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.

 

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.

 

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.