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Friday, October 31, 2014

Rick Vetter

Rick is a staff research associate in the Entomology Department at teh University of California, Riverside, for 31 years.

Features

[Spider Control] Southern Expansion

Spiders

The brown widow spider, a non-native urban spider, is becoming more common in the southern U.S.

November 30, 2012

Fig. 1. A brown widow showing somewhat typical coloration.

The brown widow spider, Latrodectus geometricus, has spread to many tropical and subtropical areas worldwide, probably originating from Africa. It became established in the United States in the 1930s, but did not spread beyond peninsular Florida for decades. However, in the first decade of the 21st century, it experienced an explosive range expansion such that it is now well-established in the southeastern U.S. from Texas to South Carolina with sporadic finds being reported in North Carolina and possibly Virginia; it is also very common in southern California. The brown widow is an urban pest species so it is important for the PMP to be familiar with this now widely spread creature.


Identification. One big hindrance in dealing with the brown widow is identifying the spider because there is so much color variation. The abdomen of the adult female is typically pale tan, similar to the paper covering of a yellowjacket nest, but can vary from almost white to almost black. The dark forms can be mistaken for black widows but if well fed, the spider’s black color won’t be as shiny as a black widow body, more of a dull, matte finish. However, it would be almost impossible to tell an underfed black widow from a dark brown widow.

The brown widow also has abdominal stripes: typically, a longitudinal line extending about halfway from the back end with two isolated dots that are wider than long toward the front. The sides of the abdomen usually have light-colored lines that look like a small hand with one finger extended upward holding a large, dark rectangular blotch. However, in the lighter versions, these lines and blotches are washed out and, in the darker versions, much detail is obliterated.

The brown widow’s belly has an hourglass whose most common form is orange or reddish in the middle with orange fringes. However, much confusion occurs because everywhere brown widows exist in the United States, there are black widows whose juvenile coloration involves abdominal stripes and an hourglass that deepens from white to yellow to orange to red as the spider matures. This is particularly difficult for people in the West because the western black widow starts out life as a tan and white spider; it is often misidentified as a brown widow. The eastern black widow spider has a black background, therefore, is less likely to be misidentified as a brown widow. However, people also misidentify orb weavers and false black widows as brown widows.

The male brown widow is very small, much smaller in proportion to the female than are male and female black widows. Most people probably wouldn’t even see the male brown widows, or if they did, they wouldn’t be concerned about them.
 

Fig. 2. Four brown widow abdomens showing the variation that they can display.

 

Nonetheless, there is hope. The most diagnostic way to identify a brown widow infestation is by the egg sac. Whereas black widow egg sacs have a tough, smooth surface (tan in the West and white in the East), the brown widow’s egg sac is covered with little silk spikes such that it looks like a big pollen ball or a World War II harbor mine. The only other spider making a similar egg sac is a green lynx spider, which shouldn’t be around homes. The egg sacs are usually conspicuous because the brown widow takes up residence in somewhat exposed sites under structures like patio chairs. They also can produce an egg sac every four days if well fed. So it is common to run into larger clusters of several to even dozens of egg sacs.


Attention Getters.
In many cases, a homeowner is often completely unaware of the presence of a pest such as termites or carpenter ants. However, the brown widow is a pest that has made its presence well known among southern homeowners to the point where they range from severe nuisances to major headaches. For the typical homeowner, the spiders are merely abundant in their backyards. However, for some business people, brown widows can infest merchandise in a major way. Brown widows are attracted to the underside of automobiles and it is not unheard of if someone sprays a car with pesticides that 50 brown widows drop from underneath. This makes selling both new and used cars a challenge as brown widows are typically not standard equipment on a motor vehicle.

A study was recently published in the Journal of Medical Entomology looking at microhabitat preference for brown widows. In southern California, collections were made for brown and black widows where the location and approximate height off the ground of the spider or its nest were recorded. These collections were done in different habitats such as urban homes (day and night), parks, zoos, horticultural centers and plant nurseries, around agricultural property, in tree crops (citrus, apples, avocados), and natural areas.

