Back when my TDI was parked and waiting for buyback, I had squirrels living under the hood - I would find nut shells on the battery box. I placed some mothball kind of things under the hood, and that seemed to dissuade them.
Yeah, they do gradually evaporate, so they need to be replaced from time to time. The amount of time they last depends on how big they are to begin with - a large block would last longer than smaller balls.How long do the mothballs last? Do they "evaporate" (disappear) or just lose their scent (and effectiveness)? Helps to know when to replace them...
Yeah, the mothballs worked for me only because the TDI was parked waiting for buyback, and I would only drive it once a week or two in order to keep it all in working order. For an active car, the sonic thing sounds way better - I wonder if it bothers pets however.Mothballs work (usually placed in a sock and then within the engine bay when not operational). They will breakdown/dissipate over time. The drawback of course is the rather pungent odor. That's why again in more recent history I've opted for a Sonic Deterrent, even if it meant running an exterior extension cord (which I've also used for an under-hood waterproof 12V Battery Tender Jr. in Winter). I use a similar plug-in sonic in my shed with success.
The sonic is of course well above human frequency acuity.
e.g. Amazon.com : Loraffe Under Hood Animal Repeller Rodent Repellent Ultrasonic Rat Deterrent LED Strobe Lights Keep Mice Away from Car Pest Control for 12V 24V Automobile Truck RV, Rodent Defense Vehicle Protection : Garden & Outdoor