Cascade particle system LOD

I don’t fully understand what you mean, HOWEVER, if you go into the STATIC MESH settings of the meshes that you use for the Niagara/Cascade particles, you can edit the LOD behavior there:

Click “CUSTOM” to expose the available LODs.

Then search for the “SCREEN SIZE” to adjust the LOD-Picking behavior.

To add LODs, check the “LOD SETTINGS”:

NOTE: “Auto Compute LODs” must be disabled to customize the screen spaces yourself.