Spiders
Up to several thousand species of spiders are thought to exist in the United States; obviously many of these species are found in the Pacific Northwest. Thankfully, many of these spiders stay out of doors, doing their part in the ecosystem as predators of various insect species that, without spiders, could raise serious risks themselves to nature and to human health. It is only when spiders, or the webs they create, attach themselves to our homes, business structures, and personal property that they become a concern.
In general, control of spiders involves a combination of inspection,
identification, and prevention and sanitation methods.
Knowing the spiders that are problematic in and around a
structure can assist a homeowner to know how to control their presence.
Spider populations indoors can been kept to a minimum through
various methods of weather proofing and screening, reducing the amount
of vegetation around a structure, modifying outdoor lighting to reduce
attracting the insects that spiders want to eat, keeping firewood away
from structures, can all help in reducing the risk of spider
infestation. In worst case
scenarios, especially where properties are located in or near wooded
areas ideal for spider populations, regular professional treatment
services may also be a necessary option to pursue.
In most cases, general spider populations build during the year as
flying pests such as flies, mosquitoes, and other insect populations
grow. Most spider
populations in the Northwest peak around September 1 and gradually
decrease thereafter until local fall frost conditions set in.
While the vast majority of spider species in our area are not a threat
to human health, questions do come up regarding some of the more
notorious species. The
following provide a brief synopsis of some of these species:
Well known for the distinctive black color with the orange hour-glass
shaped mark on its abdomon, the female black widow spider (Latrodectus
spp.) can inject a neurotoxin with its bite that requires medical
attention. Black widow
spiders tend to shun light, preferring living in cracks, crevices, and
other sheltered locations, and are most active at night.
Most frequently they can be found in places such as firewood
piles and other outdoor areas of storage or refuse, in cracks in brick
walls, or in crawl spaces.
They build a web that has no regular pattern to it and generally will
hang upside down in the web waiting for prey.
Black widows can be found with some frequency in western Oregon from the Umpqua and Rogue Valleys southward, mainly in urban areas. Apparently due to the damper, cooler climates, their native range diminishes quickly as one travels north along the I-5 corridor so that black widows are extremely rare from the mid-Willamette Valley (Salem area) northward. In some areas of eastern Washington and Oregon, where the climate is drier, black widows tend to be reported with a greater frequency in spot locations.
The males of the black widow spiders (which are smaller, much lighter in
color) do not produce venom upon maturity and are not a threat to
humans.
There is another common spider species, the brown cellar spider, which
is shaped much like a black widow, which frequents crawl spaces under
houses throughout western Oregon and Washington.
This spider is dark brown in color with no markings on it.
Unlike the black widow, the brown cellar spider is not poisonous
to humans and is thus not considered a health risk.
Of
all the biting spiders found in urban areas, the yellow sac spider (Chriacanthium
spp.) may be the cause of most human spider bites in western
sections of the Pacific Northwest.
It is extremely prolific and, in structures, can be found in
attics, crawl spaces, and building interiors along ceilings and ceiling
corners.
Yellow sac spiders, generally about ¼ inch in size and yellow to light
green in color, build flattened silk tubes for shelter in ceiling
corners. Their venom is a
cylotoxin that can result in symptoms from localized redness, swelling,
and pus developing in the wound.
In rare cases symptomatic reactions such as cramping or nausea
may occur that would require medical attention.
Ulceration of the wound may result, taking several weeks to
completely heal.
The hobo spider (Tegenaria agrestis) may be guilty of most of the
spider bites that are misdiagnosed as brown recluse spider bites in the
Pacific Northwest. Hobo
spiders produce venom that, when injected, can result in a necrotic
lesion developing that can destroy skin tissue and take several months
to heal. Because the
reaction between a hobo spider bite and a brown recluse spider bite are
so similar, it is generally not possible without having the spider in
hand to determine the cause of the bite and resultant tissue damage
between the two arachnids.
Many other causes of necrotic lesions also exist (tick bites, skin
cancers, Lyme disease, etc.) which may have nothing to do with the hobo
spider.
It is important to know that, in spite of some isolated medical and
media claims made over the years, the brown recluse spider (Loxosceles
spp.) is not established in the Pacific Northwest and thus cannot
rightly be blamed for such kinds of wounds in this part of the country.
The range of the brown recluse spider is limited to the southern
Plains states and lower Mississippi valley.
The last known confirmed sightings of a brown recluse spider in
the Pacific Northwest were in Prosser, Washington in 1978 and in Central
Point, Oregon in 2000; in both cases the spiders came in with personal
items shipped from Kansas where the brown recluse is extremely
plentiful.
Hobo spiders, along with other scientifically related species such as
the domestic house spider (Tegenaria domestica) and giant house
spider (Tegenaria gigantea), build funnel webs at ground level to
catch their prey. They are
extremely difficult to positively identify since they look—to the
untrained eye—so similar to the other Tegenaria species.
It is prudent to consult academic web sites or a state
agriculture entomological specialist for a positive identification if
one suspects that a hobo spider has been found.
Additional information on the hobo spider can be found by clicking here.