Spider Research

[From the February 2013 issue of the UC IPM Green Bulletin newsletter]

During the last 10 years, a new widow spider has moved into parts of Southern California. The brown widow spider, Latrodectus geo­metricus, is closely related to the well-known

Figure 1. Mature adult female black widow spider showing hourglass. The brown widow adult female has a fainter hourglass as well. (J.K. Clark)

black widow spider, L. hesperus, (Figure 1) that occurs throughout much of California.

A recent survey of widow spiders in South­ern California led by retired UC Riverside entomologist Richard Vetter revealed new information about the distribution of brown widows. Currently brown widow spiders are known to be common in Los Angeles, Orange, San Diego, Riverside and San Ber­nardino counties. They have also been re­ported in Ventura and Santa Barbara coun­ties, and experts believe they may eventually move up the coast of California and also into the Central Valley.

Vetter's group also found that, like the black widow, brown widows are outdoor spiders rarely found inside homes. They also found brown widows tend to inhabit more exposed outdoor habitats such as under eaves or win­dow ledges, garden furniture with solid—not mesh—tops, or recessed handles of plastic trash bins. Black widows, in contrast, prefer more protected habitats such as in garages or sheds, under debris or woodpiles, or in a protected hole in an outer wall. Unlike black widows, brown widows weren't found in nat­ural dry habitats or agricultural areas; they prefer urban environments and structures.

Researchers report anecdotal evidence brown widows may be displacing the black widow spider in some urban habitats. This is probably good news for residents, because, despite the growing numbers of brown widow spiders, their bites are infrequently reported compared to black widow bites and rarely cause severe symptoms in humans; there is only one verified case of a brown widow bite in Southern California, and re­ported symptoms were mild.

Brown widow spiders are mottled brown in all stages (Figure 2) and resemble immature black widows (Figure 3), so some skill is required to distinguish the two species.

Figure 2. A mature adult female brown widow spider showing mottling. (R. Vetter)

Figure 3. Immature female black widow spider. (J.K. Clark)

The easiest way to identify an infestation of brown widows is to find the egg sacs, which have pointy protuberances (Figure 4) in contrast to the more round black widow egg sac (Figure 5).

Figure 4. The distinctive egg sacs of brown widow spiders with pointy protuberances. (R. Vetter)

Figure 5. Egg sacs of black widow spiders are smooth. Small, newly hatched spiderlings surround this egg sac. (J. Clark)

For more about identifying brown widows, visit the UC Riverside Center for Invasive Species Research (CISR) Web site. For more about widow spiders and managing them, visit the CISR Web site or see Pest Notes: Black Widow and Other Widow Spiders. Details of the survey are reported in J. Med. Entomol. 49(4):947-951.

This article was originally published in the February 2013 issue of the UC IPM Green Bulletin. See this and other articles at http://www.ipm.ucdavis.edu/greenbulletin/index.html.