Adventures in Commensal Rodent Management & Control

This article, by Adette Quintana (initiating author) and Jerry Cates (editor and principal contributor), was begun on 13 November 2020, first published on 14 November 2020, and last revised on 18 January 2021. © Bugsinthenews Vol. 21:11(01).


Summary: In the process of our ongoing research in rodent biology, we often encounter cases that don’t fit the general mold. Our attempts to square the circle on those outlier cases teach us great lessons. In this article we share those lessons with the public. This article, still in it’s infancy, will be continuously fleshed out in the coming days, weeks, months, and years.


Before we delve into specific case histories, we should briefly discuss the biological features of the rodents under investigation here.

North America hosts three species of commensal rodents. These are, specifically, the common house mouse (Mus musculus Linnaeus, 1758), the black rat (Rattus rattus Linnaeus, 1758), and the brown or Norway rat (Rattus norvegicus Berkenhout, 1769).

Taxonomically, each of these mammals is a member of the family of rodents (in the order Rodentia) known as the Muridae (from the genitive form of the Latin root mus = muris, having the meaning “mouse”.) The Muridae is the largest family of rodents and mammals, and embraces 700+ species of mice, rats, and gerbils, many of which are native to Eurasia, Africa, and Australia. 

Each of the above-mentioned commensals is also a member of the subfamily Murinae (comprising 519 species of rodents known collectively as murines). The Murinae was coined by the German entomologist and zoologist Johann Karl Illiger in 1811, to distinguish the Old World rats and mice from those of the New World (the Cricetidae, all of whom are native to North and South America.) The murines exhibit a molar pattern having three rows of cusps, as distinct from that of other muroids, which have but two. 

Though initially confined to their native lands, the three commensal rodents described here are now scattered almost worldwide. Each, in fact, is now absent only in the most forbidding of locales, such as Antarctica. 

Dispersal of these animals to practically all corners of the globe resulted from their affinity for human foodstuffs as sources of nutrition, and for man-made shelters as nesting spots. It is common today to refer to these as preferences, but that does not appear to be the case. Neither human-related nutriment nor man-made abodes constitute the primary choices for these rodents, yet they readily adapt to both in order to secure year-round sources food and lodging when naturally-occurring food and habitat is scarce or non-existent. 

This innate adaptability, combined with an uncanny ability to remain hidden from sight, led their human benefactors to take them wherever humans travelled. This the latter did mostly without intending to do so, as commensal rodents have been recognized throughout history as pernicious pests.


The case histories posted below are true and accurate accounts of difficult to resolve rodent infestations in residential and commercial structures. Lessons learned are analyzed and described in detail. Identities are redacted to protect our clients.


Case History 1068:

This client called on EntomoBiotics Inc. in the early years of the new millennium, seeking our assistance in dealing with an on-going rodent issue at the commercial structure he had purchased a few years earlier. There he had operated a successful service-oriented firm and rented out office space to a list of other professionals.

An Unusual Structure with an Ordinary Problem…

This was an oddly-constructed structure which, though in excellent condition, was already several decades old. What made it odd was its unique spatial arrangement. It extended outward, horizontally in space over a dry river bed that in normal times became flooded several times a year. Suspended on reinforced concrete piers some 25 feet above the river bed, it remained dry even during high-water conditions.

The parking lot in front of the building was connected to the structure via a broad walkway. Beneath was strung a series of water, electrical, and sewer pipes to and from the street, under the parking lot, and into and out of the structure through its underside. Each pipe was fitted with a rodent exclusion flange where it transited from the walkway to the building.

Rats inside this structure were making noises in the ceiling above the owner’s head when he called. In fact, rats had historically managed to invade this building with uninterrupted regularity, for as long as the owner had knowledge. The owner’s receptionist was married to a pest management professional who provided general pest control services here. That gentleman had no experience in rodent control, and his meager efforts in that field had not been successful.

