Placed or Spaced?

Features - Rodent Control

New science shows it’s time for a shift away from an outdated equipment paradigm.

October 15, 2021

By Matt Frye, Jody Gangloff-Kaufmann, Bobby Corrigan, Hank Hirsch and Dusana Bondy

You just acquired a new food-handling facility and your company pledged to protect it from rodents. Where do you begin? Depending on your customer’s in-house policies and third-party auditor guidelines, setting up a rodent program could be simple: Place multi-catch traps at 20- to 40-foot intervals along the interior perimeter, and bait stations 50 to 100 feet apart on the building exterior and fence lines (if permitted by the rodenticide label). The question is, will this “Three Lines of Defense” approach with interval device spacing offer the best protection for your customers?

INTERVAL DEVICE SPACING. Let’s first take a look at the approach to rodent management that utilizes interval spacing of devices. This strategy was developed in the 1940s and is based on estimates of mouse and rat foraging ranges (Corrigan 2002). The thought process in creating the program went something like this: House mice are known to travel between 15 and 30 feet when foraging. By placing devices at 20-foot intervals along interior perimeter walls, foraging rodents will never be more than 20 feet from a device at any time, creating a protective barrier. The same philosophy was applied to exterior stations that were spaced further apart to account for rat foraging behavior.

Without testing or evaluation to determine if this approach effectively protects facilities from rodents, it was adopted by third-party auditing agencies and used to develop in-house management programs. For the past 80+ years, guidelines have remained relatively unchanged — despite advances in our understanding of rodent biology, behavior and management. And where guidelines have changed to provide flexibility in device placement (NPMA 2018), interval spacing of devices remains the norm due to customer expectations, existing auditing guidelines or a client’s outdated in-house Standard Operating Procedures. The question remains: Does interval device spacing provide the best protection for customers?

PUTTING IT TO THE TEST. To evaluate this question scientifically, we worked with RK Environmental Services (Westwood, N.J.) and Abell Pest Control (Etobicoke, Ontario, Canada) at seven food distribution centers in the New York Metropolitan area and five near Toronto, Canada. Sites ranged in size from 8,500 to 198,000 square feet, and all had a history of rodent activity. A list of characteristics was used to describe the area within 5 feet of each device, including 76 characteristics for interior multi-catch traps (i.e., Victor Tin Cat Mouse Trap) and 27 for exterior bait stations (i.e., Protecta EVO EXPRESS). Characteristics represented conditions ecologically attractive to rodents such as food and water resources, and structural features of a building that can affect rodent movements, including the presence of cover and openings. We compared these traits to trap capture for interior devices, and bait consumption for exterior stations over a period of 10 to 69 months, depending on data availability from each site.

TRAPPING RESULTS. Interior devices were checked monthly at 12 sites for a combined total of 636 months (all months at all sites). During that time, 1,491 mice were trapped. The number of mice caught in a trap at one time ranged from one to eight, but 84.6 percent of captures were a single mouse. Most devices trapped mice only one time per year (see chart).

The percentage of devices that caught mice in a facility ranged from 13.3 to 91.7 percent, with an average of 45.2 percent. This means, on average, less than half of the traps in a facility actually captured mice each year. The percentage of traps with two or more captures ranged from 0 to 83.3 percent, with an average of 29.7 percent.


Photo 1. Dense vegetation provides cover for rodents to forage.

Several ecological and structural characteristics near traps influenced whether that device would catch mice. Specifically, increased monthly rodent capture was associated with interior traps placed in a corner, shadowed during the day, on the south or west side perimeter wall, near a rodent-proof single door, or near a poured concrete wall. Some of these findings meet with predictions about rodent behavior based on their biology. For example, corners and shadows offer protection for foraging mice, while south- and west- facing walls buffer against cold temperatures in winter months. Higher captures near rodent-proof doors and poured concrete walls could be a response to the need for cover or escape routes in areas with no entry/exit points. In other words, rodents foraging near these pest-proof structures have no other options for cover, and might enter traps in response to disturbance.

Support for this hypothesis comes from related results of decreased monthly capture associated with devices near a loading dock door with the leveler not pest proof, and near a corrugated wall with insulation. In these settings, rodents would have sufficient openings to hide or escape detection while foraging, and therefore may not enter multi-catch curiosity traps. Taken together, these results justify the use of pest exclusion to seal openings as a method to not only prevent rodent entry, but enhance rodent management. Finally, devices near a warm mechanical room were associated with decreased rodent capture, perhaps because devices were not placed close enough to the warm elements of these spaces, but rather only hugging the mechanical room’s interior perimeter walls.

BAITING RESULTS. Rodenticide consumption data was available from four sites (73 bait stations), and included 2,769 observations. “No feeding” accounted for 36.8 percent of observations, with minimal feeding of “one corner removed” observed 19.4 percent of the time (see Table). Monthly bait consumption was higher for stations located on the west side of the building, and near unmaintained, dense vegetation. As with interior areas, the west side of the building is associated with warmth during winter months, while dense vegetation provides cover for rodents to forage. (See Photo 1.)

