[Rodent Control] Exterior Rodent Control Programs

As discussed in Part I of this three-part series (August PCT, page 48), the conventional rodent control program for many food and pharmaceutical plants and warehouses involves surrounding the exteriors of buildings with rodent bait/trap stations. Sometimes, certain facilities will supplement their perimeter wall stations with additional stations positioned at various spacing along property lines. The property line stations may be situated along the inside of a property fence (called fencerow stations) or simply be installed along the property’s border line (called property line stations). (See photos on pages 116 and 117.)
As is true for exterior bait stations positioned along perimeter walls of buildings, federal agencies involved in regulating food safety in the United States (i.e., FDA and USDA), do not require that property line rodent stations be used. However, some independent inspection organizations and companies (e.g., American Institute of Baking, Silliker Labs, etc.), may strongly encourage the use of property line stations (ref. 1).
Thus, there is a wide range of differences between facilities that employ property line stations and those that don’t. The goal of part II of this article is to consider those factors that should be considered for whether or not property line baiting programs should be employed.

HISTORY. Interestingly, no research exists that demonstrates the value, if any, of baiting property line areas (relative to providing additional rodent protection to a facility) to that already provided by the perimeter wall stations.
This practice was established during the 1940s and 1950s for protecting grain storage silos and similar structures from rodents via the recommendations for controlling building area rats and peripheral area rats that might disperse to building areas from the peripheral areas over time (ref. 8). Recall that rats can disperse considerable distances in search of food and harborage as compared to smaller mammals such as mice, voles and shrews.
Eventually, these rat control recommendations became universal rodent control recommendations for the food industry and became known as the “three lines of defense” (refs. 3,10).

TIME TO RE-EVALUATE? There is no question that exterior perimeter wall bait/trap stations are an essential component of food safety programs for current day food manufacturing. And for some facilities, additional property line baiting programs may also be appropriate and necessary. However, it may be time to re-evaluate the practice of property line baiting programs as a general operational format for all plants.
There are three reasons for this. First, these programs may be environmentally detrimental. Second, property line baiting efforts may partially or entirely offer little additional protection, and thus be an unnecessary added expense to a client. And, third, these programs detract from the desire of a food company to implement a maximal IPM program that incorporates as few pesticides as possible.

ENVIRONMENTAL CONCERNS. Many large-scale food handling establishments (food manufacturing plants, super-size distribution centers, pharmaceutical plant “campuses,” etc.) built in the United Sates throughout the past 50 years were careful to select their property areas in which to construct their plants. Obviously, a large amount of space is necessary for these types of operations. Often areas incorporating 25, 50 or more acres are needed. (See photo on page 116.)
For security reasons, the entire property of most facilities is enclosed within a fence. The distance from the property line fence to the facility’s building perimeter can range from the width of commercial truck driveway to several hundred yards. (See photo on page 117.)
With the exception of those facilities located in completely urbanized locations, those areas along the property border and fencerows are not usually subject to the same level of vegetation management and manicuring as the building’s immediate exterior. Thus, vegetation in these areas typically remains and proceeds undisturbed through its natural seasonal cycles of growth.
The vegetative environments of these areas may include grasses, shrubs, bushes, trees, rocks, boulders and a wide range of soil profiles. As can be seen in the photos on pages 116 and 117, it can range from a grassy field to a woodlot. In some cases, the property line may include waterways such as drainage creeks, river embankments or even seashores. Certainly, these different environments have established local ecosystems of their own and contain wild mammals, birds, reptiles, amphibians and invertebrates.
When bait stations are installed in such areas, they are for the most part designed to exclude larger non-target mammal wildlife species (e.g., dogs, cats, foxes, coyotes, most birds, etc.). But what about smaller mammals that can enter these bait stations as readily as target rodent pests (i.e., the house mouse, Norway and roof rats)?

