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Spiders

http://ipm_www.ncsu.edu/insect_notes/Urban/spiders.html

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Lawrence F. London, Jr. - Venaura Farm - Chapel Hill, NC, USA
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Title: Spiders
rsc-1
Prepared by:
Michael G. Waldvogel and Charles S. Apperson
Extension Entomologists

Dated August 1995
Placed on the Web 2/8/95 by the Center for Integrated Pest Management

SPIDERS

Introduction

Spiders have a widespread, but largely undeserved reputation as being dangerous to the health and welfare of people and their pets. In truth, spiders are extremely beneficial because they prey on many insects that we consider to be pests in our homes and gardens. Not all spiders build webs to snare their prey. Some species ambush their prey from tubular tunnels built in the ground or under rocks. Other species simply build a loose collection of webbing in which they live. The types of spiders that most frequently invade homes fit into these latter categories. They may remain hidden for most of the day and feed primarily at night.

Biology of spiders

Spiders are not insects. They are close relatives of ticks, mites and scorpions, which all belong in the group called arachnids. Unlike insects, which have three main body sections and six legs, spiders have two body sections and eight legs. The eyes, mouthparts and legs are found on the front section of the body, known as the cephalothorax. The second section, the abdomen, bears the openings for breathing, the digestive and reproductive systems, and the external organs used for spinning silk or webbing. Most spiders are identified by size, color, markings on the body and the number (usually six or eight) and arrangement of eyes. Female spiders wrap their eggs in a silken spun sac. Some species carry this egg sac, while others deposit it somewhere within their nest. Hatchling spiders (spiderlings) often produce a silk thread that allows them to be carried by wind currents to other areas.

How dangerous are spiders?

Encounters between people and spiders are usually accidental and bites are a defensive response by the spider when its web or nest is disturbed. Most spiders produce venom therefore, they could be considered "poisonous". The venom is stored in glands that empty into the spiders fangs' or chelicerae. For the most part, spiders bites are of no consequence. However, just as bee and wasp stings may trigger allergic reactions in some people, the same can be true for spider bites. Young children, the elderly and hypersensitive individuals are more likely to react strongly to a bite. In North Carolina, there are few spiders that can inflect serious and painful injury is extremely small. The two best-known dangerously poisonous spiders are the black widow spider and the brown recluse.

Black Widow Spider

The black widow spider, Lactrodectus mactans, is probably the most widely recognized of the dangerously poisonous spiders found in North Carolina. Most people are familiar with the large, shiny black body and red "hourglass" markings on the underside of the abdomen (see picture), but it is important to note that the coloration and markings are only found on adult females. Young black widow spiders are tan-to-gray in color and have orange and white "racing stripes" on their abdomens. These marks may remain visible on adults. The female has a body length of about 1/2", with a total length (including legs) of about 1-1/2". The male spider is smaller than the female and, like young spiders, has red and white markings on the back of its abdomen.

Black widow spiders are found in protected places, such as under rocks, wooden boards and in dense plant growth. They frequently nest in electrical, water and telephone equipment boxes outdoors. Around homes, these spiders may be found in crawlspaces, beneath appliances, usually in corners that are dark and damp. The female produces an egg sac containing 250-750 eggs that hatch in two to four weeks, depending upon temperature. The spiders spin a web that is used to ensnare prey, mostly insects, but sometimes other spiders as well. The trapped victim is bitten, then injected with venom to paralyze it.


Black widow spider venom affects the nervous system. Because the bite itself is relatively mild, some people may not even realize immediately that they were bitten. The venom causes pain in the lymph nodes. Other symptoms of a severe bite include nausea, elevated blood pressure, sweating, tremors and increased white blood cell counts. The wound may appear as a bluish red spot, surrounded by a whitish area. Both male and female spiders bite, but adult females produce the most severe bites. An antivenom serum is available through a physician, but it must be administered soon as possible after the bite occurs.

Brown Recluse Spider

The brown recluse spider, Loxosceles reclusa, is native to the Midwest (most commonly found in Kansas, Missouri and Arkansas), but it has spread to the eastern United States and elsewhere. The adults are about 1/4-1/2 inch in length. The body may be tan to dark brown, making it similar in appearance to many other spiders, but the key characteristics of the recluse are the arrangement of its six eyes into three pairs, and the familiar dark violin or fiddle-shaped mark on the top of the cephalothorax (see picture). The "neck" of the fiddle points toward the rear of the body. Brown recluse spiders are nocturnal in their feeding habits. Outdoors, they are most common under rocks, debris, wood piles, etc.. Indoors, common hiding places are bathrooms, attics, cellars and storage areas. Problems most often occur when people are accidentally bitten by a recluse hiding in stored clothing, inside shoes/boots or cardboard cartons. The hands, underarms, lower abdomen and the ankles are the areas of the body most likey to be bitten.

A brown recluse bite may go unnoticed for 6-8 hours, before a reddening, swelling and blistering of the wound starts to appear. The occurrence of additional symptoms of a recluse bite will depend upon the amount of venom injected by the spider. A severe bite can produce a necrotic lesion (i.e., an area of dead skin tissue) that may require surgery. Although an antidote has been developed, it is not widely available and it appears to be most effective before the lesion develops. In some cases, antibiotics and the drug Dapsone may be used successfully to treat the bites without surgery, but these decisions are made after careful diagnosis by a physician. Prompt medical attention is critical to successful chemical or surgical treatment of the bite.


Control of Spiders Indoors

Finding spiders indoors usually means that there is an ample supply of these insects in the area. Any real attempts to get rid of spiders should focus on eliminating these insects. The long term solution is sanitation, i.e., reducing or eliminating conditions that attract insects, e.g., high moisture and ready access to food of some sort.

There are a number of short-term solutions to spider infestations, as well. Knocking down and removing webbing, or mechanically removing/killing the spiders should be sufficient. If you're concerned that more spiders will show up (or hatch from an unseen egg sac), then you could resort to applying an insecticide along baseboards, in corners, and inside storage closets. Select a pesticide that is labeled for use against spiders indoors. Always read the insecticide label for complete instructions on how and where to use the product. Treating your crawlspace is an option, but exercise extreme caution when applying pesticides in such confined areas without adequate ventilation and personal protection. Crawlspace treatments are often best left to pest control professionals. The availability of particular insecticides may change from year to year, so consult the latest edition of the North Carolina Agricultural Chemicals Manual or your local county Cooperative Extension office for advice on selecting insecticides.

Tips on Avoiding Spider Bites

Follow these suggestions to reduce the chances of being bitten:
  1. Always check for spiders before sticking your bare hand(s) into dark corners or areas.

  2. Always wear work gloves when handling boxes, firewood, lumber and other items that have been stored/stacked undisturbed for some time.

  3. Stored clothing should be shaken vigorously to dislodge any spiders and inspected carefully before wearing.

  4. If you think you have been bitten by either a black widow spider or brown recluse, carefully apply ice or a cold-pack to the bite and seek medical assistance. If you can catch the culprit spider, bring it along for positive identification.
References used in preparing this publication:
Goddard, J. 1993. Physicians Guide to Arthropods of Medical Importance. CRC Press, Boca Raton, FL.
Kaston, B. J. 1953. How to Know the Spiders. Wm. C. Brown Co., Dubuque, Ia.

The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsements by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. All pesticide recommendations provided in this publication apply to pest control in North Carolina. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain information about product usage in your locale and examine a current product label before applying any chemical. The information presented on pest biology is based on conditions in North Carolina and may not be applicable to your locale. For further information, we recommend that you contact the Cooperative Extension Service office in your county.