Mice In Cars? A Hastings solution: the car corral
If you don’t have a mouse-proof garage for you car, you may be in for some trouble.
Living in a natural area, we keep finding mice in our cars, often even if we only park a car overnight. It seems the US and Japanese car manufacturers are so far out of touch with nature that they build cars as if they would never see a wild mouse. Hoods are insulated with cardboard that makes great nest material. Vents open to the air in such a way that mice can climb up in the air ducts of the car and get stuck deep in the guts of the dashboard and die. The smell of a rotting mouse on a 95°F day is enough to ruin any appreciation you ever had for your vehicle. That smell can last for months when you turn on the air blower. The damage mice do to wiring, glove boxes, hoses, and insulation can be extreme. Mechanics at a Monterey auto repair shops report costs of up to $1,500 per car for damage by mice to wiring, or taking apart the entire air conditioning duct system in the dashboard, or worse.
Not that mice are entirely free of things that get into them (photo); they often have these fly larvae that burrow into their skin and develop into a relatively huge lump on the mouse and eventually a “bot” fly.
We have tried a number of what turned out to be useless rural mythological techniques to keep mice out of cars. These include aromatic bars of soap, mothballs, electronic ultrasonic beepers, other dead mice, and a variety of commercial predator scents. Cats are often useless; deer mice are amazing climbers (photo) and can get in the relatively small crevices of cars and readily escape cats. Poison means they often eat the poison and climb up into your car to die (see above). We have run both live traps (Sherman Live Traps; http://www.shermantraps.com) and have used Museum Special snap traps (http://www.forestry-suppliers.com ) in combination. You never know. Some mice readily walk into a live trap box, others won’t do that but will take peanut butter from a snap trap. If you check the traps every day, you often find a pair of mice; set at least two traps in the car at one time. Even if the mice leave a mess, the mess will be on a plastic sheet on the car floor inside. If you want to capture them alive, we have marked mice and found they can return in a couple of days even if released up to a mile away.
The only hope is to build a physical barrier. Nearly 30 years ago, I experimented with mice* and found a way to contain 
them (see figure). The trick was an overhang of smooth metal. I used aluminum flashing nailed at right angles to the top of a fence. Mice could not cross the fence in either direction. I suppose you could build a large fenced area to store your car, but the gate would be a challenge.
Here at Hastings, David Ribble built a similar fence in our streamside habitats, but it was just an upright aluminum fence. It kept the native deer mice in separate populations for years. Oddly, they would not walk around the fence; it went out in the open grassland and the mice did not venture out very far. They evidently could not, or would not, climb the smooth 24″ tall aluminum flashing.
Then a few years ago, Feynner Arias , a kind man and natural area manager living in Big Sur, showed me a way to keep mice out of a car in storage (see photo). The same aluminum flashing that kept the mice out of the fenced area is also generally impossible for them to climb. But, having to dig the flashing out of the soil every time you moved the car seemed like a lot of work.
We have a cement pad where we park a car and I realized I could put the flashing in a frame we could remove quickly each time (see photos). The corners are shorter sections that fit in the curved groove I routered into the wood. Just gently bend a 12″ section of flashing to fit in the groove and inside the long walls that meet at the corner. The groove and the other walls will hold up the corner section. Clip the upper edges of the inserted corner piece up at the top to make the outer joint between the wall and corner piece seamless. It is critical that the overlap is very smooth at the corners; remember the mice can climb up grass stems. The clasps for the corners are purchased at a good hardware store. We used 2×4 redwood with 12″ flashing and (eventually) even Gorilla glue failed to hold the flashing in the groove. So, I simply toe-nailed 2″ finish nails through the flashing and into the wood, holding the nail gun down in the groove to push the flashing outward at the base. We added shelf brackets to keep the flashing from bending in the wind. We drilled a small hole in the flashing at the top of each bracket and use some galvanized wire looped through the holes in flashing to keep it attached to the top of each bracket.
SUGGESTION: SAFETY CONSIDERATION: One might consder a split garden hose or a vinyl edging along the top of the flashing, and maybe the re-bar ‘mushroom’ for each upright to prevent injury in case of falls near the fence. We have stumbled and fumbled and in all cases the aluminum folded up and didn’t cause any injury. But, you never know.
Gravel Corral
We built another corral where the boards are placed on gravel. For this corral, we recycled some flashing, and we remove only the one (rear) panel when we take the car out. See photos (below).
When you have it on gravel, you must tamp the gravel back in place on returning the vehicle to the corral and occasionally along the other 3 sides. No holes under the gravel; mice like to burrow.
For the past 2 years we have kept mouse traps open in the cars and have not caught mice in the cars. To check on this, we occasionally have forgotten to corral a car at night corra and sure enough, in the morning we have a mouse or two in the traps.
If anyone is interested in developing my car corral into a commercial venture, I would be happy to partner and advise, do testing, etc. (stromberg@berkeley.edu).
* Stromberg, M. R. 1979. Experimental analysis of habitat performance and observations of deermice (Peromyscus) in southern Wisconsin. Ph.D. Dissertation, Univ. Wisc. Madison. 224 pp.
  | corral gravel Front corner; see also the photo below. This could be one, continuous piece of flashing as it doe not need to come apart when you move the car in or out. Gravel corral front |
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Corral Gravel Rear Rear of corral. This rear board is removed each time the vehicle is take out of the corral. I just set it aside and then put it back later, tamping in the gravel where the wheels have pressed it down, so there is no tunnel under the board. |
Others have built variations on this theme: here are some photos people have shared with me:
Mouse corral from the front. The challenge was to design the corral so it would fit our narrow carport and still allow the four side doors of the Prius to be opened. Removable lightweight side panels, each about 7 feet long, were our solution. A small cell phone antenna was attached to the Prius’s front license plate frame, and a flexible rod painted with fluorescent paint was mounted inside the front center post of the mouse corral, making it easy to center car in corral and know where the front bumper is.
Rear removable gate of corral
Corral with side panels and gate removed
Side panel removed so that both front and
back doors can be opened
Passenger side panel in place; note dark
brown steel flashing on lower edge, contacting the cement.
Construction of side panels: made of rigid closed cell foam with “wings” for stiffness made of galvanized sheet metal, covered on the outer surface with aluminum flashing (gold color in the above photo). All glued together with foamboard adhesive, no hardware. The glued on flashing adds to the rigidity of the foamboard. Each side panel weighs only a few pounds, can be lifted with a couple of fingers of one hand. The bottom edge of each panel has had steel flashing cemented on, so each panel sits on the edge of this steel flashing, as visible in preceding photo.
The gate is also made of closed cell foamboard. Bottom of gate is a pressure treated 2×4; the gate’s framework is lightweight wood. Gate is light, can be picked up with one hand.
Inside construction of gate
and one in Cave Creek Canyon, in the Chiricahuas, not far from the Southwestern Research Station