All that was Needed, we surmised, was a Simple, Two-Step Process…

When we first sought to assist this client, our experience in rodent control, though measurably greater than that of many pest control firms, was still sparse. True, we had practiced pest management in Texas for several decades. During that time we’d successfully performed rodent control hundreds of times. Still — like most of our peers — we viewed “rat killing” as uncomplicated intuitive work that did not require unusually specialized knowledge. It invariably followed, we naively believed, a simple two-step process that combined mechanical exclusion with the strategic placement of rodenticides. So, we thought, solving this building’s rat problem would be a snap…

Our inspection of the building’s basic shell — focused on its underside and outer walls — found no obvious portals rats could exploit to get inside. For us to gain access to the void directly under the walkway would have required specialized climbing gear, and since the rodent exclusion flanges were visible and clearly in place, inspecting that area appeared unnecessary. Accordingly, we scoured the structure’s roof, expecting to find one or more open access ports aloft. Sure enough, we found several glaring openings where the eaves met the roofline. That confirmed our deepest suspicions, that the rats inside the building were coming in via the roof.

Mechanical Exclusion & Rodenticides should Do The Trick…

We immediately sealed these open access ports with 1/4th inch hardware screening. Then we recommended that the receptionist’s husband obtain some of the better versions of the rodent bait stations then on the market, that he then select one or more of the rodenticide baits available, place those baited stations at intervals around the building, and service them on a regular basis. At that point, thinking our job was done, we issued an invoice for our services. Now we could turn our attention back to all those other clients of ours, the ones with “more complicated” pest issues.

Time passed. We made a few gentle inquiries of the client via email, asking how things were going, but received no replies. Evidently, all was going well…

Then the Client Called Again, with a Shocking Revelation…

Years later, however, this same client contacted us again. He began the conversation with a startling message. Never, he told us, in all the intervening years following our roof exclusion work, had rodent activity in his building fully ceased. Yes, they’d appeared to be under control for a short while after we’d left, but — before long — they’d returned with a vengeance. No longer trusting us to solve the problem, he’d called in a long list of other pest control firms — including the “big boys,” touted to be the most professional, scientific-oriented firms in the world — in hopes that one of them might bring along the magic he and his building needed for a permanent fix.

Each of those companies used rodenticides and “more extensive” exclusionary procedures to bring the rodent invasions to a halt, but to no avail. Unfortunately, many of those control episodes were also accompanied by noxious odors emanating from the building’s ceilings and walls. Certain tenants, tiring of those nasty smells, were threatening to move out, and who could blame them?

After all those years of battling rats, time and time again, with not one glimmer of success, the owner felt his only option was to sell the building and move his company elsewhere. But how in good conscience could he do that? Unless he could solve this problem first, whoever bought that building would face the same seemingly intractable situation…

Ready to Sell the Building, He Gives Us Another Chance…

Firmly ensconced upon the dreadful horns of this dilemma, he came across an article we’d published about a string of restaurants we’d freed of chronic rat invasions after a long list of other pest management firms had failed. Yet we — just like those other companies — had failed him, too. What was the likelihood we’d do any better now? Hoping we’d learned a few things in the interim, he’d picked up the phone and dialed our number.

He was right, of course. We had learned a lot about rodent control since servicing his business years earlier. If you think, however, that we’d learned enough to immediately execute a permanent fix to his rodent invasion, you’d be wrong. We still had a lot more to learn, and his case gave us the perfect opportunity to do just that.

Great Lessons from the Recent Past…

One of the great lessons we’d learned from that string of rat infested restaurants alluded to earlier was that exclusion, though important, is not the cure-all many pest management firms tout it to be. In fact, in the vast majority of cases exclusion — particularly when the focus is on portals aloft (e.g., on the roof) — is way down on the priority list. It isn’t so much that closing up all the gaps in the roof isn’t important, because it is, particularly if the gaps involved are relatively easy to find and seal.

