The document discusses two species of widow spiders found in California - the western black widow and the brown widow spider. It provides details on identifying each species at different life stages through their coloration and markings. Both spiders are characterized by a rounded body shape and cobweb-like webs used to catch prey. While the native black widow poses the greatest threat to humans, the venom of the introduced brown widow is typically milder though it continues to spread its range. Managing spider encounters involves reducing clutter that provides them places to hide.

1. PEST NOTES Publication 74149
University of California
Statewide Integrated Pest Management Program
Agriculture and Natural Resources October 2009
There are two species of widow spiders in California, the western black widow and the brown widow. Both are in the genus Latrodectus and are characterized by a similar body shape, reclusive habit, and haphazardly constructed cobwebs.
The western black widow spider—a native species—is widespread and is the spider posing the greatest threat to humans in the Western United States. It is well known in many localities, and nonprofessionals can identify it easily.
In the first decade of the 21st century, the non-native brown widow became established in Southern California, and although it isn’t nearly as dangerous as the black widow, it causes alarm because of its potentially deadly relative.
BLACK WIDOW SPIDERS
Several species of black widow spiders are common in North America, but in the Western United States, the only species is the western black widow, Latrodectus hesperus. Its habitat ranges from British Columbia to Mexico and throughout the Rocky Mountains to the western portions of the Great Plains.
In California, it is a common desert spider that can survive very hot, dry conditions. However, black widows also can be found in mountainous terrains above the 5,000-foot elevation in Southern California where snow covers the ground every winter. Outside of California, they are common in urban Colorado and in Central and Eastern Washington state.
Because the holes, cracks, crevices, trash, and clutter associated with human structures attract the western black widow, these spiders are often very common around homes, barns, outbuildings, and rock walls. In supportive habitats, a mature female can be found every few feet.
Identification
The mature female western black widow spider (Fig. 1) is about 1/2-inch long, not including the legs, and has a rounded abdomen and very characteristic coloration. She is shiny jet black all over her body and legs except for a red pattern on the underside of the abdomen, which looks, in perfect specimens, like an hourglass. Some specimens have a brownish or plum-colored tinge, but usually these are females that are so well fed the black pigment on the abdomen has expanded until it looks brown instead of black.
The red hourglass can vary from two perfect triangles whose points merge to make a perfect hourglass to two triangles separated by a space, a triangle and small bar, or just minimal almost imperceptible red coloration. The false black widow, which is discussed below, is chocolate brown and never has red coloration, although many people frequently mistake it for a black widow.
As easy as it is to identify an adult female black widow, the immatures look nothing like the mother (Fig. 2). When baby black widow spiderlings emerge from their egg sac (Fig. 3), they have tan legs and tan cephalothorax, the body part to which the legs attach, while the abdomen is mostly white with a few black spots.
As the spider grows, the background coloration of the abdomen becomes olive gray, and there is a longitudinal white stripe on the top of the abdomen and three diagonal stripes on the flanks with a small black dot at the uppermost portion of each diagonal stripe.
Like all spiders, as the spiderlings grow larger, they molt in order to shed their restrictive exoskeleton. With successive molts in females, the white stripes become thinner, the olive gray darkens toward black, and eventually the spider acquires its well-known black coloration. Some mature females retain one or two conspicuous, indented white lines on the front surface of the abdomen that look like a corporal’s chevrons.
In the youngest spiders, the space where the hourglass develops starts off being a whitish shield. As the spider
Integrated Pest Management for Home Gardeners and Landscape Professionals
Black Widow and Other Widow Spiders
Figure 1. Mature female western black widow spider.
Figure 3. Western black widow spiderlings and egg sac.
Figure 2. Immature female western black widow spider.

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grows and goes through several molts,
the color of this shield turns from white
to yellow to orange to red and changes
from a shield with thick middle to a
hourglass with a thin, tapered middle.
In contrast to the female, the male black
widow (Fig. 4) retains the coloration
of the juvenile. After it matures, it
stops eating, and its abdomen shrinks,
because its only task at that point is
to mate. The male still retains its one
longitudinal stripe and set of three
diagonal flank stripes on each side the
abdomen. The males are much smaller
than the females in body length al-though
sometimes their legs are almost
as long as the adult female.
