Many spiders requiring control by pest management professionals are not native to North America. Here’s what you need to know.
With all the threats, real and hypothetical, that this country frets about, a silent invasion has been occurring for centuries with little fanfare. Many of the spiders that are common to our homes are actually non-native species that have become established in North America and have spread across the continent.
Some probably came over with the first European settlers and have been here for more than two centuries. Several are worldwide tramp species, found on almost every continent. Quite often, they take over a habitat so profusely that they are the dominant species. For example, the Los Angeles County Museum of Natural History has been performing an area-wide spider survey for more than seven years; of the five most common spiders submitted by homeowners, only the western black widow is a native species.
CELLAR SPIDERS. Several species of cellar spiders have become well established in North America, all of which have long, spindly legs. They are one of the major reasons why homeowners request spider control measures around their homes.
The most common species is the long-bodied cellar spider, Pholcus phalangioides, which has a uniformly grey body and is a common resident of basement and window wells. It has some dark markings on the front part of its cephalothorax which people mistake as a violin and, therefore, this spider is mistaken for a brown recluse. It has been on this continent for more than a century, originating from Europe, and is found throughout much of the United States.
In the southwestern U.S., the marbled cellar spider, Holocnemus pluchei, has a brown sternum and brown longitudinal stripe on its belly, with all leg joints possessing a white ring surrounded by black rings. It is common around homes, found inside in the corners of the ceiling as well as in the eaves of the house. The marbled cellar spider is of Mediterranean origin and is thought to have entered the country in the San Francisco Bay area around the middle of the 20th century. It has spread throughout coastal California, into Arizona and New Mexico. In many places in Southern California, it is the most common spider around house eaves.
From Texas to Florida, another cellar spider with no common name, Crossopriza lyoni, (from southeast Asia) has an abdomen that looks like the end has been chopped off because it ends sharply with a flat surface rather than tapering gently. These cellar spiders can be major nuisances because of the cobwebs they leave behind.
BROWN WIDOW. The brown widow spider, Latrodectus geometricus, is found in most tropical areas of the world. It was described from South America, but its origins are probably Africa because that is where its closest relatives live. For many years in the United States, it was restricted to peninsular Florida. However, in the first decade of the 21st century, unexplainably, it spread dramatically. It is now found from Texas to South Carolina.
In Southern California, it has become an extremely common urban spider from San Diego to Santa Barbara. It probably is not done colonizing new territory so it would not be surprising to see it continue to expand its range. Homeowners who used to admit to having a handful of black widows on their property are now complaining that they have dozens to a hundred brown widows taking over their backyards. The spiders will tolerate highly exposed areas like underneath wrought iron railings, garden pathway lighting fixtures, in rolled up leaves of plants and in gardening clothes. In California, the brown widow is frequently mistaken for immatures of the native western black widow and vice versa.
Bites from these spiders are rare because the brown widow readily assumes a death position with its legs curled up close to its body. Minor disturbances will cause the widow to fall to the ground to escape interaction with people. Although there is one report of a hospitalization from a brown widow bite, a series of 15 bites recorded in Africa resulted in just minor effects (it hurts when bitten) and leaves a minor red mark. Despite the high density of these spiders around human habitation, historically, bites by the spider are rare.
WOODLOUSE SPIDER. There are about 200 species of woodlouse spiders in the world, mostly in Europe and the Canary Islands, but only one species, Dysdera crocata, has been found outside this area (and how!). It has a cinnamon-colored cephalothorax, a tannish gray abdomen and long fangs, which it unfurls and directs toward you in defending itself. Its bite is harmless, though, with symptoms disappearing within an hour or so.
This spider is found on many continents throughout the world. It is still spreading in North America, only being found in the Pacific Northwest in the last decade. It occasionally gets into houses but, most often, the woodlouse spider stays outside. However, because of its long fangs and willingness to open its fangs toward you, people have been intimidated by them.
YELLOW SAC SPIDER. There are two species of yellow sac spider in North America (there are about 160 species worldwide). The species that gets into homes most often is Cheiracanthium mildei. This spider is from Europe and was not found in the U.S. often before the 1950s. However, since then, it has spread coast to coast.
Yellow sac spiders are common in homes. They make a little sac in which they rest during the day, waiting for nighttime when they come out to hunt prey. These little sacs are typically hidden in crevices; in nature, they curl leaves to make the sac, pulling the edges together with silk. In the home, they make their sacs in tight places such as between bookcases or pictures and the wall, in folds of curtains that are left open and under furniture.
The bite of a yellow sac spider is painful, hurting like a bee sting. People who are bitten usually then seek out the source of the pain and locate the spider. Although older literature states that yellow sac spiders can cause mildly necrotic skin lesions, recent research has shown that this is incorrect. After the initial painful bite, the symptoms are redness, itching and slight swelling, all of which disappear in a few days.
HOBO AND RELATED SPIDERS. Several related species of European spiders of the genus Tegenaria have become common urban house spiders. These include the hobo spider, Tegenaria agrestis; the giant house spider, T. duellica (or T. gigantea, depending on which authority you speak to); and the barn funnel weaver or common funnel weaver, T. domestica.
The hobo and giant house spider stepped off the boat in the Pacific Northwest around the 1930s and the hobo spider has been moving steadily eastward. The hobo spider is found from British Columbia to southern Oregon and eastward to Montana, Wyoming, Colorado and northern Utah. The giant house spider is restricted to the Pacific Northwest from Oregon to British Columbia, west of the Cascade Mountain range (the "wet side"). The common funnel weaver is found throughout the United States and, indeed, the entire world. These spiders make funnel webs around homes, in basement window wells and fields. Their web is a flat sheet that tapers back to a hole under a dirt clod, or piece of wood, or a missing piece of mortar between two bricks. These spiders are virtually restricted to their webs as juveniles but when they mature, they start to roam and are found in homes. In the Pacific Northwest, male hobo and giant house spiders start turning up in residences around August and disappear around Halloween. The females lag about a month behind, being active from September to November.
The hobo spider was blamed for causing skin lesions similar to the brown recluse but subsequent research has challenged this notion because the original research that incriminated the spiders could not be duplicated by another researcher. Other discrepancies are that 1) hobo spiders are not considered toxic in Europe where scientists have been active documenting the local spider fauna for more centuries than the United States, and 2) no other member of the genus is considered dangerous; usually related spiders (such as recluses and widows) have similar venom components.
METALTELLA SIMONI (NO COMMON NAME). Metaltella simoni is a South American spider that was first discovered in Louisiana in the 1940s and now is found throughout the Southeastern United States and in Southern California. It is a medium-sized spider (about 6 to 8 mm in body length), dark in color and makes a web that is more like cotton-candy silk than the shiny, thin fishing-line silk of other web weavers such as the black widow. It has widely set eyes on a blocky head. It is harmless but its numbers can get excessive such that pest control is requested.
FINAL THOUGHTS. Non-native exotic spider species will continue to establish themselves in North America. For those that are successful, they may reach a status of urban pests because it seems that humans provide the necessary altered habitat with perennial landscaping or structures that support spider webbing to allow them to survive once they hit our shores. This will probably not change soon and it would be expected that more will follow.
The author has been a staff research associate in the Entomology Department at the University of California, Riverside, for 31 years. He can be reached at firstname.lastname@example.org.