NON-TARGET VICTIMS. Various non-target small mammals are common in vegetative and undeveloped natural areas. The possible species found in these areas varies considerably depending on the general region and on a locale’s specific environment (and all the factors affecting that environment).
The table above presents a partial list of non-target small mammals that may inhabit fields, fencerows, wooded lots and other undeveloped, or semi-developed property-line environments associated with commercial structures. Note that with the exception of the shrews (insectivores), all of the mammals listed are rodents. (Even still, shrews will also feed on bait blocks inside bait stations.)
To the average person, some of these border fields may appear as mere “weedy lots.” But the point of the table above is to illustrate that any one (and more likely, several) species of these non-target small mammals may occur in a field that is also a property line/fencerow area to a large commercial structure such as a factory, food warehouse, etc. In trapping studies of ordinary-looking “grassy fields,” mammalogists have trapped up to nine different species of small mammals all inhabiting the same general area (refs. 5,6,9).
Thus, in today’s rapid development of idle, undeveloped fields by commercial and residential developers, one can see it is fairly easy for non-target species to quickly become vulnerable to exterior rodent control operations (see photo on page 112). What is not known is whether or not all the species in Table 1 would enter bait stations should they encounter them. But from field observations over the years by pest professionals, as well as from some research, we know that a significant number of them do (ref. 4).
Moreover, most of us (myself included) cannot, with a casual look of a mouse carcass, distinguish between house mice, harvest mice, oldfield mice, golden mice, or among the different types of voles and shrews.
Without the benefit of a course in mammalogy or ecology some might comment, “Who cares? They’re just field mice” or “They’re just varmints! Any of these could pose a threat to my client.” But the small mammal diversity of an area is important. Various species of rodents play an important part in all types of ecological processes. Most serve as significant prey species for hawks, owls, coyotes, foxes, badgers and weasels. Many serve as disseminators of seeds. Some play important roles in the local soil ecology via their burrows and nests, and their consumption of various soil invertebrates.
Perhaps most importantly is that there is so much we have yet to discover about many of these small mammals. In other words, we don’t know what we don’t know yet. We cannot assume that all these mammals are inherently beneficial to ecosystems. We may learn one day that some of these non-targets are significant health pests to humans, livestock or other wildlife.
But until we learn more, should any non-target small mammals be potential victims of property-line baiting programs when they forage into bait stations? Additionally, should the various predators that feed on these small mammals be subject to incurring dosages of rodenticides intended for the commensal species (dosages that might be accumulative and thus lethal)? (refs. 2,7)

A NECESSARY PROGRAM? For certain facilities, property line baiting programs are unnecessary. Consider property lines that range up to several hundred yards away from a building’s perimeter walls. Such distances are well out of the typical foraging range of many of the small mammals listed in Table 1, although it is possible that some environmental or construction events may occasionally trigger dispersal.
In certain cases, fencerow/property line rodent control efforts (which may or may not include bait toxicants) may be justified (to be discussed in Part III). These cases may include, for example, facilities where property lines containing vegetative covers are in relatively close proximity to the building (e.g., within 25 yards), and there is significant commensal rodent species pressure on the building’s exterior walls.
 
IPM/GREEN MOVEMENT. More and more larger scale food and pharmaceutical corporations are now inquiring as to how they can “go green” on as many of their production avenues as possible. It’s no secret how wildly successful the organic food industry has been the past five years — only one indicator as to how the “green movement” is expected to increase in importance in the public sector in the next couple of decades. This movement not only involves a desire for less pesticide use in environments, but a consideration to not impact non-target animals via primary and secondary avenues associated with pest management programs that are necessary to control pest species.
It is unheard of, for example, that when we implement control programs around buildings for city pigeons, sparrows and starlings, that anyone considers it acceptable that robins, bluebirds, hawks, owls, cardinals or any other bird might be inadvertently harmed by any of the programs employed for the bird pest species. As we continue our urban sprawl into undeveloped areas for commercial scale development, why should it be any different for any of the small non-target mammal species?

NEXT MONTH. Can we have our cake and eat it too? I think so. We need not sacrifice food safety considering that our NPMA logo states we are “guardians of the environment.”
Next month, I will conclude this three-part series with some suggestions and considerations as to the future of exterior rodent control programs relative to the use of rodenticides when needed, and the use of alternative programs such as non-chemical monitoring and trapping programs.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@giemedia.com or 765/939-2829.

References
(1) American Institute of Baking. 2001. Consolidated Standards for Food Safety. AIB. Manhattan, Kansas.
(2) Brown, R.A. 1994. Assessing the environmental impact of rodenticides. Pp. 363-380. In: Rodent Pests and Their Control. A. Buckle and R. Smith, Eds. CAB International. Wallingford, UK. 405 pp.
(3) Corrigan, R.M. 2003. Rodent Pest Management: Pages 265-291 in: Food Plant Sanitation. Y.H. Hui, et. al (Eds). Marcel Dekker, New York, N.Y. 745 pp.
(4) Erickson, W.A., R.E. Marsh, and W.L. Halvorson. 1990. A roof rat bait station that excludes deer mice. Wildl. Soc. Bull. Vol. 18 (3): 319-325.
(5) Forsyth, A. 1985. Mammals of the American North. Camden House Publ. Ontario. 351 pp.
(6) Hamilton, W.J. and J. O. Whitaker. 1979. Mammals of the Eastern United States. Cornell University Press. 346 pp.
(7) Kaukeinen, D.E., C.W. Spragins and J.F. Hobson. 2000. Risk-benefit considerations in evaluating commensal anticoagulant rodenticide impact to wildlife. Proceedings of the 19th Vertebr. Pest Conf. Univ. of Calif. Davis.
(8) Marsh, B. T. and W.B. Jackson, 1963. Perimeter burrow rat control methods. Grain Age. January 18.
(9) Mumford, R.E. and J.O. Whitaker, J.R. — 1982. Mammals of Indiana. Indiana University Press. Bloomington, Ind. 537 pp.
(10) Troller, J. A. Sanitation in Food Processing. 1983. Academic Press. New York, N.Y. 456 pp.