Why spiders enter the home
Although some spiders will enter the home at certain times of the year for mating purposes, for the most part they will tend to set up home indoors purely based on the amount of available food and shelter.
The wide range of prey that different spider species tend to hunt will mean that they can be found in almost any environment from dank cellars to conservatories or sun rooms too hot for most species to thrive in for any length of time. Reducing the amount of suitable shelter and food sources then can greatly reduce the amount of spiders you might find in the home.
Dark, cluttered and damp rooms with plenty of undisturbed space will tend to attract hunting spiders for example that hunt on the ground and prey on other pests such as woodlice, silverfish or flies. Airing out such rooms and reducing damp where possible is the easiest way overall to dissuade these types of pests.
More open areas such as kitchens or refuse areas tend to attract flies, wasps, hornets and the like and so in turn will encourage web spinning spiders to set up home nearby. Keeping containers sealed and food safely away will discourage flying pests and in turn the spiders eager to catch them.
Keeping clutter to a minimum then as well as ensuring that other pests are kept at bay is perhaps the single most effective and simple means of keeping spiders out of the home overall.
All manner of different chemicals, plants and substances are said to deter spiders and a whole range of other bugs from entering the home. Which ones do and don't actually work however is a matter of some debate and many are probably in fact specific to certain types of spiders in specific geographical areas.
Perhaps the most commonly recommended tip of this kind is to leave horse chestnuts around the home, which spiders are averse to the scent of. Simply poke holes into the chestnuts and place a few of them on interior and exterior windowsills and around entrances. This is generally said to work fairly well for common house spiders although smaller spiders or those that tend to stay off the ground likely won't be as easily dissuaded.
Lemon or mint based oils, cleaners or furniture polishes are also said to work due to the fact that spiders dislike the taste of them although again this method seemingly has obvious limitations. While it may keep spiders away from furniture and areas that have been sprayed heavily, they are of course a temporary solution at best.Similarly cotton balls soaked with almost any strong smelling chemicals are said to work by many although need replacing regularly to retain effectiveness.
Tobacco is another widely used deterrent which is said to be effective for spiders. The best method being to boil some tobacco for a few minutes, strain out the strands, add a little detergent or washing up liquid and spray the resulting liquid around the outside of the home and any areas that pests frequent. This not only keeps pests away but will kill most that come into direct contact with it. There are also manufactured tobacco based pesticides now available in some countries. An important note when using tobacco based deterrants however is that they cannot be used on certain plants without killing them, so in general keeping to use around the outside of the home is recommended.
Man's best friend
In many cases pets are perhaps the most effective means of controlling all kinds of household pests including spiders. Cats particularly will eagerly kill anything that crosses their path including spiders prey such as flies and moths as well as spiders themselves.
As well as killing pests indiscriminately whenever the opportunity presents itself, pets also discourage pests by their very presence. They not only tend to move around in areas that humans will tend not to frequent, discouraging anything from setting up home, but will encounter and scare pests that might otherwise have stayed in full view and come into contact with humans later.
A Passing Intrusion?
During certain times of the year the males of many spider species will set off in search of a mate, often leading them directly into or through a house.
While this is undoubtedly a nuisance this is generally only a passing phase that only lasts a few weeks, after which the males tend to die off and the females begin to lay eggs.
If you notice a spider running across the floor seemingly aimlessly then, this is probably the case rather than that they intend to set up home where they are not wanted.
Of course this will likely come as little comfort to arachnophobes.