Although brown widow spiders have a rather docile reputation, they are the most venomous of the five different types of widow spiders in the U.S.; that is, when one looks at LD50 values, values that indicate what dose of venom is sufficient to kill half of all mice in a small sample.
In a study by McCrone (1964)1, LD50 values of several widow spider species was compared by testing the venom from different species on mice held in captivity. The study showed several things worth remembering. For instance, the amount of venom, in milligrams, that could be recovered from one of each widow spider varied with a factor of almost three, with the brown widow spider as the spider with the least venom.
The table below shows how much venom could be recovered from each type of spider. That's the venom column, and it states how many mgs could be recovered from one spider. The LD50 column is how much venom was necessary to kill 50 % of the mice in a sample, and the lethality factor is the venom retrievable from the spider divided by the LD50. This number signifies how serious an average bite is when venom is injected.
Download full table here. (a) is mactans, and (b) is tredecimguttatus.
The two common black widows in North America are the mactans and the Northern black widow, which can be found from Florida to Canada. The red widow is found in Florida only, whereas the Latrodectus mactans tredecimguttatus is not even found in North America (it is found in Southern Europe, the Middle East, and Asia). In the study by McCrone (1964), all species except the tredecimguttatus was found in Florida.
When one looks at the actual danger imposed by brown widow spiders, the brown widow seems more dangerous than all the black widow spiders—at least among the North American species.
However, in practice, the brown widow is considered less dangerous; the main reason being that the brown widow spider is sometimes too small to be able to penetrate human skin and deliver any venom at all. Secondly, although a weakened person might feel a lot of pain from a brown widow spider bite, its venom, even in a full dose, is not capable of severely damaging a healthy person.
In fact, there are only a few documented cases of effects of brown widow spider bites in the U.S. Only one of them should have been serious (Brown, et al., 2008)2, while the two others were only minor. In each of the two cases with minor reaction, the effect of the bite was located in the area around the bite site.
In all three cases, the bite occurred after the spider had been accidentally pressed against the skin of the person bit.
When the widow spider was first described, it was located in South America and South Africa. Since then (1841), it has become a cosmopolitan species, and recently it has spread to islands such as Japan, Australia, New Guinea, and Indonesia. As the brown widow likes residential structures, it often gets into close contact with humans. Humans fear the brown widow much more than is actually necessary.
Brown widow spiders build webs close to the ground, where the insects it eat are mostly caught. The webs are rather irregular and build of indelicate silk. Besides insects they eat other spiders and everything else smaller than itself, but mostly insects.
According to McCrone & Stone (1965)3, the only place in the U.S. where the spider could be found in 1965 was in Florida. During the late seventies the brown widow had spread further north, and in 2001 it was found as far north as South Carolina.
Today (late 2012) it can be found in Georgia, South Carolina, Florida, Louisiana, Hawaii, Texas, California, and Mississippi. In Louisiana, the majority of specimens found are in New Orleans. As New Orleans has undergone quite a bit of rebuilding since Katrina, a lot of building materials have been transported to New Orleans, which explains why so many brown widow spiders are suddenly found there.
Some of the places brown widow spiders prefer to stay, and from where they can be collected, are greenhouses, trash cans, car garages, and storage facilities.
1. McCrone, JD. Comparative Lethality of Several Latrodectus Venoms Toxicon Vol. 2, pp. 201-203 (1964).
2. Brown et al. (2008) The brown widow spider Latrodectus geometricus C. L. Koch, 1841, in southern California The Pan-pacific entomologist 85(4) pp. 344-349 (2008)
3. McCrone, JD. & Stone, KJ. The widow spiders of Florida. Arthropods of Florida and neighboring land areas. Florida State Collection of Arthropods. Gainesville, FL (1965)