas in and around the home. Wearing clothing that covers skin and thick gloves is useful when entering potential nesting sites of western black widow spiders, especially when moving wood, furniture, or cleaning in dark and undisturbed areas. Areas should be well lit periodically as these spiders do not respond well to an increase in light. Regular vacuuming of isolated areas in the house prevents webs from forming in an undisturbed habitat.
Scout for sticky, irregular webs in isolated areas of the home. Circular tubes like pipes, lawn chairs, etc., can be ideal nesting areas.
When found, these spiders and their webs can be crushed or vacuumed while wearing protective equipment such as heavy leather gloves. Egg sacs can be crushed or frozen.
Insecticides do not have a large impact on egg sacs but can be effective on immature and adult forms of the western black widow spider. Insecticides sold for general control of spiders can be applied by spraying the exterior foundation of the house, window seals, and doorways. However, chemical management is highly variable and may not be effective. Crushing or removing adults from the home in addition to freezing egg sacs are more effective management approaches.
Cranshaw, W. 2013. Western Widow Spider. Colorado State University – Extension. Available https://extension.colostate.edu/topic-areas/insects/western-widow-spider-5-605/
Reeves, J., E. Allison, and Peggy, G. 1996. Black Widow Spider Bite in a Child. American Journal of Emergency Medicine. 14: 496-471
Utah State University (n.d.). Top 20 Arachnids. Utah State University – Extension. Available https://extension.usu.edu/pests/uppdl/top-20-arachnids
Utah State University. (n.d.). Black Widow Spider. Utah State University – Extension. Available https://extension.usu.edu/pests/schoolipm/structural-pest-id-guide/black-widow