Spiders
Spiders are arachnids, a group of arthropods that are wingless, possess eight legs, and have no antennae. Ticks and mites also belong to this group. There are approximately 50,000 known species of spiders and they are found in nearly every habitat worldwide. In the United States there are approximately 3,500 identified species of spiders. Fortunately, most spiders play a beneficial role by consuming nuisance insects that impact humans.
Vector Significance
- Spiders possess a venom that is used to kill and digest prey
- Most spiders are of little public health concern since few possess venom that will affect humans
- Spider bites can result in uncomfortable pain and swelling
- The black widow is the ONLY spider of medical importance in Alameda County
- This species has a powerful neurotoxic venom, and bitten individuals may require medical attention
- The brown recluse spider is not found in Alameda County or California
- All cases of bites related to brown recluse spiders in California are due to travel to the Midwest US
- Because brown recluse spiders are not native to California, the risk to the public is very low
Vector Control Services
- Provide inspections of properties and give advice on how to lessen spider abundance
- Provide identification of spider specimens
Requesting Services
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What can the public do?
- Monitor the corners, ceiling, and door jams for the webbing/egg sac and vacuum up immediately
- A night-light will attract insects which attract spiders, so avoid leaving your porch light on at night, or use a motion detector sensor
- Plants, which provide food and harborage for insects and spiders, should be kept back from the perimeter of your living structure
- Keep window and door screens in good repair
- A monthly rinsing of eves, windows, and doors will help reduce spider populations on the exterior of your home
- An effective way to remove an unwanted spider from your home is to vacuum it up with the hose and wand attachment
- Locating and removing eggs sacs will reduce future spider populations
- As a last resort, pesticides may be used to control spiders
Spiders
Types of Spiders
Western Black Widow Spider (Latrodectus hesperus)
- Females of this species have a rounded black body (½ inch) with a red hourglass on the underside of the abdomen
- The male black widow is smaller and has a shiny black abdomen like the female but lacks the hourglass
- Both sexes may have yellow and red marks on their backs in immature stages
- The Western black widow spider is found throughout Alameda County
- These spiders are shy, nocturnal, and stay hidden during daylight hours
- Preferred habitats include garages, attics/basements, burrows, debris, and dense vegetation
- Medical attention may be required if there is a reaction to a bite
Brown Recluse Spider (Loxosceles reclusa)
- They are native to the Midwest United States and are not normally found in Alameda County
- This species is about 3/8 inches long, has a uniformly colored abdomen that varies from cream to dark brown and has a dark brown violin shape on its back
- The unique eye pattern consists of six eyes in pairs with a space separating the pairs
- All Loxosceles spiders have venom that can cause necrotic skin lesions
- Although these spiders are not established in Alameda County, interstate travel creates the possibility of an unwanted introduction
- Suspect brown recluse spiders found by County residents should be submitted to the District for identification