One more variation involves the longi-tudinal
stripe that runs up the middle of
the top surface of the abdomen in im-mature
black widows. Sometimes it has
a vivid red stripe within the confines
of the white border. This coloration can
cause anxiety for anyone who isn’t fa-miliar
with widow immatures, because
they might incorrectly identify it as the
Australian redback widow spider. This
widow has red markings on the top but
otherwise is uniformly black rather than
mottled as an adult. Redback widow
spiders aren’t found in North America.
Egg Sac
The egg sac of the western black widow
is a yellowish, tear-drop shape, tapered
at the top and bulbous at the bottom.
The margins of the sac are well de-fined,
as opposed to some spider egg
sacs, which look like fluffy cotton balls,
making it difficult to determine exactly
where the egg sac starts. The egg sac is
very tough and difficult to rip apart.
A female western black widow typical-ly
lays about 300 eggs per sac. Because
they can store sperm from their first
mating, they can produce more than 10
egg sacs without subsequent matings
without a decrease in the number of
eggs or a reduction in the percentage of
eggs that will hatch into spiderlings.
Habitat
The western black widow is found al-most
everywhere in California where
people live. Although they can be found
in homes, black widows typically are
outside, around the home and in clut-ter.
In garages, they usually make webs
by doors, near vents, and in other places
where they are guaranteed lots of insects
for food. Because most people don’t toler-ate
large numbers of insects in their liv-ing
spaces, widow spiders usually won’t
find sufficient prey to survive in homes.
Black widows are shy spiders that seek
retreats such as a hole between two
bricks or a pipe hole in a wall where they
can hide during the day and then come
out at night. In natural settings, you’ll of-ten
find them in rodent burrows and rock
faces. The spider makes a web of tangled
silk extending from this retreat hole.
The web doesn’t have a very recogniz-able
pattern although it does have verti-cal
support threads above and below the
central areas where the spider sits while
it waits for prey at night. The lower sup-port
threads also alert the widow to the
presence of a prey item blundering into
the web. In most cases the widow spider
will seek a retreat near the ground as
the home base for her web, which con-nects
to the retreat, allowing the spider
to emerge to catch both flying and
crawling prey at night. However, some
spiders will make a retreat well above
ground level, such as in the eave of a
house, and then drop down 10 or more
feet before building their web.
Widow spiders come out at dusk. After
making improvements to their exist-ing
web, they take up a position in the
middle, their underside facing upward,
to wait for prey. Any large disturbance
of the web that indicates something
larger than a prey item causes the spi-der
to quickly move toward the safety
of its retreat.
The silk of a mature black widow is
very strong; running a finger through
the web that a large spider has made re-sults
in an audible ripping sound. Dur-ing
World War II, black widow silk was
used to make crosshairs for gun sights.
Medical Aspects
The black widow bite itself is painless
or may feel like a little pinprick. Almost
all medically important black widow
bites are from the adult female, which is
much larger than the male; the female
also has stronger biting muscles and a
larger venom reserve. At the site of the
bite, you might see a little red mark or
red streaking away from the bite. With-in
an hour, symptoms start to appear.
Bite victims might suffer from some but
not all of the following symptoms: rigid
stomach muscles, which some medical
professionals have misdiagnosed as ap-pendicitis;
sweating, sometimes of just
the bitten body part, such as a bite to the
hand that results in only the arm sweat-ing
profusely; pain that can be local,
radiating, or regional; urine retention;
and—less commonly—numbness, agita-tion,
fever, and patchy paralysis. An-other
symptom is bite victims will move
or rock back and forth incessantly to try
to lessen the pain from the venom injec-tion
process. However, these symptoms
are the most severe manifestation; many
black widow bite symptoms merely
resemble the flu. Black widow bites don’t
cause conspicuous swelling, necrosis, or
deterioration of tissue around the bite.
As a neurotoxin, the venom of a black
widow affects the nerve-muscle junction
in the body. Normally the body’s neurons
work like a light switch; they make the
muscle, or “light,” turn on and then off
again, so that the muscle can relax and
be ready to contract again if needed. The
venom causes the muscle to repeatedly
contract. It would be like flipping on a
light switch and not being able to turn it
off again.
Figure 4. Mature male western black
widow spider.

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If bitten, seek medical attention im-mediately.
