Spider cases at the UW Insect Diagnostic Lab often boil down to clients wondering if they’ve found either a brown recluse or black widow spider. Wisconsin is home to approximately 500 species of spiders, and essentially all of these are harmless, beneficial creatures that provide us with an astonishing amount of free pest control*. While we’re outside the native range of the dreaded, but horribly misunderstood brown recluse spider (Loxosceles reclusa), Wisconsin is actually home to a native black widow species—the northern black widow (Latrodectus variolus). In Wisconsin, these spiders are rare and have been documented in fewer than 10 counties. Most Wisconsinites will go their entire lives without seeing a northern black widow out in nature and if you’re lucky enough to spot one of these elusive creatures, you’d almost certainly encounter a lone individual. In contrast, there are several other widow spiders that can be much more common in other parts of the country.
The vast majority of black widow records in Wisconsin are from the east-central counties. Door county historically stands out as having the most confirmed sightings and perhaps takes the place of Wisconsin’s black widow “capital”—although only a few sightings occur in most years. The northern black widow doesn’t seem to occur farther north under natural conditions, which suggests an inability to the survive colder winters in the northern part of the state. Along these lines, Door county’s unique geography and the moderating effects of Lake Michigan may explain why the majority of reports come from that part of Wisconsin. Similarly, other confirmed reports of the northern black widow tend to be from nearby counties bordering Lake Michigan. Away from the lake, northern black widows have also been documented in prairie areas in Sauk and Crawford counties, where the microclimate on south-facing slopes may favor their survival.
In a typical year, the UW Insect Diagnostic Lab might get a report or two of black widow spiders, but 2017 stood out in the sheer number of reports. For spiders like the black widow, I keep in touch with Dr. Mike Draney, an arachnologist at UW-Green Bay. During some stretches of 2017, Mike and I were emailing reports of black widows to each other once or twice a week! These reports generally came from Door county and nearby areas. In addition, these spiders were also documented in two additional Wisconsin counties for the first time last year: Brown county and Sheboygan county.
It’s possible that winter weather patterns might explain the distinct bump in northern black widow sightings last year. Leading up to 2017, Wisconsin faced two consecutive mild winters, which might have favored insects, spiders, and other creatures that fare better in slightly warmer climates. While the state did see an increase in reports last year, it’s not yet known if that trend will continue in 2018 as the state just emerged from a veritable winter season.
*A recent study estimated that spiders around the globe consume approximately 400 – 800 million tons of prey annually. Nyffeler, M. & Birkhofer, K. Sci Nat (2017) 104: 30. https://doi.org/10.1007/s00114-017-1440-1