The problem is, no matter how hard you work to make sure all the gaps are sealed, Father Time has a way of opening up old seals and making new ones where you’d least expect them. Besides that, many of the gaps in the roof actually serve a useful purpose, in that they help ventilate the home’s attic space. The better the attic is ventilated, the less moisture collects in places where wood rot can take place, so aggressive sealing projects can and often do result in more harm than good. If it is possible to let the lesser gaps remain without creating a recurring rodent invasion of the attic, that’s the best way to go.

Nature-based vs. Man-made nesting habitats…

But is that possible? Based on a hypothesis we’ve developed over the past few years, we think it is. That hypothesis goes like this: rodents, including the commensal species Mus musculus, Rattus rattus, and Rattus norvegicus, prefer to nest in nature-based habitats over man-made ones. We arrived at this conclusion based on repetitive observations that these rodent species only appear to invade man-made structures when (1) they are forced out of over-crowded nature-based habitats located nearby and (2) subsequently fail to find suitable nature-based habitat in the vicinity of those man-made structures that they can move into.

In other words, man-made structures are not the first choice of these rodents when they are looking for a place to nest and raise a family. If they can find nature-based habitat instead, they prefer to make their nests there.

But, Why?

This begs the question of how such a choice preference came into being. The evolution of rodent behavior is invariably driven by factors that improve propagation. Nature-based nesting habitat would have to confer benefits favoring survival that are lacking in nesting habitat within man-made structures. We recognize that rodents have adopted a repertoire of behavior patterns that aid them in hiding their presence for that very reason. Simple logic suggests it is easier for rodents to hide in nature-based habitats than in man-made ones.

If the forgoing is correct, rodent control methodology is likely to be misplaced when it focuses exclusively on exclusion of rodents from man-made structures. Instead, it should begin with eliminating rodent nesting activity — maternity nesting in particular — within all the suitable nature-based habitats proximate to those man-made structures. Eliminating the former automatically protects the latter. If over-crowding of the preferred nature-based habitats never occurs, concern about any consequent secondary invasion of nearby man-made structures becomes moot.

Based on that hypothesis, the comprehensive rodent control program being developed by EntomoBiotics Inc., known as E2M2C(TM) and described in more detail HERE, and HERE, is oriented primarily toward eliminating all the rodent maternity nesting within the grounds of the sites we service.

To accomplish that we first place a set of specially designed, tamper-and-weather resistant rodenticide dispensers around the perimeters of all the man-made structures we are charged with protecting from rodent invasions. We then service those placements according to a protocol that enables all the rodents foraging there to easily find and consume palatable but lethal dosages of rodenticide throughout the program’s service interval.

The E2M2C(TM) Program Worked, Just as Expected…

We placed a set of E2M2C(TM) stations around the perimeter of this building, including the area below that was subject to flooding. Immediately the rodents inside the building stopped making noises in the ceiling areas, proving that the stations had done their jobs well.

We were not surprised, as that’s the usual experience with these stations. They are specially designed to make rats and mice gravitate to and enter them the moment the rodents see them. Once inside, they find fresh, clean and tasty rodenticides, in several different flavors and consistencies that are tailored specifically to please the most sensitive of rodent palates. Rarely does a rat or mouse enter one of these stations leave without first consuming a neutralizing meal.

The business owner thanked us for solving his rodent problem, and agreed to keep the E2M2C(TM) program in place. We had explained to him that, just because the maternity nests inside the building and in the grounds had been neutralized, his rodent problems were not permanently resolved. Newly arriving rodents, from surrounding areas, would soon be establishing new nests in the grounds if the stations were not kept in place to neutralize them before they could set up housekeeping. Once that took place, it would only be a matter of time before they invaded his business structure again. The stations would prevent nesting in the grounds, and by extension, would also prevent nesting in the building…

As usual, we swapped out the originally placed stations, 30 days later, with freshly sanitized, fully re-provisioned stations. The old stations were then bagged up and sealed, after treating them with a miticide/insecticide that neutralized all the mites, fleas, and ticks that visiting rodents may have left behind. This ensured that when those bagged stations arrived back at our laboratory, they would not have any live parasites inside. Protecting our lab personnel from the risks those parasites pose is imperative.

