An example of incorrect rodent proofing. Gaps under doors that measure 1/4-inch high, or openings 3/8 inches wide, can permit mice to get through. Such gaps should be sealed with a true pest-proof door sweep, NOT a common vinyl weather strip as shown here.
Matt Frye, NY State IPM Program

The most important element of rodent management is exclusion. If a structure is fully protected, whether by sealing holes or fixing weather stripping, it often alleviates the need for catching, trapping or baiting. The industry’s most well-known rodentologist, Bobby Corrigan, knows that it’s this aspect of the battle against rodents that the layperson is less tuned in to, and more often negligent in considering. Corrigan offers the essential fundamentals of exclusion while providing important tips on how PMPs can fine-tune their exclusion skills.

CAPABLE ADVERSARY. “It’s amazing how intelligent rats are in figuring things out,” Corrigan says. “We’re learning more and more that these animals are clever explorers and once they lock on to food by smelling it, or they find a good harborage, they become highly skilled gymnasts.”

Corrigan says, that unlike many pests, rodents have the smarts to not only utilize tools, but also make decisions. This is due not only to their innate cleverness by design, but also by their biology and anatomy. But despite their skills as explorers, Corrigan is quick to remind technicians that rodents do indeed leave behind evidence that can help thorough PMPs locate weaknesses in a structure that provide rodents entry and clues to how those weaknesses are exploited. But one cannot expect the customer to see what a technician is trained to spot.

“I’m always stressing in my training that since we’re hired and we’re the professional we have to have vision that obviously the public does not have,” Corrigan says. He offers the example of a restaurant’s gnawed-away weather stripping, where rats have created a point of entry and how in most cases it will go unnoticed by building custodians. “The rats are gaining access there, going in, getting what they want and then going back to the sewer. We are trained to see it. They are not.”

This is not a problem exclusive to the United States, but all over the world, Corrigan explains. “Human beings are not able to grasp that if you keep them out in the first place, then you don’t have to catch, bait, trap and so forth later.”

TROUBLE SPOTS. Corrigan offers what he says are the four primary trouble areas rats and other rodents are expertly adept at exploiting. “First, any gaps at the ground level,” he says. Those are the areas most overlooked by the layman and amateur technicians. But a good PMP knows to look high as well as low, because, as Corrigan explains, rooftops are another common point of entry.

“Rats, of any species by the way, are adept at climbing buildings and getting up on roofs where they can come in from the top down, rather than from the ground up,” he says.

Doors, also at ground level, are often obvious culprits for allowing rodent entry, usually because they’ve either not been pest proofed, or they’re being left propped open by residents or employees.

The fourth possible trouble area covers a wide range of ways in which a structure’s integrity has been compromised. “Who knows how many points of penetration we have in commercial buildings or homes,” Corrigan says. “These can be any hole that goes right through a wall that’s not sealed correctly. Rodents are going to find that, and they’re going to take advantage of it and enter.”

Inspect areas where pipes and wires enter buildings to determine whether pests can enter around them.
Jody Gangloff-Kaufmann, NY State IPM Program
TOOLS AND TRAITS. A rodent’s anatomy not only gives it the ability to explore and infiltrate small obscured places, but is also responsible for providing the trained eye of a technician the evidence they’re looking for when tracking the possible invaders. Corrigan explains rodents’ most helpful characteristics, anatomically speaking, are their pointed muzzles and their body shape.

“Their muzzle comes to a long point, and tapers down,” Corrigan says. “And at the edge of that are very sensitive ‘nerve hairs’ if you will, that help them determine what they’re going to do — how to proceed.”

In addition to whiskers, Corrigan says rodents also have more nerve hairs growing out of the top of their heads behind their ears. They also help rodents make “decisions.” He’s quick to point out that this should not be dismissed as simple animal instinct. “We’ve learned that it’s more than instinct that tells them to decide when to go forth or not go forth into a hole, crevice or crack. We now know it’s something beyond instinct.”

So, the muzzle of a rat has more utility than that of a sensory area. It’s also, Corrigan explains, a tool to assist in choices for burrowing. “In the wild rodents use the muzzle to locate workable spaces beneath something heavy such as a rock,” he says. “They bring this same ability to everyday doors where they might be able to gain access beneath.

“In the wild rodents use the muzzle to create a space beneath something heavy such as a rock,” he says. “They bring this same ability to the city where there are structures they can get under. They try to burrow beneath heavy objects because of the protection they offer.”

But if a rodent can’t find a way under, to gain access to an area, they have other powerful tools, their incisors, that they use to find alternate entrances. Corrigan explains that a rodent’s incisors are powerful and can create access where access didn’t previously exist. For example, in a space around a structure where wood or other building material meets pavement, it’s impossible to burrow down beneath the structure to get in. In this case, the incisors can be used to gnaw away at the building material where it touches the ground, chewing to create a hole big enough to accommodate its passage.

And as Corrigan points out, those holes need not be big or even rat-sized, because of how the rodent’s anatomy allows it to squeeze its way into areas much smaller than its full body size.

“They have flexible tube-shaped bodies,” Corrigan says. “Their shape and flexibility allows them to squeeze through tight areas you wouldn’t expect. However, this is not to say that rats can “flatten” themselves out to squeeze under doors or other shallow horizontal spaces. “Because they’re flexible in the same way we are. Their backbones allow them to twist and turn and get the job done.

