Photo of Steatoda grossa, © 1999, Darwin K. Vest, Eagle Rock Research

STEATODA SPIDERS AS

COMPETITORS/PREDATORS

OF THE HOBO SPIDER



Spiders of the genus Steatoda belong to the spider family Theridiidae, or the cobweb weavers. Also known as Combfooted Spiders, this family includes a number of well known arachnids, including the American house spider, Achaearanea tepidariorum, and the widow spiders Latrodectus spp.. Steatoda spiders are found throughout the world, in both temperate and tropical climates. At least eight Steatoda spp. can be found within the U.S. range of the hobo spider. Steatoda spiders are small to moderately small (3-9 mm) spiders with oval abdomens; they may be reddish, brownish or black, with most species exhibiting a white band at the front of the dorsal abdomen which may resemble a collar. Steatoda spiders construct a strong, irregular web, somewhat resembling the webs of widow spiders; this web is very sticky, making it a highly effective snare for hobo spiders.

Steatoda grossa, the false black widow spider (pictured upper left), is the most well known of the Steatoda spiders. Found in cosmopolitan areas around the world, and on both coasts of the United States, grossa is a larger (9 mm) Steatoda which, as its common name suggests, may resemble a black widow spider (with no hourglass). Specimens can be reddish to purplish brown in color, with pale yellow markings on the dorsal abdomen, but many specimens are so dark that these pale markings cannot be distinguished. Steatoda grossa is a common and well known "house spider" in many areas, constructing its webs in and around buildings, rock walls, and other structures. It has long been known that the "false black widow" will ensnare, kill and prey upon actual black widow spiders in its natural habitat. Steatoda grossa has been observed to ensnare and prey upon hobo spiders in the Seattle, Washington, area and almost certainly serves as a very important predator of the hobo spider in the coastal areas of the U.S. Pacific Northwest: It ranges inland as far east as the Sun Valley, Idaho area, but is rare or absent from the Upper Snake River Valley of Idaho.

Steatoda hespera, the western Steatoda or western bud spider, is a smallish (7-8 mm), brown cobweb weaver that is often misidentified as a small "brown" black widow: Pictured at right, this species is common throughout the eastern half of the U.S. range of the hobo spider. It now appears that this little arachnid may be the single most effective predator of the hobo spider in southeastern Idaho, and possibly other infested areas. One specimen living in a household was observed by Eagle Rock Research to ensnare, kill and devour seven hobo spiders during a Photo of Steatoda hespera, © 1999, Darwin K. Vest, Eagle Rock Research single season. The remains of hobo spiders are frequently found in and below hespera webs, and Eagle Rock Research estimates that this species may destroy as many as one million hobo spiders per year in Bonneville County, Idaho, alone. This species is extremely common in and around basements, garages, outbuildings and wood piles in southern Idaho, and is not particular about where it builds its web; Steatoda hespera snares are most often built close to the ground, but webs are also found in lofty areas, such as around porch lights. Steatoda hespera populations vary from year to year, and "blooms" of this species occur every few years, resulting in large numbers of specimens being found around typical households. Interestingly hespera may, like its European cousin S. paykulliana (European cobweb spider), harbor a venom which is significantly toxic to mammals; never-the-less, as of yet there is not a single incidence on record of hespera biting a human. The western Steatoda is a spider which tends to stay in its web, and is thus susceptable to insecticides; this spider is probably the most effective hobo spider deterrent present in much of the U.S. range of the hobo spider, and its presence should be encouraged. Eliminating or reducing populations of hespera with pesticides, etc. in hobo spider affected areas is a deleterious practice, which will likely result in very much unwanted results.

Steatoda triangulosa, the triangulate Steatoda, or triangulate bud spider, pictured at left, is a cosmopolitan species found in various areas of the world. Photo of Steatoda triangulosa, © 1999, Darwin K. Vest, Eagle Rock Research This spider is moderately small (5 mm), reddish to reddish-brown in color, and exhibits a well defined pattern of light colored triangles on the dorsal abdomen. Steatoda triangulosa is widely distributed within the U.S. range of the hobo spider, and is found in close proximity to established hobo spider populations in such areas as Moscow, Idaho and Bountiful, Utah; it is rare or absent in the upper Snake River Valley of Idaho and other hobo spider affected areas. It can be expected that triangulosa plays a significant role (though not to the degree of S. hespera and S. grossa) in reducing hobo spider populations in areas where it shares the hobo's range. As with other Steatoda spiders found within the U.S. range of the hobo spider, Steatoda triangulosa populations should be encouraged.


Hyrum the hobo spider, ©1997 Darwin K. Vest, Eagle Rock Research



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©1999 Darwin K. Vest, Eagle Rock Research