Spiders are on the march – now is the time to start thinking about having your house treated for spiders. Treating the roof void with a permethrin dust and the perimeter of the house with a safe insecticide will greatly reduce the infestations of spiders in your house. Tip: Once the house has been treated it is a good idea to go around the inside of the house with a vacuum cleaner to remove any live spiders from the inside [these ones will not have been affected by the treatment]. Also about 2-3 weeks after the treatment you can remove the cobwebs from the outside of the house.