Black widow and brown widow spiders are subjects of public curiosity, particularly regarding their interactions. A common question is whether the invasive brown widow actively hunts and preys on its black widow relative. Understanding their characteristics and behaviors reveals their complex ecological dynamics.
Identifying Widow Spiders
Black widow spiders (Latrodectus mactans and Latrodectus hesperus) are known for their shiny black bodies and a distinctive red or orange hourglass marking on the underside of their abdomen. Females typically measure about half an inch in body length. They generally prefer dark, undisturbed, and secluded environments close to the ground, such as within woodpiles, under stones, or in cluttered areas of sheds, garages, and basements. Their egg sacs are smooth, white, and pear-shaped.
Conversely, brown widow spiders (Latrodectus geometricus) exhibit a wide range of colors from light tan to dark brown or nearly black, often displaying a mottled pattern. They also possess an hourglass marking on their underside, typically orange or yellowish. Brown widows are similar in size to black widows, with females also around half an inch in body length. Their preferred habitats include more exposed, human-made structures and cluttered outdoor spaces like empty containers, mailboxes, and the undersides of outdoor furniture, especially in urban settings. A distinguishing feature is their unique egg sac, which is spherical and covered with numerous silk spicules, giving it a tufted or spiky appearance.
Competitive Encounters Between Widows
Brown widows are an invasive species, originating in Africa or South America, now established across many parts of the world. In areas where they have become established, particularly in the southern United States, brown widows actively displace native black widow populations. This displacement is not solely a result of competition for resources like food or habitat, but also involves direct predation.
Research indicates that brown widows exhibit an aggressive tendency towards black widows. In controlled experimental settings, brown widows were observed to be 6.6 times more likely to attack southern black widows than other related spider species. Studies involving pairings of sub-adult spiders showed brown widows killed and consumed black widows in 80% of encounters. While adult interactions varied, with brown widows killing black widows in 40% of trials, black widows were never observed initiating aggression.
This aggressive behavior, coupled with their higher reproductive rates, contributes to the brown widow’s ecological success. Brown widow females are roughly twice as fertile as black widows, producing more egg sacs and reaching reproductive maturity at a faster pace.
Impact on Local Spider Populations
The dominance of the invasive brown widow can lead to shifts in local spider populations and alter ecosystems. In some regions, especially in Florida, native southern black widow populations have experienced local extinctions due to the brown widow’s proliferation. This phenomenon reflects a broader ecological struggle where an invasive species outcompetes and directly preys upon a native relative.
From a human perspective, the displacement of black widows by brown widows presents a nuanced outcome. While brown widow venom is potent, the amount injected during a bite is generally less than that of a black widow, resulting in typically less severe symptoms for humans. As brown widows become more prevalent in urban environments, the overall risk of serious injury from spider bites could potentially decrease. Urbanization appears to play a role in the brown widow’s invasion success, as these spiders thrive in developed areas, which may somewhat limit their impact on black widow populations in more natural, non-urban habitats.