The Eight-Legged Health Threat

Certain spider species, such as the infamous black widow, are classified dangerous to human health. To control them, thorough inspections, followed by careful management tactics, are required.

Spiders can affect man’s health in three ways: annoyance or fear, allergic reaction, and envenomization. Although each varies in its degree of seriousness and medical effects, man’s health may be affected by each. Interestingly, spiders and their venom have long been used for medicinal purposes as well.

The success of the movie Arachnophobia was due in no small part to the “fear factor” — that strange but not uncommon characteristic of man that leads him to the brink of sheer terror only to be able to step back in relief with the realization that no true harm has befallen him. Some people may see a spider and not be bothered. Others see a spider and get heebie-jeebies. To a few, however, the sight of a spider induces fear to the extent that it results in a debilitating malady that can render them both physically and psychologically ill.

Certain spiders are considered dangerous to human health. Such spider species, including the black widow, the brown recluse and the hobo (aggressive house), are officially classified as “dangerous” by the Centers for Disease Control. Other spiders can and do fight — more than 50 species have been identified as having bitten people, although most of these bites result only in localized swelling and pain. The sighting of a single black widow or brown recluse in a building, however, is cause for concern and triggers an immediate need for action. This article will deal with the control of four types of dangerous spiders and provides procedures that are helpful in successful remediation of infestations when they occur.

IDENTIFICATION IS CRITICAL. It is imperative that the spider(s) involved in an infestation are positively identified prior to instituting the often-intense procedures required to control a dangerous species. To assist in the identification of spiders, consult the PCT Field Guide to the Management of Urban Spiders.

Misidentification often plays a role in cases involving the brown recluse spider. Frequently, people claim to have been bitten by the brown recluse spider at work and efforts are directed at treating the building only to find out that brown recluse spiders are not present in that building. In other cases, people claim to have seen brown recluse spiders or have them identified by a “credible” source, and upon further investigation, the spiders actually seen turned out to be wolf spiders or grass spiders.

It is important when dealing with dangerous spider cases that the exact species involved be investigated. A lot of different factors may cause symptoms that are diagnosed as a “spider” bite, and physicians are often quick to blame spiders in the absence of other clues. Such occurrences generate panicked and upset customers who demand action. Yet, if the “feared” spider species is not actually present, whose time and effort get wasted?

Always take the time to find and identify whether dangerous spiders are actually present in or around the building in question. For brown recluse and yellow sac spiders, the placement of numerous insect monitoring traps is the best course of action. Left out several nights, these traps will reveal whether such spiders are present. Male and immature hobo spiders will also wander into monitoring traps.

For web-building spiders, such as the black widow and the hobo spider, one has to look for the webs, and from there, find the females residing within them. Both spiders tend to construct their webs near ground level and usually in some type of sheltered location. Black widows spin scaffold-type webs in which they hang upside down. A hobo spider’s web consists of a flat, horizontal sheet at the one end of which a funnel is located. The funnel often disappears into a hole in the wall or into a large crack, such as that formed by two closely placed boxes.

Yellow sac spiders spin ½-inch long silken retreats that are usually located in corners, especially at the ceiling/wall juncture, and beneath furniture.

Once the presence of dangerous species has been confirmed within the home or building, steps can then be implemented to eliminate, where possible, the infestation. (In cases involving brown recluse spiders, total elimination is often not possible, especially in older structures and in homes with cedar shake roofs.)

COMMON CONTROL PROBLEMS. Certain procedures are useful in controlling the most dangerous spider species. As with most pests that invade from the exterior, achieving long-term relief from infestations of these spiders requires a focus on the outside of the building.

As much potential harborage as possible needs to be removed from next to and near the foundation. Such efforts should extend to all parts of the property where possible.

Heavy vegetation, especially ground covers, plays a role in attracting the insects used as food by the spiders. Clutter, formed by piles of items or debris, provides the shelter desired by species such as the brown recluse, black widow and hobo spiders. When searching for black widow spiders around buildings, the author has found that these spiders will mostly be found in association with firewood and piles of items. Vegetation lying against foundations or landscape timbers also provides a protected site for black widows. Eliminating piles of lumber, bricks and debris limits the number of sites dangerous spiders can use. Firewood should also be moved away from the home and stacked off the ground. Always wear heavy gloves when moving items to inspect for spiders.

Cracks and holes in the building’s exterior need to be sealed. Even cracks up to the roofline need to be sealed, particularly when the brown recluse spider is involved. Weep holes, when present, should have screening pushed inside them to prevent spiders and larger insects from entering. Exterior lights should be changed to sodium vapor lamps (commercial buildings) or yellow “bug” lights (homes) to attract fewer flying insects.

Vacuuming is important for removing spiders, webs and egg sacs. This technique is especially beneficial for web-builders, such as the hobo and black widow spiders. A vacuum is very handy for removing brown recluse spiders as they are uncovered during inspection, such as from inside boxes, behind furniture or beneath attic insulation. Care should be taken when removing the vacuum bag for disposal.

