Exterior Baiting Programs Around Commercial Properties: Time for Re-evaluation? Part I

Editor’s note: This is the first in a three-part series. The series will examine the benefits, techniques and potential ecological and toxicological risks regarding the exterior use of rodenticides, as well as attempt to dispel some of the misconceptions associated with exterior baiting programs.

In this article, an overview of exterior baiting programs for commercial facilities and perimeter wall baiting programs are discussed. Next month in Part II, fencerow (or property-line) baiting programs will be reviewed. In Part III in October, possible alternatives to conventional exterior rodent control programs and/or modifications to the use of rodenticides to minimize any potential threats to the environment and to non-target animals are considered.

Every day, hundreds of thousands of commercial structures around the world are protected from mouse and rat invasion via the use of exterior baiting programs utilizing hundreds of tons of rodenticide baits. In most cases, these baiting programs involve the use of bait stations (or bait boxes). And in the United States, a majority of the bait stations are classified as "tamper-resistant rodenticide bait stations."

The types of commercial structures that most often receive large-scale protective rodent control programs include food- and pharmaceutical-manufacturing plants, food-handling warehouses and large-scale grocery distribution centers. But other facilities, such as fast food restaurants, zoological parks, office buildings, shopping mall peripheries, municipalities, multi-family housing complexes and other buildings may employ exterior bait stations for both remedial and prophylactic rodent control.

Typically, exterior rodent baiting programs designed to provide protection to commercial structures involves one, and sometimes two, tier(s) of protection. The first and most frequently employed tier is the use of bait stations positioned around a building’s perimeter foundation wall (see Figure 1 above). The spacing intervals should be determined by rodent pressure on the building. But in some cases, and for some food companies, the density of bait stations may be determined as per a corporate policy, or from various private inspection company guidelines (e.g., AIB, Sillika Labs, etc.). Around facilities that do not occupy large tracts of land, or may be not be subject to much rodent pressure, usually only a single tier of protection is employed.

But for large-scale operations that may contain up to several hundred acres of property associated with the facility, or for facilities in which the building is surrounded by a suspected source of rodents (e.g., nearby woods, water sources, unkempt neighboring properties, etc.), a second tier of bait stations is often added and employed along the facility’s property perimeter fence line. If a fence does not exist along the property line this second tier of bait stations is usually optional and up to the discretion of the facility management and the servicing pest management professional based on the rodent pressure on the property. These stations may also be installed on a temporary basis, or only along certain areas of the property. (Property line baiting programs is discussed in Part II of this article.)

REGULATIONS AND GUIDELINES. It is important to note that the use of exterior bait stations is not mandated by any federal or state agency. In other words, neither USDA nor FDA requires the use of bait boxes to protect commercial food-handling establishments. These agencies stipulate that food-handling facilities must not allow food to be subject to pest contamination. How that is accomplished is entirely up to the food-handling establishment. But certainly, both USDA and FDA assume the food establishment will employ pest proofing, sanitation, mechanical devices and, where needed and within federal guidelines, pesticidal approaches. In other words, these agencies assume and encourage IPM programs.

Interestingly, various non-regulatory, independent inspection companies (i.e., the American Institute of Baking, Sillika Labs, and others), have established relatively specific guidelines for the use of exterior bait stations and mouse traps employed inside facilities.
Unfortunately (and to the frustration of many servicing pest management professionals), the guidelines of different inspection companies are not only different from one another; they are also arbitrarily based. Moreover, it is not uncommon to hear of food plant QA managers, and pest professionals servicing food-handling establishments, misinterpreting these guidelines as regulations.

BENEFITS AND TRADEOFFS. There is little doubt of the value that exterior bait stations provide in protecting buildings, occupants and food from invading rodents. Hundreds of millions of rodents have been prevented from establishing interior infestations via exterior rodent control bait stations.

But what, if any, are the trade-offs and potential risks to the environment from the use of exterior baiting programs? This question has come to the attention of regulatory agencies (e.g., EPA), environmentalists, company public relation personnel, progressive pest management companies and some sections of the public in general.

EXTERIOR PERIMETER WALL BAITING PROGRAMS.

The Premise. Rodents living in the "natural zones" nearby buildings and building peripheries (e.g., yards, fields and wooded areas), or low-maintained areas (e.g. rail embankments, ditch banks, vegetative median strips, etc.), must forage for food and water, and may also attempt to utilize any new harborages they encounter. Also, rodents may disperse from field areas as a result of some disturbance to their habitat (e.g., the harvesting of fields, development construction projects). In either case, it is common for these rodents to periodically encounter building perimeter walls and doors.

Nearby large-scale food-processing plants, field rodents may also routinely follow their noses in the direction of any food odors emanating out from exhaust vents, doors and windows. In some cases, it is likely these currents of food molecules "suck" rodents into buildings, as the rodent will attempt to follow the molecules to their source.

If a foraging rodent encounters boards lying on the ground, a crevice beneath a doorway or a box containing holes (e.g., a bait box), it is likely to seek the protection these sites offer from the rodent’s predators. Or, on a cool autumn night, the rodent may respond to the highly beneficial current of warm air escaping from beneath a door or from around an unsealed pipe.

The frequency at which rodents interact with building environments over time can be referred to as "rodent pressure." The rodent pressure upon any commercial building may vary from very low to very high. And it can vary from one building to another, even for buildings located within a mile or so of one another. Too, the pressure can shift from one season to another, and from one year to another. Overall, it is difficult to quantitatively pin down rodent pressure on commercial buildings.

Some food plants and warehouses experience only a couple of mice each year inside their buildings, and minor feeding activity on their exterior bait stations. Other facilities however, experience non-stop battles with rodents attempting to gain entry into their buildings.

