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The Western Black Widow

Latrodectus hesperus 

Adult female western black widow

A mature western black widow found in the north Denver area. She was protecting an egg sac. (not pictured)

Identification

L. hesperus’ most recognizable characteristic is the distinct red hourglass on the underside of the abdomen. This hourglass is made more prominent by the mature female’s inky, black, shiny body. Juvenile females may be gray or brown with lighter bandings on the entire abdomen. The red markings may extend to the top side of the abdomen. As the female matures she gradually becomes blacker until only the hourglass remains on her underside. The western black widow has been known to even lose the hourglass entirely in maturity. 

Male Identification

Male L. hesperus spiders tend to be longer and more slender, notably lacking the bulbous abdomen of his female counterpart. They can be one-half to one-third the size of a female. One characteristic of the male is the presence of large pedipalps. While these appear to be large fangs, they are in fact mouthparts used for holding and guiding prey into the mouth. Male L hesperus’ fangs would unlikely be able to penetrate the skin of an adult.

Misidentification

Other species of spider share some characteristics of L. hesperus. Common cobweb spiders can have the same body shape, and unorganized mesh-like webs made of strong silk. The most common species mistaken for the western widow is from the genus steatoda. Spiders from this genus are often, fittingly, called “false widows.” All of these cobweb spiders lack the tell tale red markings.

This image is of a triangulate cobweb spider Steatoda Triangulosa. It is apparent why this genus is so commonly mistaken for Latrodectus.

Juvenile female L hesperus

A juvenile female. Note the lighter banding and red markings on the the topside of the abdomen.

Male western black widow

This is an adult male. You can easily see the distinct pedipalps, as well as the red markings on the top side of the abdomen.

Cobweb Spider S triangulosa

Habits and environments

Female L. hesperus spins a web of strong silk which emanates from a retreat where she can stay hidden, emerging to retrieve prey. She prefers to live close to the ground and in areas that are dark, dry, and have sufficient insect traffic. Garages, window wells, crawl space entrances, abandoned rodent burrows, and dense debris piles are among her favorite places to dwell. The western black widow is exceptionally shy and semi-territorial. In nature they have been found as close as .5 meters apart, but never occupying the same space vertically. That is to say they do not live on top of one another.

Mating

Males locate female webs by detecting pheromones from her web. Once a female is found the male initiates courtship behavior. These include web plucking and abdominal tremulation. If the female is receptive she will come out to the male. Despite common belief, cannibalization of the male is uncommon, typically only occurring when the female is experiencing food insecurity.

When the female is fertilized she lays up to 200 eggs and confines them to a sac which she suspends in her web. Egg sacs are roughly one centimeter across and are white to peach color. This is the time when L. hesperus becomes most protective and will boldly defend her offspring. After two weeks the eggs hatch and the spiderlings emerge shortly after. Most of these do not survive. Some estimates state 1-12 of these will survive to 30 days. Females reach sexual maturity after four to six weeks. Under optimum conditions, she can live two years or more, however the normal life cycle is one year.

Adult L hesperus with egg sacs

Not the a very high quality image here, but two egg sacs can be observed. They are being tended by the adult female who is difficult to see.

Bites

L. hesperus rarely, if ever, bites outside of her web. Bites are most likely to occur when an egg sac is present in the web and the female is defending. Bites may go unnoticed at first but will typically produce localized swelling and discomfort. The western widow produces a neurotoxin that can cause muscle soreness and chest tightness. Sometimes abdominal pain and nausea occur. Symptoms usually subside in two to three days. There is variance in reaction among mammals. Horses and cats tend to be quite susceptible to the toxin, while dogs reportedly show little to no symptoms.

Control

Harborage management is an important step for keeping black widow populations away from the home. Keeping debris and shrubs managed is an excellent way to limit preferred living areas of L. hesperus. Lighting dark areas of the basement and crawl spaces can also be helpful. 

Female Western Black Widw

Additionally, having the structure treated with a pesticide that has residual efficacy is very helpful. Pyrethroids are extremely effective, and have low to no toxicity to mammals when used in accordance with label instructions. These treatments are most effective in the spring and fall when L. hesperus tends to be the most active and likely to migrate.

Sources

Article written by Kenny Kisch

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