Brown widows were very common around homes. They were found under structures that provided some cover from above such as under inexpensive, plastic patio furniture, in air conditioning units or under barbecues. Whereas black widows like to hide during the day completely out of sight, brown widows made exposed retreats on the horizontal supports of wood fences or on stone walls if there was just two inches of cap material extending outward from the wall. They also made nests under wrought iron railings which provide little shelter. Also of concern, the spiders favored the curled lips of potted plants and recessed handle nooks of plastic bins, places where people stick their fingers and exert pressure to lift or move the object, increasing envenomation risk. About eight brown widows or nests were collected per hour which was 20 times higher than the number of black widows in the same locations. This corroborates anecdotal observations of many homeowners who almost uniformly mention that they used to have a few black widows in their yards, but now find dozens of brown widows instead. Unfortunately, to determine whether brown widows are actually pushing out black widows, one would have had to be clairvoyant and have performed this same study years ago in anticipation of a brown widow takeover.

Both species of widows were found most commonly within three feet of the ground. Brown widows sometimes made nests in the eaves of houses, 8 to 10 feet off the ground so it is possible to find them there.

Fig. 3. Three brown widow egg sacs.

One very useful piece of information for the PMP is that brown widows are not found in the living spaces of homes unless someone brings inside a potted plant or a children’s toy in which the spider was hiding. Also, although black widows are often found in garages (usually near areas of high insect traffic like by vents and doors), brown widows were almost never found in garages in southern California, except where the garage door was never closed.

Brown widows were also absent in natural areas in southern California in dry chaparral or around old decrepit agricultural outbuildings on the university campus; these areas were well infested with black widows. Brown widows were also very rare in agricultural tree crops. It was actually surprising to find them in crops but they would bind together two or three leaves of a plant or make a home inside a naturally curling leaf. They were found in citrus, apple and avocado trees but were so rare that this should never cause concern for anyone growing harvestable crops.


Toxicity.
Finally, everybody wants to know about their bites. Because the spider has the word “widow” in its name, the general public immediately assumes that brown widows are just as dangerous as black widows. Of black widow spider bites, 80 percent result in moderate or severe envenomations; bite victims experience severe pain that radiates into their back, stomach and legs, sweating of isolated limbs, rigid stomach muscles and may rock incessantly in their beds to try to stop the pain. In complete contrast, the most common symptoms of a brown widow bite are pain at the time of the bite and a burning sensation at the bite site. Although there are a few reports of more severe brown widow bites, the typical brown widow bite is mild. Paradoxically, if brown widows displace black widows, the threat of widow envenomation may become lower due to replacement by the less toxic species.


The Future. In a decade, the brown widow has spread over a very large portion of the southern United States. No one knows if its spread will continue or if it has reached its limits. Also, as with many non-natives, they may explode on to the scene, swamp an area with individuals, then later, their numbers settle down and they continue on a sustainable population level much lower than before. Georgia entomologists are already seeing a resurgence of black widows and decrease in brown widows. However, brown widows will probably persist at some noticeable level and be a nuisance spider species that the PMP needs to address.

 

The author is research associate at University of California, Riverside, and can be contacted at rvetter@gie.net.

 

References

Vetter, R. S. How to identify a brown widow spider. http://cisr.ucr.edu/identifying_brown_widow_spiders.html

Vetter, R. S., L. S. Vincent, D. W. R. Danielsen, K. I. Reinker, D. E. Clarke, A. A. Itnyre, J. N. Kabashima, and M. K. Rust. 2012. The prevalence of brown widow and black widow spiders (Araneae: Theridiidae) in urban southern California. Journal of Medical Entomology, 49: 947-951. (A PDF is available from the author upon request.)

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