Nevertheless, any rodent inside a commercial food-handling building is a violation of federal, state and local food safety and health code laws. And so, rodent control programs are established for exterior zones in attempts to stop rodents before they gain entry, as well as for interior areas to capture those rodents that make it past the exterior programs, or that may have arrived within deliveries or on pallets, etc.

The Conventional Perimeter Wall Model. To help keep invading rodents from field areas in check, most commercial facilities install bait stations around their exterior perimeter walls. In the majority of cases, the exterior stations are positioned around the entire building perimeter at spacings ranging from 25 ft/7.5m to 100 ft/30m (see Figure 2 on page 54). The closer spacing is typically employed for those facilities experiencing the heaviest rodent pressure.

Over the past several years however, this model is beginning to be challenged. For example, why, if only one wall of a building is subject to rodent activity, should the other three walls receive the same number of bait stations? Some might argue — and perhaps rightfully — that such a treatment is hardly within the principles IPM. Perhaps such treatments are more a blanketing of a building with rodenticides (i.e., pesticides) based upon erring on a maximum protection approach instead of an approach of minimizing the property’s pesticide load.

This is not to say the other three walls need no protection; but perhaps they need only half or a quarter of the units as that of the active wall, or, only strategically placed trap stations nearby likely rodent entry zones. (Trap stations will be covered in Part III.)

Bait Formulations. Rodenticidal baits used for exterior baiting programs around food plants are often subject to excess moisture from rain and snow, or from wet/dirty rodents or other small mammals and birds visiting the stations and baits. Thus, bait blocks are the most widely used formulation for these programs because they provide good durability to the elements as well as being acceptable to rodents (see Figure 3 on page 58).

Moreover, block baits can be secured on either vertical or horizontal rods inside bait stations offering additional protection of the blocks from the elements (the wet floor of a bait station following rains or melting snow). And, secured blocks greatly reduce the potential of rodents carrying the bait out of the station (which defeats the entire effort in using a tamper-resistant bait station in the first place). In general, this practice is now standard operating procedure for most pest professionals servicing commercial facilities. Interestingly, this practice exceeds the tamper-resistant baiting criteria as set forth by the Environmental Protection Agency.

Packet-style baits (often referred to as "toss packs," "throw packs" and "place packs") are sometimes also used inside exterior bait stations. But some rodents may not gnaw through the packaging to get to the bait. Nor do packet-style baits provide long-term protection from wetness once a rodent has torn the packet open. Furthermore, unless secured, bait packets can be dragged out of the station and spilled on the ground by a rodent. Obviously, this renders the pesticide available to birds, dogs, cats, and all other types of wildlife.

Still, in cases of extreme wetness or excessive heat that destroys the paraffin blocks (and where exterior traps programs may not manage high rodent pressures), secured packet baits may be the appropriate bait formulation. Packet baits can be secured inside stations by hanging the packet from the top end of one of the station’s interior dividing walls using ordinary paper binder clips. Some commercial facilities use the packet style baits for only those months of intense heat or wetness, and then switch out to bait blocks.

The Importance of Record Keeping. Records kept on all exterior bait (or trap) stations, and interior mousetrap captures over the course of 12 to 16 months, can serve as guiding template as to how, and to what amounts the exterior rodent control stations might be set up at the facility.

If over the course of a year or so, the interior capture records indicate only certain walls or areas within the building are experiencing rodent activity, it truly is within the IPM approach to let the data dictate the exterior rodent control model.

Conversely, if the records over time show that rodents are captured all around the building in an unpredictable and ongoing manner, then, the correct path is to have all walls contain protective devices (either bait stations or trap stations) spaced appropriately so as to protect the plant sufficiently (ref. 2, 3).

FINAL THOUGHTS. Are exterior baiting schemes based on sound science? How were the current exterior baiting models developed? And, are they based on early research, or any type of "sound" science? Surprisingly, the answer is no.

The models appear to have evolved from the recommendations of various government (and other) rodent control publications during the 1940s and 1950s (ref. 3).

These publications provided general guidelines for placement locations and spacing of rodent traps and bait containers for mouse and rat infestations, presumably based on the foraging ranges ("territories") of rodents. These recommendations were then broadly adopted for grain storage and warehousing facilities. Eventually, they were adapted for the food industry (for example, AIB 2001), and the "standard perimeter defense model" based on the early guidelines remain in place today (ref. 1, 3, 4).

But as mentioned earlier, this model is now the subject of some discussion, challenges and debate. For example, the numbers of bait or trap stations as recommended in early publications may be appropriate for those facilities subject to ongoing rodent activity and pressure.

However, for those structures subject to low rodent pressure, and also practicing effective pest exclusion practices (tight doors, well maintained foundation walls, etc.), the "standard" perimeter defense program may be excessive and unnecessarily expensive. This is especially true for some of the modern "super facilities" that now exceed one million square feet in size.

Moreover, and as will be discussed in Parts II and III of this article, these current programs also may provide a potential risk of rodenticides into local environments.

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. AIB Consolidated Standards. Food Safety. 51 pp. Manhattan, Kan.

(2) Corrigan, R.M. 2001. Rodent Pest Management: A Practical Guide For Pest Management Professionals. GIE Media. Cleveland. 355 pp

(3) Corrigan, R.M. 2003. Rodent Pest Management. Pages 265-291 in: Food Plant Sanitation. H.Y. Hui, B.L. Bruinsma, J.R. Gorham, W.Nip, P.S. Tong and P. Ventresca (Eds.). Marcel Dekker Inc. New York, N.Y.. 745 pp.

(4) Weier, J. 2002. A comparison of the rodent control guidelines between the independent inspection companies serving the food industry. Presented at the PCT Rodent Summit, Chicago, Ill. March 15, 2002.