Contact sprays, applied either directly to observed spiders or to spider webs, are very effective at killing black widows. Common chemical ingredients for contact insecticides effective against spiders include synthetic pyrethroids such as cyfluthrin, sypermethrin, or lambda-cyhalothrin, but any insecticide labeled for spider control can be used. Spraying spider webs is important in controlling hidden spiders, since the spiders will eventually walk through the web and contact the insecticide. Note that while contact insecticides are lethal to spiders, they will not affect any black widow egg sacs.
A wettable powder or microencapsulated, slow-release, residual insecticide is effective when applied to areas that black widow spiders frequent, such as corners, behind and under furniture, behind stored items and the like. The same type of insecticide can also be applied to areas outside the home that serve as entrance points for black widows, such as cracks, crevices and doorways. These are preventive measures meant to discourage spiders from coming into the house and taking up residence there, but may not be as effective for controlling a spider population that is already take inside the home.
The methods already discussed are useful for managing small black widow populations, but an infestation of numerous black widows requires a different approach. In this case, total-release aerosol foggers -- sometimes called "bombs" -- usually containing synergized pyrethrin or synergized pyrethroids such as resmethrin, sumithrin and cyfluthrin, represent a much more efficient approach. Foggers can reach parts of the home that can be difficult to reach with a contact spray.
In rare cases, an infestation may be so severe that it will be necessary to contact a professional exterminator. Exterminators can use more potent insecticides that are illegal for consumers to purchase, so this is the best option for treating a critical black widow infestation. Additionally, exterminators have extensive experience in treating spiders and will know the best techniques for controlling infestations while taking care to minimize putting the inhabitants of the home at risk of exposure to toxic chemicals.