You can place a cold pack
on the bite to relieve the pain. An
antivenom for black widow bites is
available that works for all species that
have been tested, worldwide. Response
is fast, and bite victims can go from
intense pain back to normal in 30 min-utes.
The antivenom is based on horse
serum, so physicians need to monitor
for anaphylactic shock. American phy-sicians
are somewhat reluctant to use
antivenom for this reason and might
prefer to have the bite victim simply
endure the symptoms, which can be
similar to a bad flu episode and that
usually dissipate in a few days.
BROWN WIDOW SPIDERS
The brown widow spider, Latrodec-tus
geometricus, is found worldwide
in subtropical habitats. It probably
originated from Africa although there
is some conjecture that it also could
have been native to South America. In
North America, for many decades, it
was found only in Florida, where it was
rather common. However, in the first
decade of the 21st century, the spider
began appearing from Texas through-out
the Gulf Coast states and up the
Atlantic Coast into South Carolina.
While it was expanding in the South-eastern
United States, it was being col-lected
in great numbers in Southern
California. Initial news reports exag-gerated
the impact of the brown widow.
However, unlike the black widow, this
spider isn’t much more toxic to humans
than the typical spider.
Identification
The brown widow spider is a mottled
collection of tan, brown, and gray (Fig.
5). It has the longitudinal stripe on
the abdomen and the three diagonal
stripes on the side similar to immature
western black widow spiders. However,
the coloration is more of a tan whereas
the western black widow is more olive
gray.
Also in the brown widow, the black
marks at the top end of the 3 diago-nal
abdominal stripes are large and
squarish whereas in the western black
widow, the black mark is more round
and much smaller. Additionally, the
hourglass of the brown widow spider is
more of an orangish color or orange in
the middle with a yellowish border.
In many ways, the mature brown
widow female looks very similar to an
immature western black widow, so you
need to have some skill to identify the
two accurately. However, the brown
widow egg sac, as described in the next
paragraph, is a sure way to confirm
which species you’ve found.
Egg Sac
The egg sac (Fig. 6) of the brown widow
is so characteristic, the spiders them-selves
need not be seen in order to
verify their presence in the home. The
sac has protuberances of silk all over
its surface, and some say it looks like a
large pollen grain.
Brown widows produce about 80 eggs
per egg sac and are able to make 20 or
more egg sacs in a lifetime, several in a
short time period. It is not uncommon
to collect a female brown widow with
several simultaneously developing egg
sacs.
Habitat
The habitat of the brown widow is
similar in many respects to that of the
black widow. They generally reside in
cluttered areas outside such as wood-piles
or in cluttered areas of garages,
but you’ll also find them in more
exposed areas such as on chain-link
fences where black widows normally
wouldn’t be found. Their webs also are
cobwebby.
The brown widow appears to be dis-placing
the black widow in many of its
habitats. Brown widows are common
in Los Angeles, Orange, and San Diego
counties. Because this spider has been
found in Southern California only
since about 2002, it probably still is
expanding its range and might eventu-ally
spread up the coast toward Santa
Barbara or into the Central Valley.
Medical Aspects
The bite of the brown widow is much
milder than the western black widow.
In one study in Africa, where the
brown widow might have originated,
the most common symptoms in 15 veri-fied
bites were that the bite hurt when
it happened, and it left a red mark;
none of these patients developed the
typical dynamic symptoms of black
widow envenomation. However, there
is one American record of a verified
bite where the patient developed more
severe symptoms and required hospi-talization.
Although its venom is as potent as
black widow venom, drop for drop, the
brown widow probably doesn’t inject
very much venom during a bite, mak-ing
it much less dangerous than the
native black widow spider.
The recent arrival of the brown widow
in Southern California is not cause for
alarm. In fact, if the brown widow is
indeed displacing the more toxic black
widow, there actually might be a reduc-tion
in spider bite risk compared to
previous decades.
Figure 5. Mature female brown widow
spider.
Figure 6. Brown widow spider egg sacs.

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FALSE BLACK WIDOW
The false black widow, Steatoda grossa,
(Fig. 7) is not a true widow spider;
however, it is in the same family, Theri-diidae,
as the widow spiders and easily
can be confused with them. It shares
the same rounded-abdomen body form
and web-making traits. It is slightly
smaller than a mature western black
widow spider, is chocolate brown, and
never has red coloration on its belly.