And Then, It Failed…

The weeks that went by, with no more rodent activity being reported at this business, just confirmed how well the installed E2M2C(TM) program was working. Then, out of the blue, the business owner called to report bad news: “The rats are back!” he exclaimed.

We installed wildlife cameras in the ceiling void and confirmed that, yes, rats were back in this business structure’s attic. Our first thought was that possibly there were so many rodents in the neighborhood that all the rodenticide in the stations had already been depleted. This could be easily remedied by replacing the stations in place with fresh ones. We did that, and in a few days the noises stopped in the ceiling, and our wildlife cameras stopped imaging live rodents. Yet, we were puzzled to find that when we opened the used stations back at the lab, less than half the rodenticide had been taken.

And failed again…

Still, more weeks went by again without any new rodent incursions, leading us to suspect the earlier, unexpected infestation was a fluke. But, no, it wasn’t. in less than two months, it happened again. And, then, again!

The business owner was now at his wit’s end. We’d failed him again, just like we had unknowingly failed him years before, and just like all the other pest management firms that he’d hired in-between.

This situation was disappointing. Still, undaunted, we thrive on solving vexing mysteries. On the wall in the hallway at our laboratory hangs a quotation from Carlos Castanada’s 1974 book, “Don Juan, Tales of Power,” that we take to heart every day:

“The basic difference between an ordinary man and a warrior is that a warrior takes everything as a challenge while an ordinary man takes everything as a blessing or a curse.”

Viewing this as a curse was never an option. As a challenge, it made us take a more concentrated, in-depth review of the ecosystem in which this business was immersed.

Re-evaluating the Surrounding Ecosystem…

The E2M2C(TM) program’s success is based on placing and servicing a set of specially designed rodenticide & pesticide dispensers within an ecosystem’s wildlife foraging zone. Those dispensers must be so constructed as to attract targeted pests as soon as they enter that zone, so that they enter the dispenser right away, and on entering are provided with effective, essentially irresistible neutralizing agents. Those dispensers must also be capable of protecting those agents from the elements so that, for as long as six months, they remain as effective and irresistible to the pests they target.

The E2M2C(TM) stations we placed at this business have confirmed, all over Texas, their ability to meet all the criteria listed above. Why didn’t they work here? Our answer: they must not have been properly placed within the ecosystem surrounding this business. Yet their rodenticide provisions were being consumed, and those provisions were only accessible by rats and mice, so at least some of the rodents that foraged here were being neutralized. Just not all…

Missing An Important Rodent Subset…

In other words, we were missing a subset of rodents that were arriving here and getting into the business without foraging in the normal way. If they were foraging normally, they would have found the E2M2C(TM) stations. And, like all the other rodents that dropped by, they would have been neutralized shortly afterward.

They must be getting into the building through a previously unknown route. Yes, there could be other possibilities, too, but most of those involved a failure of the E2M2C(TM) stations to work as designed, or of the rodenticides in them to properly attract or neutralize the rodents that entered them. We soberly considered all those other possibilities. In the end we discounted them — at least in the interim — simply because our experiences all over Texas testified to the ability of these stations to both attract and neutralize every rodent that happened by, for as long as four months after the stations had been deployed, even in harsh, challenging environments.

So, we set out to find the unknown route…

As mentioned earlier, this building projected out, on stilts, over a normally dry river bed. Underneath, attached to four of the stilts, was a HVAC bay some 10 feet off the ground. Electrical cables and coolant lines connected the HVAC compressors to the building interior through ports in the building’s underside. Some of those ports were large enough to permit rodents to pass through them. Could this be the previously unknown route?

