VERTEBRATE PESTS: Getting To The Bottom Of Slab Mice

As the house mouse’s name implies, this little mammal typically establishes its nest within structural elements such as walls, ceilings, floors, voids, appliances, equipment and stored materials.

But the house mouse also readily nests and harbors in the soil beneath and around buildings. House mice will populate the slabs beneath many different kinds of buildings. I call these "slab mice."

Slab mice can be particularly troublesome and expensive to clients in the food and health-care industries. This is because sub-slab mice forage above ground and tear into and contaminate food bags, boxes and containers of all types. Even if the food is not attacked directly, the mice may deposit fecal pellets onto the floor or onto bases of pallets. When the pallets are shipped out they may be subject to expensive rejections whether or not the product sitting on the pallet is contaminated.

Are slab mice any different from the mice that nests above ground in the structure or in equipment and furniture? Or are they merely adaptable opportunists taking advantage of sub-slab harborage spaces? Although it is possible certain populations of mice may be genetically or behaviorally inclined towards subterranean burrows, I think it’s more opportunity than biology.

The house mouse is a mammal that has its origins in the open grassy fields of central Asia where it naturally occupied subterranean harborages of varying types. Moreover, house mice exist in fields and wooded areas outside and disperse into our buildings on a regular basis, depending on local environmental factors.

SLABS AS A HARBORAGE RESOURCE. Once inside slab-constructed buildings, the mouse gains entry to the subterranean soil via the expansion joints or via some break in the slab. In some buildings, sub-slab areas offer the mouse superior harborages compared to those found above ground. In heated buildings, these earthen areas can be relatively warm in winter and cool in summer. Sub-slab burrows also offer exceptional protection. Consider the vulnerability of the mouse when it attempts to nest inside equipment, boxes, pallets and other items subject to disturbance by the mouse’s primary predator inside buildings (i.e., humans).

The amount of "real estate" available for the mice beneath industrial-sized slabs is relatively unlimited as compared to the above-ground harborage resource. Inside grocery stores or food distribution centers, the food resource sitting directly upon the slabs is also virtually unlimited. Mouse infestations in these accounts can become chronic and widely dispersed.

The biological and behavioral components of sub-slab mouse colonies and their sub-slab environment has not been scientifically studied (e.g., length and depth of burrows, nest locations, storage chambers, entrance sites, etc.). And unfortunately, this information is not likely to be easily gathered — at least not under real-world conditions.

Good research does exist, however, on the burrowing dynamics of the house mouse in field conditions, collected by researchers at Southern Illinois University. In a paper published in the American Midland Naturalist journal in 2001, researchers released house mice in outdoor enclosures and allowed them to dig into the earth and construct burrows and nests. In this study, the mice constructed earthen burrows ranging in length from 4 inches to 27 feet. The burrows contained one to seven entrance holes and had zero to five chambers that the mice used for either nests and/or for storing food reserves gathered from above.

How similar the burrows of sub-slab mice are to the natural earthen burrows is not known. But they likely vary from similar to radically different, depending on the nature of a given slab and building.

Recall that the fill beneath commercial slabs sometimes settles at various points, creating relatively large voids (like mini-caves) between the top of the fill and the bottom of the cement void. (It is these voids that we use foam formulations of termiticides). Thus, beneath industrial slabs, there may be few or even no "burrows" per se. Instead, regularly used "pathways" likely exist along the top of the fill that connect to exit and entry holes. Nesting and storage cavities may be dictated by the availability of settled gaps in the fill. Or the mice might supplement such gaps by digging or enlarging existing gaps and spaces nearby the entry holes.

In buildings that contain sub-slab utility tunnels, it is common for the mice to use these tunnels extensively for both nesting and travel highways.

Although the biological and behavioral aspects of sub-slab mouse populations is an interesting topic in itself, there are several important considerations relative to managing these mice.

MANAGEMENT CONSIDERATIONS. First, sub-slab mice pose challenges to pest professionals when their populations become large and widely distributed beneath the slabs of warehouses, large-scale distribution centers, etc. If these populations are not nipped in the bud, they can pose substantial costs and embarrassment to the client. Managing slab mice requires a proactive approach from both the client and pest professional.

This begins with proactive inspections. For example, it is common for the entry holes of slab mice to be located in hard-to-see or concealed spots within industrial accounts. In supermarkets, for example, mice foraging beneath the food gondolas in the aisles may be accessing the floor and shelving from cracks or joints in the slab covered over by each aisle’s shelving units.

In warehouses and distribution centers, the entry holes into the slabs may be located in any one or all of the following hard-to-see areas: 1) along perimeter area wall/floor junctions; 2) directly beneath pallet storage zones; 3) behind support piers (at the expansion joint where the pier’s footing joins the slab); and, 4) in the dark inter-aisle space that exists between two different aisles (i.e., where product pallets meet back-to-back).

For pest professionals, it is critically important to note that slab mice living in the interior portions of stores and warehouses are not subject to the perimeter wall traps and bait stations. If not detected, these slab mice colonies will continue to increase until an "incident" occurs involving the client’s product. By this time, resolving the problem can often be expensive for both the client and the