Masses of bees showing up suddenly on your property are not cause for too much alarm. Bee swarms are relatively inert. Calm reaction, a little awe, and some patience, could handle the problem.
Bee swarms are an instinctive response to overcrowding, which usually occurs in late spring or early summer. They can consist of 1,500 to 30,000 bees and are generally not dangerous. Bees swarm in a ball-like fashion protecting their queen. The bees are waiting for scout bees to return and signal that they have found a suitable location to build a nest. Once a suitable nesting location has been found, the bees will relocate to the newly found nesting site.
Do not panic if you see a swarm of bees on the side of your house. They may only be resting and could leave in a matter of minutes or just a few days. UC IPM states that bee swarms become more of a danger the longer they stay in one place. If you would like them removed, a beekeeper can come to handle them and give them a new home in a bee apiary. You should not attempt to remove them yourself or spray them with insecticide. Doing so only increases the risk of getting stung. Once threatened, the bees can become aggressive and attack. A professional beekeeper or an exterminator will use a bee suit to shield his head, face, body, hands and feet.
If a swarm remains on your property for several days, it may be thinking about moving in. Established nests are not as easily removed, however, by a beekeeper and extermination may be the best option. After the bees are exterminated, all traces of the hive and its honey must be removed so other swarms will not be attracted to nesting site in the future.
In order to prevent bee swarms from nesting on your property, make sure holes are caulked, and other entryways are sealed.