Placement of monitoring traps is critical in cases where the brown recluse, yellow sac or hobo spider is involved. These should be located along walls in protected sites. Spot treatments are applied to web-building sites, particularly on the building exterior. Such treatments are also useful when applied to the floor/wall juncture behind furniture and other items in areas where brown recluse spiders are active. Microencapsulated pyrethroids (e.g. Demand CS) seem to be especially effective against spiders; however, wettable powder formulations will also work. Perimeter treatments are often necessary where the threat of spider reinfestation is possible. These may need to be regularly reapplied.

SPECIFIC CONTROL PROCEDURES. The following details procedures that are used for each type of dangerous spider that may be encountered:

Brown Recluse Spiders

1. Confirm that brown recluse spiders are actually involved by examining a specimen provided by the customer or by placing monitoring traps.

2. All cracks behind baseboards, window and door frames, inside furniture, beneath and between cabinets, etc., should be lightly treated with a dust insecticide, such as Drione or DeltaDust.

3. All electric outlet and switch plates should be removed and the wall void behind treated with a dust insecticide.

4. All plumbing voids need to be treated with a dust insecticide.

5. In the attic, insulation should be lifted to check for spiders. Any found should be removed by vacuuming. The area under the insulation where spiders are active should be treated with dust insecticide.

6. Prior to the service, the customer should be instructed to remove all boxes from the attic, garage and/or basement, take them to the driveway and go through them. Spiders found should be vacuumed. If the customer prefers not to do this, then the professional should include a cost for doing so in the price of the service. Once the boxes have been searched, they should have all openings securely sealed with tape to prevent spiders from re-entering the boxes.

7. In basements, crawlspaces and/or the garage, all accessible cracks and voids should be treated with dust insecticide.

8. Spot treatments can be applied to floor/wall junctures in all areas where spiders may be active.

9. As many monitoring traps as possible should be placed in all areas of activity. You cannot use too many traps because each spider captured is one that cannot bite someone.

10. Outside, all exterior cracks should be treated with a dust insecticide. For moderately to severely infested structures with brick veneer and weep holes, it may be necessary to drill through the brick veneer around the perimeter of the building and treat the void behind. A dust insecticide can be used; however, better penetration may be possible using Conquer and Kicker applied using an Actisol or Micro-Injector machine. In addition, a perimeter treatment may need to be applied where the threat of reinfestation is possible.

Yellow Sac Spiders

1. Confirm that yellow sac spiders are actually involved by examining a specimen provided by the customer or by placing monitoring traps.

2. Inspect corners and under furniture for the silken retreats used by these spiders. Remove these by scraping and vacuuming.

3. Spot treatments can be applied to floor/wall and ceiling/wall junctures in all areas where spiders may be active.

4. Numerous monitoring traps should be placed in all areas of activity.

5. Outside, all exterior cracks should be treated with a dust insecticide. A perimeter treatment may need to be applied where the threat of reinfestation is possible.

Hobo Spiders

1. Confirm that hobo spiders are actually involved by examining a specimen provided by the customer or by placing monitoring traps.

2. Search for the flat webs with a funnel at one end that may be present under furniture, in the garage, basement and/or crawlspace. Remove these by vacuuming. Remember that a hobo spider’s web often has the funnel located into a hole in the wall or some other opening, such as that formed by boxes stopped close together. In such cases, the spider might escape back in the void or between the boxes. Be sure to treat inside the void with a dust insecticide or to move boxes, etc., to find and kill the spider.

3. In basements, crawlspaces and/or garages, all accessible large cracks and voids should be treated with dust insecticide.

4. Spot treatments can be applied to floor/wall junctures in all areas where spiders may be active.

5. As many monitoring traps as possible should be placed in all areas of activity.

6. Outside, all exterior cracks should be treated with a dust insecticide. For moderately to severely infested structures with brick veneer and weep holes, it may be necessary to drill through the brick veneer around the perimeter of the building and treat the void behind. A dust insecticide can be used; however, better penetration may be possible using Conquer and Kicker applied using an Actisol or Micro-Injector machine. In addition, a perimeter treatment may need to be applied where the threat of reinfestation is possible.

Black Widow Spiders

1. Confirm that black widow spiders are actually involved by examining a specimen provided by the customer or by searching for a spider in its web.

2. Find and remove as many spiders, their webs and egg sacs as possible using a vacuum. Be sure to wear heavy gloves when moving items under which spiders might be residing.

3. Spot treatments can be applied to corners and other potential web-building sites where spiders may be active, especially in the garage and basement.

4. In crawlspaces, a space treatment can be applied to kill as many spiders as possible. Be sure to close all foundation vents and to extinguish the furnace pilot light if it is located there. A dust insecticide may also be applied to the sill plate areas and other sites in the crawlspace where spiders are seen.

5. Outside, a microencapsulated or wettable powder pyrethroid works well as a spot treat to potential web-building sites. In addition, all items around and under which spiders might reside should be inspected and the spiders treated directly.

Stoy Hedges is a contributing author to PCT.