Black Widow Spider
Species in the genus Latrodectus are commonly known as widow spiders. A
number of different species of widow spiders occur in the United States.
The black widow spider has a potent neurotoxic venom and is considered the
most venomous spider in North America. However, the female injects such a
small dose of venom that it rarely causes death. Reports indicate human
mortality at well less than 1% from black widow spider bites.
Identification
The adult female black widow spider has a shiny, jet black, spherical
abdomen with two connected red triangles on the underside that form a
characteristic hourglass marking. Note, however, that the hourglass color may
range from yellowish to various shades of orange or red. Adult females are
about 1/2-inch long, not including the legs (about 1-1/2 inches when legs are
spread). Adult males are harmless, about half the female’s size, with smaller
bodies and longer legs. The male’s abdomen usually has red spots along the
upper midline and white lines or bars radiating out to the sides. Newly hatched
spiderlings are predominately white or yellowish-white, gradually acquiring
more black and varying amounts of red and white with each molt. Juveniles of
both sexes resemble the male and are harmless.
Adult female northern black widow spiders are shiny black or brown-black
with a row of red spots on the top of the abdomen along the midline. Two
reddish triangles resembling a split hourglass are present on the underside of
the abdomen.
Life Cycle and Habits
Mating takes place in spring or summer. A common misconception is that the
female usually consumes the male after mating. Such behavior rarely occurs.
The female lays eggs in a silken sac that is globular shaped and about 1/3 to
1/2-inch diameter. Sacs are white at first, later turning tan or gray. Each sac
contains 25 to 250 eggs, and several egg sacs may be produced over the
course of a summer. The egg sacs are suspended in the web and guarded by
the female. The egg incubation period usually lasts about 4 weeks. The
spiderlings hatch and molt (shed their skin) one time while inside the egg sac.
They then disperse by ballooning—extruding silk threads and being
transported by air currents. Their growth to maturity requires 2 to 4 months
depending on the availability of prey. Spiderlings molt several times before
reaching maturity.
The black widow spider is a cobweb builder whose silk is very strong. The
female constructs a web of crisscrossed silk threads with no recognizable
pattern and with a dense area of silk, usually to one side, that serves as the
spider’s daytime retreat. At night, the female hangs belly upward in the center
of the web. She does not leave her hidden web voluntarily. The web typically
is situated near the ground in a dark, sheltered site. Webs often are one foot in
diameter.
The web serves to trap the spider’s food, which includes a variety of insects
(cockroaches and beetles) and other arthropods. Outdoors, black widow spider
webs are usually built in woodpiles, rubble piles, under stones, in hollow
stumps, and in rodent burrows. These spiders commonly occur in outbuildings
such as privies, sheds, and garages. Indoors, they prefer undisturbed, cluttered
areas in basements and crawl spaces.
The northern black widow spider is similar to the black widow except its
habitat is marginal land with sparse vegetation. It is found in stumps, hollow
logs, and piles of debris, and only rarely indoors.
Bite Symptoms
The severity of an individual’s reaction to the black widow spider bite depends
on the area of the body bitten, amount of venom injected, and their sensitivity
to the venom. The venom travels in the bloodstream throughout the body and
acts on the nervous system, causing varying degrees of pain. Some people
report very intense pain. There typically is no necrosis (sloughing) of tissues
and no conspicuous swelling.
The bite of a black widow spider initially may go unnoticed, but some people
report a short stabbing pain. At first, there may be slight local swelling and two
faint red spots, which are puncture points from the fangs. Pain soon begins
and usually progresses from the bite site to finally localize in the abdomen and
back. Severe cramping or rigidity may occur in the abdominal muscles. Other
symptoms may include nausea, profuse perspiration, tremors, labored
breathing, restlessness, increased blood pressure, and fever. Symptoms often
diminish after a day or so and cease after several days. Serious long-term
complications or death are very rare.
First Aid
If bitten, remain calm, and immediately seek medical attention (contact your
physician, hospital and/or poison control center). Apply an ice pack directly to
the bite area to relieve swelling and pain. Collect the spider (even a mangled
specimen has diagnostic value), if possible, for positive identification by a
spider expert. A plastic bag, small jar, or pill vial is useful and no preservative
is necessary, but rubbing alcohol helps to preserve the spider.
A hospital stay may be recommended, particularly for those with a heart
condition or with health problems. A physician may administer a specific
antivenin to counteract the venom or calcium gluconate to relieve pain.
Control
Control efforts should target black widow spider webs because that is where
the spider spends most of its time. Control is best achieved by following an
integrated pest management (IPM) strategy, which involves using multiple
approaches such as preventive measures, exclusion, sanitation, and chemical
treatment when necessary. IPM requires a thorough inspection of the building
to locate the pest. An inspection preferably should be done at night because
the black widow spider is nocturnal.
Preventing Spider Bites
In order to prevent spider bites, be sure to wear gloves and a long-sleeved
shirt when handling stored cardboard boxes, firewood, lumber, and rocks. Be
sure to inspect these clothing items for spiders before putting them on. Shake
out clothing and shoes before getting dressed.
Exclusion
Install tight-fitting screens on doors and windows to prevent entry of black
widow spiders. Also install door sweeps. Seal or caulk cracks and crevices
where spiders can enter the house. Install yellow or sodium vapor light bulbs
outdoors since these attract fewer insects for spiders to feed upon.
Sanitation
In order to reduce black widow spider populations, it is very important to
eliminate their potential hiding places indoors and outdoors. Black widow
spiders are often found in undisturbed, cluttered areas indoors, so discard old
boxes, old clothing, lumber, and other unwanted items in basements, crawl
spaces, garages, and outbuildings. In such areas, store any items off the floor
and away from walls. Remove piles of lumber and rubble outdoors. Remove
ivy and other heavy vegetation from the foundation. Do not store firewood
against the house. Note that these measures also reduce harborages for the
spiders�?prey.
Vacuum thoroughly indoors to remove black widow spiders and their webs
and egg sacs. After vacuuming, immediately place the vacuum cleaner bag in
a plastic bag, seal tightly, and discard in a container outdoors—this prevents
captured spiders from escaping into the home. Wash off the outside of the
house using a high-pressure hose, paying particular attention to window wells
and other undisturbed areas where webs are built.
Insecticides
There are many labeled pesticides for spider control. Some are labeled for
homeowner use, while others are labeled only for the licensed, certified
pesticide applicator.
Insecticide treatments should be applied so that the chemical contacts spiders
in their webs. A non-repellent insecticidal dust is useful to treat webs because
the dust clings to the silk and is likely to be contacted by the spider. Individual
exposed spiders can be killed with a non-residual aerosol spray, but any egg
sacs will be unaffected.
A wettable powder or microencapsulated formulation of a residual insecticide
can be applied to corners, behind and under furniture, behind stored items,
etc. to prevent establishment of new spiders.
Residual liquid sprays applied to the outside perimeter of the home are not
very effective because of the black widow spider’s web-sitting behavior.
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