E2M2C(TM) stations had been placed on this HVAC bay for months, in greater numbers (sq. ft. per station) than for the perimeter of the building itself. That argued against this bay as having a part in the previously unknown route, so, after a careful review, we discounted it entirely.

Revisiting the Walkway Void…

That left only one possibility. The void under the walkway connecting the front parking area to the building also provided a cover for underground electrical, water, and sewer pipes that emerged from the soil beneath the parking area and projected forward, under the building, and then turned upward to pass through ports in the building underside to service the structure with important utilities.

As noted earlier, each of these pipes passed through intact rodent exclusion flanges that appeared to be so well constructed that no reasonable investigator would expect rodents to be able to circumvent their intended purpose. For that reason we discounted these pipes, early on, as being the route by which rodents could have entered the building in the normal fashion. By “normal fashion” we mean by traveling over the pipes in the way rodents do while moving from one place to another. But, in the interest of conducting a full investigation, we examined these flanges once more.

Were The Rodent Exclusion Flanges Defective?

As expected, we found the rodent exclusion flanges to be in good repair. No rodent could travel past them. But we also noted that several of these pipes, particularly those that served as conduits for electrical cables, were disconnected from their couplings at various distances from the flanges. The electrical cables in them appeared, at first glance, to nearly fill the pipe voids, but on closer examination — conducted after donning climbing equipment and venturing into the hollowed area under the walkway — we found that the cables in two of the conduits left enough space for a medium sized rat to pass into it.

In all honesty, that seemed an unlikely route for a rat to follow. If one tried, the adventurous rat would have to pass through nearly 50 feet of dark conduit, with barely room to squeeze through, in a void presumably filled with stale air, before entering the building itself. What are the chances? Slim… indeed. Yet, it was now the only genuine possibility we could find. So, we sealed the open conduits, both at the ends projecting out of the soil under the parking area, and the ends leading through the exclusion flange and into the building.

Finally, Success…

Shortly afterward, all rodent activity inside this business ceased. It has now been several months since, with no further rodent activity. To our knowledge, this is the first time in 15 years that no rodent activity has taken place in this building…

In summary, at least at this point in time, we are forced to assume that the rats getting into this building were arriving there via underground electrical conduits. These conduits emerge from the parking lot, under the walkway. They then pass through the rodent exclusion flanges and, from there, into the underside of the building.

It defies credulity to believe that rats would use this route to enter the building, and that they had been doing so for over 15 years, during which time this route of entry was never discovered. Intrigued, we conducted additional investigations to help explain how this might have taken place.

The Rats That Defied Credulity…

When we traced the electrical conduit pipes that extended from the flange to the underside of the building, one of them was found to be uncoupled just before it entered the structure. The air in this pipe would not necessarily be stale, and might even be subject to frequent flushing by the winds that passed under the building. Rats entering the conduit at the walkway side of the rodent exclusion flange would find the environment in that pipe somewhat, if not entirely, habitable. Once they got to the distal end where it decoupled just before entering the building, the rodent would be prepared to continue upward, into the remainder of the conduit, to enter the building.

But did those rodents come from the grounds, or from the pipe emerging from under the parking area? To test this question, we placed wildlife cameras in the void under the walkway to see if rodents were inhabiting that area. Remember, we had already sealed the conduit pipe, both the end emerging from the parking area, and the end extending through the flange to the underside of the building, so any rodents that showed up on the wildlife cameras would have to come from the grounds.

Raccoons, an opossum, a ringtail, and rock squirrels were imaged under the walkway over the next three weeks. Not a single rat passed through that area. By inference, then, we conclude that (1) the rats that had been entering this business came from the pipe emerging from under the parking area and not from the grounds near the building, and (2) the E2M2C(TM) stations presently in the grounds are working at peak efficiency, inasmuch as no rats entered the walkway void from the grounds during the surveillance project.

More to come…


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