The false black widow is a European
immigrant that has become extremely
common in Pacific Coast homes from
San Diego to British Columbia. It
doesn’t seem to need as much food as
the black widow, so the spiders are
more likely found inside cupboards and
underneath refrigerators or cabinets.
False black widows make an egg sac
(Fig. 8) that looks like a cotton ball with
indistinct margins. Unlike black widow
spiders, baby false black widow spider-lings
are dark like their mothers when
they emerge.
Because they are more common in
homes, they frequently are involved in
bite incidents. They have mild venom
and cause symptoms similar to a mild
black widow spider bite. When black
widow antivenom mistakenly has been
used on false black widow bites, it ap-peared
to eliminate the venom effects.
MANAGEMENT
One of the easiest ways to minimize
encounters with widow spiders is to
reduce clutter around your home, which
deprives them of places to make retreats.
However, it is impractical to eliminate
them completely by removing all clutter.
Regularly vacuuming or sweeping
windows, corners of rooms, storage
areas, basements, and other seldom-used
areas helps remove spiders and
their webs. Vacuuming spiders can be
an effective control technique, because
their soft bodies usually don’t survive
this process.
In the garage, keep items such as
gardening clothes and gloves in bags
closed with zipper locks or twist ties.
Store seasonal items such as winter
clothes or Christmas decorations in
boxes that you can tape shut and can
place off the floor and away from walls
in order to exclude spiders. When
cleaning up clutter in garages and oth-er
storage areas, be sure to wear gloves
to avoid accidental bites.
Areas of concern include children’s
pedal-powered toy vehicles made of
molded plastic that have open spaces
facing downward where spiders can
crawl in. Picnic tables and other large
pieces of furniture where you place
your fingers underneath to lift also can
be a source of exposure.
Spiders can enter houses and other
structures through cracks and other
openings. To prevent spiders from com-ing
indoors, seal cracks in the founda-tion
and other parts of the building
and gaps around windows and doors.
Good screening not only will keep out
many spiders but also will discourage
them by keeping out the insects they
eat. However, baby black widows have
no problem crawling through regular
window screen mesh.
Be careful that you don’t carry spiders
indoors on items such as plants, fire-wood,
and boxes. Stack woodpiles
away from your house, and never pick
up pieces of wood unless you are wear-ing
gloves.
Eliminate places for spiders to hide and
build their webs by keeping the area
next to the foundation free of trash, leaf
litter, and accumulations of other ma-terials.
Removing ivy and other heavy
vegetation growing around founda-tions
and trimming plant growth away
from your home and other structures
will discourage spiders, in general,
from taking up residence near the
structure and then moving indoors.
Outdoor lighting attracts insects,
which in turn attracts spiders. If possi-ble,
keep lighting fixtures off structures
and away from windows and door-ways.
Sweep, mop, hose, or vacuum
webs and spiders off buildings regular-ly.
Insecticides won’t provide long-term
control, so generally you shouldn’t use
them against spiders outdoors.
Because widow spiders are nocturnal, a
nonchemical method of eradication is
to search for them at night with a flash-light
and kill them with a shoe or rolled
up newspaper. If you are concerned
about wildlife and feel comfortable
doing so, you can remove individual
spiders from indoor areas by placing
a jar over them and slipping a piece of
paper underneath to seal off the open-ing
when you life the jar up. Release
the spider about 100 feet from your
home into a natural area.
One aspect that makes controlling
widow spiders difficult is that they, like
many spiders, exhibit a behavior called
ballooning. When the spiderlings are
very small, on warm days when there
is an updraft they climb to the top of a
fence post or piece of vegetation, raise
their abdomens into the air, and release
a small filament of silk.
When the updraft currents overtake
the forces of gravity, the spiderling is
carried into the air to another location.
This may only be a few feet away, or it
could be miles. Ballooning spiderlings
Figure 7. Mature false black widow
female.
Figure 8. False black widow egg sac
(left) and black widow egg sac (right).

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have been captured at 10,000 feet from
the ground and 200 miles offshore.
Because spiderlings will be dropping
down on your property continually,
eliminating them will be a task that
needs to be done repetitively through-out
the year.
