When you enter California, you go through an agriculture inspection station. There, they will either see/ask if you have anything that it is illegal to import into California, for example pecans obtained outside Nevada. My understanding is that this is to prevent crop-destroying pests from getting a ride into the state (the state is surrounded by natural barriers that make it hard for pests to get into the state without help).
However, I don't believe that is an effective long-term solution. If the people want such pests out, they should figure out how to repel them, not rely on the state. Those inspection stations should go and CA should lift the ban on agricultural items that the USDA allows to be planted or imported into the US.