SURVEY SAYS? Our results demonstrate that certain ecological and structural features of a facility are associated with higher trap capture on the interior, and increased bait consumption on the exterior. Considering that, on average, fewer than half of interior multi-catch devices caught mice (see Photo 3), it seems that interval device spacing does not maximize rodent management at food distribution centers. Rather, our results suggest that assessment-based placement of devices in areas attractive to rodents can enhance food safety, offer economic incentives and provide other benefits.

Photo 3. Interior multi-catch traps near loading dock doors with the leveler not rodent-proof were associated with lower trap captures.

ECONOMICS OF INTERVALS. To understand the benefits of assessment-based device placement, let’s take the example of a facility that has 50 interior multi-catch devices. Based on our results, 45.2 percent or 23 of the devices are likely to intercept rodents, leaving 27 devices untouched, but checked by a technician at each visit. These devices may be in a brightly lit area with heavy foot traffic, making them extremely unlikely to have rodent activity. Assuming that servicing each device takes two minutes and a service cost of $1.25 to $2.50, removing the 27 unused traps could result in a time savings of 54 minutes, and a service cost savings of $33.75 to $67.50. Time saved on device-checking could be used to perform the often overlooked, proactive inspections into those hard-to-reach areas where new infestations can begin because they are out of sight, out of mind. Or it could be used to dig deeper for any conducive conditions that would justify additional control equipment.

Similar economic savings are expected for exterior rodent management, in addition to environmental benefits. In our study, 56.1 percent of observations on rodenticide bait were minor consumption (no feeding and feeding on one corner), and yet bait is often replaced monthly, regardless of the amount consumed. Discarding one bait block every month from stations with only minor feeding would result in disposal of 1,554 blocks (129.5 pounds) from these four sites alone. The amount of bait sent to the landfill would increase if more than one block were used per bait station, and would double for sites that replace bait twice per month. While the environmental benefits have not been estimated, savings in service time (3.5 minutes per device) and service cost ($3 to $5) can easily be calculated, with resources redirected toward assessment.

WHERE DO WE GO FROM HERE? Rodent management experts have long-called for an evaluation of the “one size fits all” interval-based program at food facilities (Corrigan 2002). Our results demonstrate that rodent management can be improved, and we therefore propose a paradigm shift towards assessment-based placement of devices. This approach can offer time and cost savings, enhance food safety by improving program efficacy and reduce environmental waste.

Also, dynamic programs that rely on inspection and interpretation of evidence can reduce trap fatigue, a phenomenon where device captures decrease over time if traps remain stationary, especially for stable rodent populations. Assessment-based programs also can reduce technician fatigue, where service professionals become trap-checkers that open, clean and replace bait or glueboards without assessing the rodent problem or developing management approaches specific to that location. Finally, assessment-based placement of devices facilitates compliance with the Food Safety Modernization Act (FSMA) of 2011, and specifically the Preventive Controls for Human Food Rule, which calls for proactive programs.



WHAT CAN YOU DO? A true paradigm shift will take time for policies to change and for adoption of new practices. Pest professionals will play a vital role in accelerating that change by taking action now.

  • Review third-party audit schemes to determine if assessment-based device placement is allowed at your facility. The NPMA Food Safety Audit Scheme Quick Reference Guide is a good place to start. (See here.)
  • Review in-house procedures and explain the potential benefits of assessment-based device placement to your facility contact. Gauge client willingness to deviate from interval device placement. Explain that time and cost savings will be redirected toward inspection and assessment.
  • Provide training for service specialists/technicians on detailed rodent inspections, and how to interpret evidence for enhanced device placement. Resources to get started are available from PCT magazine and industry textbooks.
  • If you have a facility that permits assessment-based placement of devices, try it!
  • Add equipment to the existing program, specifically in areas that would be attractive to rodents (for example, an interior device in a shadowed area along a west-facing wall) or areas where rodent evidence is present. Observe trap capture over time, especially compared to nearby, interval-spaced devices.
  • Or, remove equipment in areas that are simply unattractive to rodents (e.g., the exterior stations not located near any potential entry points (door gaps, pipe penetrations, unsealed wall cracks, etc.) on the north and east side walls. Unless some highly unusual scenario exists, there is no reason to expect interior catches along these same walls would increase.

Authors’ Acknowledgements: This work was supported by the USDA National Institute of Food and Agriculture Hatch project 1010711. We thank Chris Sweezy, Ricardo John, Shaun Funk and Sally Petkovich from RK Environmental for their assistance in the field and for providing trap capture data.

Corrigan, RM, Rodent pest management, in Food Plant Sanitation, ed. by Hui YH, Bruinsma BL, Gorham JR, Nip W-K, Tong PS, and Ventresca P, Marcel Dekker, New York, NY, pp. 265-291 (2002).
Frye, MJ, JL Gangloff-Kaufmann, RM Corrigan, H Hirsch, D Bondy. 2021. Assessment of factors influencing visitation to rodent management devices at food distribution centers. Journal of Stored Products Research 93: 101838.
NPMA, Food Safety Audit Scheme Quick Reference Guide. National Pest Manag. Assoc., Fairfax, VA. 6 pp. (2018). URL.