Chemical Control
Typically pesticide control of spiders
is difficult. Various insecticides are
registered for control of spiders includ-ing
pyrethrins, resmethrin, allethrin,
or combinations of these products;
however, they usually aren’t very ef-fective.
Sprays work only if you apply
them directly to the spider or their
web, since the spray residual does not
have a long-lasting effect. This means a
spider can walk over a sprayed surface
a few days—and in many cases, a few
hours—after treatment and not be af-fected.
Sprays won’t affect egg sacs, and
if you apply them to the outside pe-rimeter
of a structure, they won’t keep
spiders from moving in.
Control by spraying is only temporary
unless accompanied by housekeeping.
It is just as easy and much less toxic to
crush the spider with a rolled up news-paper
or your shoe or to vacuum it up.
Removing harborage sites such as clut-ter,
woodpiles, or heavy ground cover
is essential for reducing widow spider
populations. Sticky traps offer a nonin-secticidal
way to remove spiders from
your home as long as you can place the
traps where pets and curious children
can’t tamper with them.
REFERENCES
Clark, R. F., S. Wethern-Kestner, M. V.
Vance, and R. Gerkin. 1992. Clinical
presentation and treatment of black
widow spider envenomation: a review
of 163 cases. Annals Emergency Medicine.
21:782-787.
Goddard, J., S. Upshaw, D. Held, and K.
Johnnson. 2008. Severe reaction from
envenomation by the brown widow
spider, Latrodectus geometricus (Araneae:
Theridiidae). Southern Medical Journal.
101:1269-1270.
Kaston, B. J. 1970. Comparative biol-
AUTHOR: R. S. Vetter, Entomology, UC
Riverside.
TECHNICAL EDITOR: M. L. Flint
EDITOR: M. L. Fayard
ILLUSTRATIONS: Figs. 1 and 4-8, R. S.
Vetter; and Figs. 2-3, J. K. Clark.
This and other Pest Notes are available at
www.ipm.ucdavis.edu.
For more information, contact the University of
California Cooperative Extension office in your
county. See your telephone directory for addresses
and phone numbers, or visit http://ucanr.org/ce.cfm.
University of California
Agriculture and Natural Resources Program
Produced by UC Statewide
Integrated Pest Management Program
University of California, Davis, CA 95616
University of California scientists and other
qualified professionals have anonymously peer
reviewed this publication for technical accuracy. The
ANR Associate Editor for Urban Pest Management
managed this review process.
To simplify information, trade names of products
have been used. No endorsement of named products
is intended, nor is criticism implied of similar products
that are not mentioned.
This material is partially based upon work
supported by the Extension Service, U.S. Department
of Agriculture, under special project Section 3(d),
Integrated Pest Management.
WARNING ON THE USE OF CHEMICALS
Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations
given on the container label. Store all chemicals in the original, labeled containers in a locked cabinet or shed,
away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock.
Pesticides applied in your home and landscape can move and contaminate creeks, rivers, and oceans.
Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens
containing fruits or vegetables ready to be picked.
Do not place containers containing pesticide in the trash or pour pesticides down the sink or toilet. Either use
the pesticide according to the label, or take unwanted pesticides to a Household Hazardous Waste Collection
site. Contact your county agricultural commissioner for additional information on safe container disposal and
for the location of the Household Hazardous Waste Collection site nearest you. Dispose of empty containers
by following label directions. Never reuse or burn the containers or dispose of them in such a manner that
they may contaminate water supplies or natural waterways.
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6th Floor, Oakland, CA 94607, (510) 987-0096.
ogy of American black widow spiders.
Transactions San Diego Society Natural
History. 16:33-82.
McCrone, J. D. 1964. Comparative le-thality
of several Latrodectus venoms.
Toxicon. 69:201-203.
Muller, G. J. 1993. Black and brown
widow spider bites in South Africa: A
series of 45 cases. South African Medical
Journal. 83:399-405.
v
Vetter, R. S. and G. K. Isbister. 2008.
Medical aspects of spider bites. Annual
Review of Entomology. 53:409-429.
Vincent, L. S., R. S. Vetter, W. R. Wrenn,
J. K. Kempf, and J. E. Berrian. 2008. The
brown widow spider, Latrodectus geomet-ricus
C. L. Koch, 1841, in Southern Cali-fornia.
Pan-Pac. Entomol. 84:344-349.