Several years ago, false reports describing brown recluse spider encounters in southern California became widespread in the media. However, these reports have since been forcefully disproven by numerous entomologists, pest control professionals and other experts. Today, it is common knowledge among California residents that brown recluse spiders are commonly mistaken for other similar looking spider species that are commonly found in the state. According to a 2005 scientific survey, the three spider species that are most commonly mistaken for brown recluse spiders in California are false widows, woodlouse spiders and yellow-sac spiders. While the false black widow and woodlouse spider species are harmless to humans, yellow-sac spiders are known for their habit of invading homes, their aggression toward humans and their tendency to inflict bites to humans without provocation. In fact, many studies have documented medically significant yellow-sac spider bites, including bites that led to tissue necrosis.

Two arachnid species of the same genus, Cheiracanthium inclusum and Cheiracanthium mildei, are both commonly referred to as “yellow-sac spiders” due to their similar appearance and habits around humans. These two species are abundant in southern California, and they often appear in Ventura County homes, sometimes in large numbers. According to a 2016 nationwide survey of pest control professionals, yellow-sac spiders were the sixth most commonly managed spider species within homes. C. mildei and C. inclusum are not shy around humans, as each species is known for readily crawling on humans and inflicting very painful and unprovoked bites, but the former is more common within homes, while the latter is known for inflicting bites outdoors. The vast majority of yellow-sac spider bites occur in residential homes where the spiders are often seen moving rapidly across window sills, walls and ceilings. During the day, yellow-sac spiders rest in very small white silk sacs that are commonly found in floor and ceiling corners. Yellow-sac spider bites cause immediate and very intense burning pain that tends to last for an hour or more. Bites also cause erythema, edema, intense itching, and a local rash and/or blistering typically occurs within 10 hours. While it is exceptionally rare for tissue necrosis to result from yellow-sac spider bites, it is not uncommon for bites to cause systemic symptoms, such as fever, malaise, muscle cramps, and nausea. Both species of yellow sac spiders are around ¼ to ⅖ of an inch in length, but their relatively long legs make them appear larger. Their cream to light-yellow colored bodies sometimes bear a green tinge.

Have you ever encountered a yellow-sac spider within your home?