There are several varieties of wasp such as yellow jackets, paper wasps and mud daubers in the Southern California area.  You may have seen samples of these under the eaves of your home or office building for example.

While certain wasps are unlikely to sting unless provoked, knocking down what appears to be an abandoned nest can lead to unexpected results and a painful sting.  If you are uncomfortable with removing a nest or nests that you have,
Victory Bee Removal is qualified to safely handle the situation and remove the wasps from your area so that none are left behind.