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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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This information.was reviewed as of June 2008.  For more information about the contents please contact costner@msu.edu for webpage problems strausc@msu.edu .