Spiders are not insects, they are from the group Arachnida which includes Mites, Ticks and Scorpions!
Due to the wide ranging habit of spiders, they are somewhat difficult to control. Some insecticides are effective at controlling spiders, few are actually labelled suitable for spider control and we feel that is because spiders would be rather difficult to breed in laboratory conditions and testing chemicals on them would also be more difficult than let's say a cockroach or ant.
Spiders most commonly enter structures through gaps around door frames, window frames, air vents and pipe runs. The most popular entry point for spiders is under the eaves of buildings, thus entering the roof void from where they can venture downward into the home.
Keeping vegetation away from the building can be a good method of reducing the incidence of spiders. Bushes provide excellent web making facilities for spiders and, if close to the house, this could result in spiders venturing into the home.
Control of spiders is not easy. You may need to treat both externally and internally to gain good control of a spider infestation. Insecticidal dusts are very effective in that the dust will adhere to spider webs and the spiders will pick up the dust as they travel around their web. We have found that applying insecticidal dust under the eaves of a home can significantly reduce spider activity, but external treatments will probably need to be backed up with some spider control within the building. Once again, insecticidal dust is probably the most effective control measure but it is not always appropriate to apply insecticidal dust into living areas. Roof voids are ideal for that. In the home, long term residual insecticide such as Dethlac or Rentokil Insectrol can be applied, be careful with Dethlac as it is a laquer which may stain and does not like plastic surfaces, but it is an effective long term spider killer. A safer product would be our Pro-Active C which is a water based insecticide, so unlikey to cause any damage. This can be applied to wall/floor and ceiling/wall junctions, around window and door frames, behind cookers and washing machines etc. Insect Monitors can also be very useful where you do not want to apply insecticides. These sticky traps will catch and hold most sized spiders.
New in 2014 is the Spider Free Repellent which promises to be a really popular spider repellent/deterrent with a long life of minumum 3 months and maximum 6 months. DETHLAC Manufacturer's Website