“The rule of thumb is, if the head can get through, the rat can get the rest of its body through.”

EVIDENCE OF INTRUSION. Once a PMP has trained his or her eye to identify a rat’s point of entry, there are other clues to be sniffed out, which can provide evidence of activity.

“Someone might see a hole and say, ‘How do you know that hole is still active? What if [the rats] built it there four years ago?’ Well, there are other signs that will point to whether or not the hole is in current use.

One, perhaps subtle, clue can be found in the area around the hole. If the hole is gnawed out of wood where it meets pavement, Corrigan advises to look closely at the color of the pavement around the hole as it compares to the rest of the pavement. “You might see a dark color around the hole — a dark, smoky-colored area on the pavement. That’s a sure sign the hole is active now,” he says.

Openings around utility lines often provide access to buildings and wall voids.
MATT FRYE

It’s sebum, an oily grease excreted from the underside of a rodent, that accounts for the discoloration. “Once they’re pressed down against the ground to get into that hole, it rubs the sebum off of them and onto the ground,” Corrigan says, suggesting that a PMP might take a photo of the sebum-stained area to show the client. “Then you can say — look, you need our services.”

More intel can be gathered with a close look at the gnawed-away area. Corrigan suggests looking around the hole for the rodent’s incisor impressions to help you identify what kind of pest you’re dealing with. “Then, a pest professional should actually measure the width of the impressions. If they’re close to 4 millimeters across, it’s absolutely a rat. If 2 millimeters, it’s mice.”

WHY EXCLUSION? “Rodent proofing doors, and holes and gaps — is a job for professionals. But for us, once we agree to rodent-proof a door, we have to make sure we know what we’re doing,” Corrigan says.

Corrigan explains that not only is rodent exclusion important, it’s perhaps the most important aspect of controlling rodents, because whether it’s a big or small infestation, all it takes is one rat to bring much more unwanted things with it.

“Once they’re in, everybody is in,” Corrigan says. “They’re carrying fleas, ticks, lice and bacteria on them, so even though you may kill the rodent, anything of these things they brought in with them are still there. If a rodent is carrying a virus or a bacteria, of what logic is it to let them in your house or food plant or warehouse or restaurant, and to then try to control them after the fact?”

So even if a PMP manages to trap or poison the offending rodent, the question remains, how does one catch or contain any associated ectoparasites and pathogenic microbes that came in with the rodent?

Indoor infestations, for example, will still require careful cleanup, because even if the rodent is gone, droppings, urine, rodent hairs and decaying carcasses are still left behind. This, Corrigan explains, is why sanitation and pest control go hand in hand.

“Pest proofing is actually pest prevention and sanitation is pest control,” he says. “So if we keep things clean and clutter free it helps keep control.”

RIGHT & WRONG TOOLS. The tools of exclusion can be very effective if used properly, but as Corrigan explains you have to know what the right tools are before you get started. This might take some research on the part of the technician, but suffice to say that one key point to remember is that weather stripping and pest proofing are not the same thing.

“It’s amazing how people will just weather-proof doors as a way to keep out pests,” Corrigan says. “Weatherproofing materials are not made for this purpose, so they are completely inappropriate for this job.”

One commonly made mistake Corrigan cites is the use of heavy-duty vinyl weather stripping used as an exclusionary tool. A maintenance person is not trained to know these things, but Corrigan explains that simple vinyl used in even the strongest weather strips can still be easily chewed through by rodents. That’s why Corrigan suggests two specific types of rodent-proofing door seals: high-density nylon bristles and rubber-encased steel fabric (or RESF). “Nylon bristles are most effective for small infestations of mice,” Corrigan says. “But if it’s rats and mice, the rats will get through the bristles with no problem.”

Maintenance crews are not trained in pests. Even a heavy-duty strip of vinyl used as weather stripping can be chewed through by rodents. RESF sweeps, on the other hand, create several layers of challenges to the rodent which will effectively keep them out.

But even the best exclusionary tools are useless if not installed properly, Corrigan says. This means taking careful, accurate measurements of everything including any holes, entry points, door thresholds and piping hole gaps. A simple depth measurement of the opening in question can reveal whether the rodent being dealt with is a rat or mouse.

Stainless steel mesh fabric is the most effective material to keep rodents from entering through building holes and penetrations.
“On average, the depth measurement for a mouse is about 6 millimeters. For a rat, it’s 12 millimeters,” Corrigan says.

Foams are another material often misused by those who don’t know better. While they are ineffective for several reasons, they can be part of a bigger exclusionary strategy. Foam “plugs,” however, are terrible for exclusion when used alone because they’re not cleanable and their micro-crevices harbor germs, they’re porous, they degrade quickly, they’re easily gnawed by rodents and the expansion of foam blows out narrow board holes, widening them.

To use foam properly, Corrigan suggests this three-part strategy for holes in stone or cement walls of commercial accounts: “Put just a small amount of quality foam into the hole and then add a wad of stainless steel fabric so it’s enmeshed into the foam. Then finish with a dollop of quality sealant matched to that substrate.”

Pest proofing walls, holes, cracks and crevices requires that the PMP fill the hole. A heavy-duty sealant is what Corrigan recommends, though he is quick to point out that simple hardware-store caulk is not a sealant (even if it might say so on the label).

“Do your homework and know the difference,” he says.

The author is an Ohio-based freelance writer.