Michigan
State University Extension
Home Maintenance And Repair - 01500561
06/24/03
Spiders: Biology and Control
Spiders are familiar to almost everyone. They are found virtually
everywhere - from mountaintops to caves. They are commonly associated
with water, soil, leaf litter, and low, dense plant growth. Only a few
of the over 2500 U.S. species (over 500 species native to Michigan) may
be found in human habitations.
Spiders, like insects, belong to a larger group of animals known as
Arthropods. Spiders differ from insects in several ways; as a group
spiders are recognized by their two-segmented bodies (cephalothorax and
abdomen), eight legs (insects always have six), and 4-8 simple eyes. In
addition, spiders always lack wings and antennae. Hence, spiders are
more closely related to ticks, mites, daddy-longlegs, and scorpions
than to insects.
One interesting attribute of spiders is their production and use of
silk. It is produced by a set of special glands located near the tip of
the abdomen; it is secreted as a liquid which hardens when exposed to
the air. Spiders use silk to capture prey (webs and traps), build
shelters, wrap egg sacks, and for locomotion (draglines and
parachutes).
Spiders hatch from eggs which are bundled together in sacs. The female
may carry the sack with her or tuck it away in a secluded spot. The
young spiders (called spiderlings) are miniature replicas of adult
spiders, and they grow through a series of molts. Most spiders live for
1-2 years. Since spiders are cold-blooded invertebrates, their activity
is greatly reduced or even curtailed by cold temperatures.
Spiders are beneficial to man. They are predaceous and feed on a
variety of live prey including insects, centipedes and even other
spiders. In fact, in many habitats spiders are the dominant group of
predators and frequently outnumber all other predators combined. Some
spiders use special webs or snares to capture their prey, while others
actively stalk or ambush their prey.
Spiders lack jaws for chewing. Instead, they have fang-like pinchers
and special poison glands. This poison is used to subdue the prey by
injecting it through the hollow fangs. Then, a unique enzymatic saliva
is used to predigest the muscle and internal viscera of the prey so
that it can be sucked up by the spider.
The danger of spider bites is greatly exaggerated. Most spiders are NOT
dangerous to man under normal conditions, and only a few species are of
public health significance. Spiders are frequently misunderstood; they
are non-aggressive and most will not bite unless provoked or
threatened, and even then only the larger species are capable of
piercing human skin with their fangs. In addition, the vast majority of
the spiders have secretive habits and are seldom encountered by man.
Representatives of five families of spiders are commonly encountered in
and around homes. They are briefly described below:
Cobweb spiders -- are one of
the most common groups of indoor spiders. They are small (less than
1/2"), and pale yellow, tan or gray without any distinct markings. They
build irregular webs in corners and around windows and curtains. The
webs remain inconspicuous until they are abandoned and become dust
covered.
The yellow house spider --
usually occurs outdoors among the leaves of shrubs and other small
plants, but they do build their webs inside houses from time to time.
They have been reported to bite with painful consequences and some
necrosis.
Wolf spiders -- are large,
hairy, active spiders that normally occur outdoors. They do not build
webs; instead, they wander about in search of their prey. As a result
they may find their way into houses, especially basements. Can and will
bite if molested.
Jumping spiders -- are
medium-sized, black, hairy spider (often with spots of orange or red on
the abdomen). When found indoors they are usually around windows and
doors and they move about with characteristic short, sudden jumps.
Orb weavers -- are very large,
conspicuous spiders that construct large orb-like webs for snaring
their prey. These webs are usually constructed in open areas near
gardens and houses where flying insects will blunder into the webs.
Despite their large size, these spiders are not dangerous.
Controlling Spiders
The first consideration in spider control is to determine whether or
not the spiders are living indoors. If large numbers of spiders are
seen indoors they could be more than a nuisance problem. Most of the
"domestic" spiders are small in size (usually 1/4"), uniformly colored
(pale yellow, tan or gray), and not hairy in appearance. Those spiders
which are casual invaders, and which would normally reside outdoors,
are usually large (1/2" or more), hairy, distinctly patterned (even
brightly colored), and usually jump or run quickly.
General sanitation, both indoors and outdoors, is very important in
spider control. Clean up all woodpiles, rocks, trash, compost piles,
old boards, and other debris. Exercise caution when working around any
materials that have been stockpiled for any length of time. All
garages, cellars, crawl spaces must be kept clean and uncluttered.
Control of excess moisture is also helpful. Keep crawl spaces,
basements and porches as dry as possible. Plant trees and shrubs far
enough away from the foundation to allow sunlight and wind to
penetrate.
Those spiders which enter buildings from outdoors do so through small
cracks and crevices. Thus, the sealing or caulking of these entrances
will aid in spider control. Screens, tight-fitting doors and windows
will help keep spiders out. Indoors, move and dust frequently behind
and under furniture, stored materials and wall hangings. Do not allow
objects to remain in one place too long. Vacuum up all webs. Also,
since spiders are strictly carnivorous, the elimination of household
insects such as cockroaches, bedbugs, ants and others will help
discourage spider infestations.
The best mechanical control device is the vacuum. Vacuum corners,
registers and window angles often. If you choose to use an insecticide
out of doors, apply diazinon 25% EC completely around foundation walls
and adjacent one foot of soil. Indoors, ready to use formulations of
chlorpyrifos, and propoxur is suggested. Do not use diazinon indoors.
Use proper precaution and follow the label instructions.
For a complete listing of suggested control options for all home, yard
and garden insect pests contact your local Extension Service, found
under local government in the phone book.
Read and follow instructions on the pesticide label. Heed all warnings.
Check with your physician if you have any concerns regarding your
personal health risk.
References
Revised by Tom Ellis, M.S., Department of Entomology
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status. Issued in furtherance of MSU Extension work, acts of May 8 and
June 30, 1914, in cooperation with the U.S. Department of Agriculture.
Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
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products or trade names does not imply endorsement by MSU Extension or
bias against those not mentioned.
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information.was reviewed as
of June 2008. For more information about the contents please
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