We did several tests... put them on vanilla.... take them off vanilla, put them on vanilla, take them off vanilla...

We had over 1500 rats IN our house at one point, so we could make note of differences fairly easily.. vanilla definitely helped...

We use pine bedding, the smaller/more absorbant kind helpds better.

Then finally we would put a handful of baking soda down under the pine.... at one point I thought it was causing some deaths but after additional trials we didnt lose any babies or adults, and it seems like it was a freak coincidence.... it is dusty so you want to make sure you put a good amount of bedding on top, and hope they dont kick it around much, but it helps a lot.