[Vertebrate Pests] House mouse condominiums

On your pest management route, how often do you pause to actually note the type of walls that occur within your client’s buildings such as warehouses, residential homes, apartment complexes, restaurants, schools and the various other buildings you service? Making such a wall-type observation is important in nearly all pest management service work, but it is particularly important when you are on yet another mouse callback to the same client.

And among the different types of walls, concrete hollow block walls (CHB) commonly serve as essential house mouse condominiums. So recognizing the presence of CHB and understanding their makeup and structure relating to pests can save you time and solve chronic pest problems.

But what is it about some concrete hollow block walls that make them so attractive to mice? Let’s take a look.

1. Prevalence. Over the past five or six decades, millions and millions of our structures — commercial and residential alike — have been built with concrete hollow blocks. Take a look around and you’ll see CHB everywhere. Mostly, CHB is used for supporting foundation walls, but block construction also is used for interior partitions and load-bearing walls.

So when mice invade buildings from the outside, or when they arrive inside as delivery stowaways, a concrete hollow block is usually not far away.

2. Easy and multiple access. Because of the “porosity” of the concrete, CHB provide pests with quick access to the block interiors and easy travel along the block surfaces. Rodents have body shapes, legs, feet and claws adapted for climbing, grasping and clinging. It should be obvious, then, how easily rodents can dig their sharp, pointed claws into the rough, porous block and travel up, down and over the blocks with speed and agility. This allows the mice to leave the nest, travel to food sources and return back to their hidden concrete hollow block sanctuaries with ease.

Even in brand new buildings, mice can climb and then access block wall interiors from the top-down because rarely are the top-most blocks adequately sealed (i.e., mouse-proofed) by contractors. Oftentimes, in fact, mice have moved into the block during construction phases as the block goes up in segments and well before the building is completed.

Additional access points for rodents include the cracks and breaks so commonly associated with CHB mortar joints. This is because block mortar typically only has one-third the strength in tension as concrete. And finally, pests often enter CHB walls via the gaps left unsealed around the many utility penetrations (gas lines, electrical, plumbing, cable, air-handling systems) placed through these walls.

3. Space Efficiency. Just as is true for human condominiums, concrete hollow block walls offer mice excellent space efficiency. Many voids (i.e., mouse apartments) exist within CHB in relatively small areas. As a result, many mouse families can exist within relatively small sections of hollow block walls. Consider a wall only two blocks wide by 10 blocks tall. For large commercial structures built with thousands of individual blocks, harborage is virtually unlimited. In fact, in poorly maintained commercial facilities (i.e., poor sanitation leaving many food scraps for mice), that contain CHB construction, it is easy to understand how we periodically encounter incredibly huge mouse infestations.

4. Good thermals. Due to the design of concrete blocks (i.e., air pockets and voids within each block’s walls), and the specific formulations of concrete, hollow blocks provide low thermal conductivity and high thermal capacity. No surprise then that mice and humans find this building material ideal for their nests.

As a mouse family huddles within a block void, the void holds the heat generated by the family, keeping them warm. Mice can (and do) increase their thermal needs by lining the block voids with all sorts of insulating materials they collect inside the building such as plastic wrap, paper, cardboard, Styrofoam, wrappers from snacks and candy, etc. As mice need to conduct climate control on the nest, insulating materials can be discarded, or pushed into other areas. Mice can also shift among walls and wall sections as per daily or seasonal temperatures rise and fall to suit daily temperature needs.

5. Easy housekeeping. As one nest hollow becomes fouled with feces, urine, carcasses and perhaps rotting food remnants, mice quickly can switch to a brand new apartment, perhaps next door, or below or above. Maintaining clean nests for family and colony health is an important behavioral trait among most land mammals. And with mice, if harborages are abundant, abandonment of fouled nests for new and clean nests is common.

6. CHB offer quiet concealment. If you have done enough rodent jobs, you come to learn that rodents often nest in locations that are not subject to daily disturbance and movement. This is why you find mouse nests in those forgotten boxes on the top of shelves that haven’t been moved for a year, or below a cabinet base, and so forth. For prey species, being able to move in and about nests completely protected from their enemies is a big plus.

Within hollow block walls rodents may be able to move from room to room, floor to floor and among different building areas completely protected from their urban predators (e.g., people, cats, dogs, hawks, owls, etc.).

7. Other factors. Other factors may also play a role in why concrete hollow block serves mice so well. For example, perhaps the porosity of the blocks also serve as “pheromone sinks”. That is, like heat sinks, certain blocks may retain and liberate a rodent’s or a rodent colony’s specific pheromones much in the same way wooden hollows, rocks and soil are marked in the wild by rodents to aid them in different ways.

ON-THE-JOB APPLICATION. Our job as pest professionals (i.e., not simply trap or bait applicators) is to, whenever possible, locate and eliminate the sources of pest infestations. This is particularly important with the No. 2 most successful mammal on earth — the adaptable mouse. Mice are highly successful at adapting human structures into harborages to substitute for their natural harborages of hollow logs, rock crevices and earthen nooks and crannies.

So, whenever you encounter a persistent mouse problem in a building containing block walls, here is “a GPS moment” for helping you to zero in on one of the more common sources of wall infestations:

1) Locate those walls within the areas where mice are being reported that are the warm walls due to either nearby equipment or appliances such as ovens, dishwashers, motor equipment, etc., or due to direction (i.e., south and west walls).

2) Once there, zoom further in on which areas nearby those walls mice can readily access food.

3) In the warm-wall food zone, let your eyes search for line penetrations into the block wall.

4) Inspect around the line penetrations for the classic “rub marks” that reveal the travel ways of the mice

5) If no penetrations exist, and/or all penetrations are tight, inspect the top of the CHB walls to see if the block is capped (i.e., the top blocks are filled in thoroughly with cement), or whether or not they are open or contain gaps allowing for pest entry.

Once you locate the source, snap traps and baits can be used to control the mice depending on each situation (tracking powders are not advised). But more important for long-term control, you need to advise the client to mouse-proof their concrete hollow block walls, either by ensuring all line penetrations are completely sealed off and escutcheon plates installed, and/or by having the tops of the blocks capped off thoroughly. In both cases, it would be ideal if the building owner sought the counsel of us, their pest professionals, in ensuring the walls are proofed correctly.

The author is president of RMC Consulting, Richmond, Ind.