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.