The black widow spider, easily identified by its shiny black body and the distinct red hourglass marking on the underside of the female’s abdomen, is a common concern. While the female possesses a neurotoxic venom that can cause severe symptoms, bites are typically defensive, and fatalities are rare with modern medical care. These arachnids prefer dark, undisturbed locations like woodpiles, cluttered sheds, and crawl spaces. Understanding the natural processes and non-chemical methods that control their populations offers a balanced approach to managing their presence.
Biological Predators and Parasites
In the natural environment, several organisms actively prey upon or parasitize black widows. Among the most specialized natural enemies is the Blue Mud Dauber wasp (Chalybion californicum), which paralyzes the spider and seals it within a mud nest as a living food source for its developing larvae. Other generalist predators include certain birds, such as wrens and bluebirds, and reptiles like the alligator lizard.
Other spiders, including wolf spiders and jumping spiders, may also attack and consume black widows, especially when food resources are scarce. A significant threat exists at the earliest life stage, where specialized insects attack the tightly woven egg sacs. Tiny parasitic wasps, such as Baeus latrodecti, chew their way into the sac and lay their own eggs inside the spider eggs, with parasitism rates sometimes approaching 100 percent.
Flies from the Chloropidae family are also known as egg predators, depositing their eggs into the black widow’s silk sac. This prevents the hatching of hundreds of spiderlings from a single female. The cannibalistic nature of the spiderlings themselves also significantly reduces the successful emergence of each brood, as the first hatchlings often consume their slower siblings.
Environmental Stressors
Non-living environmental conditions can significantly limit black widow populations, particularly at the extreme ends of their thermal tolerance. High temperatures, such as those exceeding 33 degrees Celsius (91.4 degrees Fahrenheit), dramatically slow the growth and development of spiderlings. This heat stress often results in smaller body sizes and increased mortality rates.
The stress from extreme heat can also trigger behavioral changes in the young, notably an increase in foraging activity and sibling cannibalism. Black widows are highly susceptible to desiccation and must seek microclimates, such as under debris or within structures, to buffer themselves from harsh daytime heat. Conversely, prolonged periods of extreme cold are also detrimental, as their metabolic processes are not adapted to freezing temperatures, forcing them to find shelter in deep crevices or inside heated buildings.
Destruction of their preferred habitat, such as from heavy flooding or prolonged drought conditions that eliminate their insect prey, can also cause population decline. Chronic scarcity weakens them and makes them more vulnerable to predators or the physiological demands of reproduction.
Non-Toxic Methods for Home Control
For control inside and immediately around a home, strategies focus on physical removal and non-toxic substances. The most direct method is physical elimination using a strong vacuum cleaner with a hose attachment to safely remove the spider and any egg sacs. It is important to immediately empty the vacuum canister or seal the bag and dispose of it outdoors to prevent any spiders or spiderlings from escaping.
A non-toxic substance widely used for physical control is food-grade Diatomaceous Earth (DE), a finely ground powder made from the fossilized remains of microscopic aquatic organisms. DE kills black widows through a physical mechanism; the abrasive particles scratch the spider’s protective outer layer, causing the spider to lose body fluids and die from desiccation. A very fine, barely visible dusting must be applied to cracks, crevices, and other areas where the spiders are known to crawl, as a thick layer may be avoided.
Other non-chemical sprays can be used for direct contact, such as a solution of equal parts white vinegar and water. The high acidity of the vinegar can damage the spider’s exoskeleton upon direct application. Certain essential oils, especially peppermint, tea tree, and eucalyptus, are sometimes used as deterrents; the strong scent may encourage them to relocate their webs. Finally, reducing clutter around the home, sealing entry points, and removing debris piles eliminates the dark, sheltered refuges that black widows seek for nesting.