Sac Spider

Introduction

The common name comes from their yellow color and the retreat they build which is a flattened silk tube or sac where they spend most daylight hours; they do not build webs. These spiders are more than nuisance pests. The 2 species found in the continental United States are probably responsible for a great many cases of indoor spider bites, especially the yellow sac spider. However, although the association between bites by members of this genus and necrosis has been cited over and over in the literature, this association is unfounded except for one case of mild necrosis involving an European species. Only 2 species are found throughout the United States.

Recognition

Adult female body length 3/16-3/8″ (4.9-10 mm), adult male body length about 1/8-5/16″ (4-8.5 mm). Color pale yellow to pale green, with chelicerae and palp tips brown, and legs and abdominal midline slightly darker. With 8 subequal eyes in 2 rows, front row slightly curved forwards and posterior row’s median eyes slightly  nearer to each other than to lateral eyes and/or slightly curved towards front. Anterior spinnerets conical with bases touching or almost so. Anterior legs (1st pair) longer than 4th pair, or 1st pair shortest of all pairs; tarsi with 2 claws each.

Similar Groups

1. 1. Trachelas tranquillus with cephalothorax orange brown to reddish on top and bottom, abdomen pale yellow to light gray with anterior median area slightly darker, and anterior legs darkest with other legs becoming increasingly paler towards rear pair.
   2. Ground spiders (Gnaphosidae) with front row of 4 eyes slightly curved to rear and middle 2 eyes slightly larger and dark (vs. other 6 eyes pale) and anterior spinnerets cylindrical in shape and distinctly longer than others.

Representative Species

1. 1. Agrarian sac spider, Cheiracanthium inclusum (Hentz). Adult female body length 3/16-3/8″ (4.9-9.7 mm), male 1/8-5/16″ (4-7.7 mm); color light yellow with chelicerae and palp tips brown, and legs and abdominal midline slightly darker; 1st pair of legs shorter than 4th pair; found throughout the United States except for most northern tier of states.

   2. Yellow sac spider, Cheiracanthium mildei L. Koch. Adult female body length 1/4-3/8″ (7-10 mm), male 3/16-5/16″ (5.8-8.5 mm); color light green to yellow-white  with chelicerae and palp tips brown, and legs and abdominal midline slightly darker; 1st pair of legs shorter than 4th pair; introduced from Europe, found throughout much of the United States with its range still expanding

Schedule a Free Consultation

Whether you need one-time service or a recurring plan, it starts with a free consultation.

Make an appointment

Biology

In the New England states, the yellow sac spider females lay their eggs in June and July in a loose mass covered only with a thin white silk sac. The egg sac is kept in her retreat indoors but it is often located in a rolled leaf outside. The female usually stays nearby on guard. Outside, the spiderlings overwinter and become adults the following May and June. They live about one year.

The 2 species of sac spiders detailed above are of medical concern because of the many spider bites attributed to them. It is estimated that they may be responsible for many bites on people indoors. However, few bite cases are reported, apparently because the spider is rarely caught in the act, captured, and/or properly identified. Their venom is a cytotoxin and primarily affects the tissues at the bite site. The following is for verified cases of bites by yellow sac spiders. The bite results in pain (similar to a bee sting) or discomfort, which lasts for an average of about 1 hour 45 minutes (range 45 minutes to 48 hours). The bite seldom results in more than localized redness, a burning sensation, and slight swelling at the bite site. Occasionally, a systemic reaction may occur which is characterized by headache, nausea, and/or vomiting. The one case of mild necrosis involved a pea-size lesion on the person’s shoulder.

It must be realized that what is generally given in the literature regarding yellow sac spider bites as causing dermonecrotic lesions similar to those caused by the brown recluse spider has recently (2006) had its validity come into question. The problem is that although the association between bites by members of this genus and necrosis has been cited repeatedly in pest management trade magazines and books, review articles, medical textbooks, and medical journals, this association is unfounded based on the 2006 examination of verified yellow sac spider bites in the United States and Australia, except for one case of mild necrosis involving the European species, C. punctorium (Villers).

First aid consists of using hydrogen peroxide or alcohol to cleanse the bite area and prevent secondary infection, applying an ice pack to reduce swelling, keeping calm, and seeing a physician if systemic symptoms occur. Remember to attempt to collect the responsible spider and take it with you for identification purposes.

Habits

Although sac spiders can be found indoors throughout the year, they more commonly enter structures in greater numbers in the early autumn when their food supply decreases and temperatures cool. They enter through loose fitting and/or unsealed doors, faulty screens, windows, vents and utility lines, structural junctures, and plant materials brought inside. They may stay in crawl spaces and structural voids feeding on the resident arthropod population, or they may enter the living space through gaps around ducts, light and electrical outlets, plumbing, and molding.

Once indoors, they may build their silk retreats in the upper corners and the ceiling-wall junctions of rooms and rest there during the day; in basements and crawl spaces, retreats are found where joists and band boards meet the subflooring. At night, sac spiders are often seen running on the ceilings and walls, but if disturbed they readily drop to the floor and seek cover

Bites usually occur when the spider crawls into clothing or bedding and becomes trapped against a person’s skin. However, C. mildei was once observed to crawl on a person’s arm and then to bite repeatedly without provocation.

Outside, sac spiders build their silk retreats under items lying or piled on the ground such as stones, landscape timbers, firewood piles, lumber, logs, and other debris. They can also be found in garages and outbuildings such as sheds and dog houses. On the outside of buildings, they can be found at structural junctures, upper corners of windows, behind shutters, and under eaves and overhanging shingles. Other places include on long grass and weeds and in the leaf litter.

Pest Control Naples

Follow the standard control procedures outlined in the introductory spider section. Pay particular attention to each and every silk retreat along ceiling-wall junctions and ceiling corners; the spiders and their retreats must be removed, and a vacuum with crack-and-crevice attachment works best. Immediately transfer the vacuum bag into a plastic bag, seal it, and take it outside or freeze it.

In severe infestations, be sure to inspect and remove retreats from the basement and/or crawl space. It may be necessary to dust open hollow-block voids along the sill plate with an appropriately labeled pesticide.

Outside, trim back trees and bushes touching the structure, keep the grass mowed to about 3″ (75 mm), and remove as many retreat sites as possible.