Along with correctly identifying a pest problem, the next most important step is to inspect and determine the extent of the problem and where it is located so it can be eliminated. Additionally, and at the same time, PMPs need to take steps to prevent the same problem from recurring. Our industry’s most well-known rodentologist, Bobby Corrigan, tells professionals to become “keen observers,” like Sherlock Holmes. This article focuses on the rodent inspection, some signs to look for and what can be learned from certain rodent evidence observed.
According to Corrigan’s Rodent Control: A Practical Guide for Pest Management Professionals, 10 rodent signs to look for include droppings; chewing damage; burrows; runways; tracks; grease marks (sebum); urine stains; visual sightings of live or dead rodents; rodent sounds (squealing); and odors. The inspection tells the pest professional where to place the control and elimination tools, whether traps or baits.
As with any pest inspection, the practitioner needs a strong flashlight to look into dark areas where rodent evidence more often than not tends to be found. I recommend a flashlight at least 110 lumens strong (the brighter the better!). I’ve found a flashlight that can be focused (making a bright single beam) is better than one that might be bright but cannot be focused. It goes without saying the flashlight needs fresh batteries. A flashlight with fading batteries may allow rodent signs to be missed and is certainly not professional.
For our inspection purposes, rodents are fortunately incontinent. When rodents need to relieve themselves they urinate and defecate when the urge hits. Droppings and urine stains tend to be heaviest where the rodents spend more of their time. A concentrated area with 30 or 40 droppings tells us control tools need to be located here first, rather than another area where three or four droppings might be present. Rodent staining can be visible, but care needs to be taken to make sure the stains are not from other substances.
The size and shape of droppings will tell the professional if he or she is dealing with rats or mice, but droppings can be confused with other sources if care is not taken. American roach droppings are almost the size of mouse droppings and can be misidentified if seen from a distance with a quick glance. American roach droppings tend to be barrel shaped and have striations, or lines, along the sides. Mouse pellets have no lines and tend to be tapered at the ends. Where the droppings are found can sometimes give a clue to the dropping source. Droppings found in hot, steamy locations might more likely be from American roaches, rather than mice so a closer, more deliberative look will be needed.
Rat droppings are larger (up to ¾ inch) and can be confused with large fly pupae. Fly pupae easily can be differentiated because of lines running around the circumference of the pupal case. Pupal cases also tend to be more uniform in shape than rat droppings, rounded at the ends and not tapered (photo 1). I’ve seen coffee beans and chocolate sprinkles identified as rodent droppings, when a closer look would have shown suspected rodent droppings to be something else.
How can you tell if rodent droppings are old or new? Fresh droppings are a brighter black color and will squish when crushed (photo 2). Older droppings are often faded, dust covered (but not always) and when crushed will disintegrate and become pulverized. Fresh droppings in high-dust locations, like a bakery or warehouse with lots of forklift/pallet jack traffic, can be faded and look old. Corrigan reports droppings as fresh as 48 to 72 hours will start to look faded and old. The squish test can help the professional differentiate old from new droppings.
Rodent droppings tend to be black, but can be brown, green or other colors. The droppings will take on the color of what they are feeding on. The color of the droppings can sometimes tell a story. I once found green droppings inside a commercial building that had no bait used inside. The bait was in outside perimeter bait stations surrounding the building. The conclusion? Mice were obviously finding their way inside prompting a close look to find how they were getting inside. In another case green droppings were found in a ceiling where no bait was being used. The mice were obviously feeding below and getting into the ceiling (photo 3).
Rats and mice like to gnaw on solid surfaces and their chewings are often easily visible, but sometimes overlooked because people don’t know what they are looking at (photos 4 and 5).
Other signs of rodent presence are grease marks. The fur of rats and mice are oily and deposits of grease marks are often left in runways and trails frequently used (photos 6 and 7, below).
We often hear experts and trainers say rodent droppings need to be cleaned up. There are several reasons why this is an important step in practicing IPM. Rodent droppings observed by health agents indicate the presence of a rodent problem needing to be remedied. Fresh droppings are indicative of an ongoing problem, while old droppings may be representative of a past problem and inadequate cleaning.
Right or wrong, health inspectors will often report droppings and not differentiate fresh from old and will simply report a rodent problem exists, when a problem may not be active. We cannot stress enough to our customers the need to clean up droppings. We can help by cleaning up small deposits of droppings, but larger accumulation deposits need removal by our customers (unless they want to pay us for additional service of course).
Author’s note: An article written by Eric Smith and me can be referenced from the PCT archives for a detailed discussion on the use of blacklights for rodent inspection. Chapter 7 of Bobby Corrigan’s rodent manual has a detailed discussion on the value and use of blacklights for urine inspection. This book is a must-have for PMPs involved in rodent control and highly recommended by this writer. Order it at www.pctonline.com/store.
Corrigan, R.M., 2001. Rodent Control — A Practical Guide For Pest Management Professionals. GIE Media, pp. 80, 87-89.
The author is retired from the Waltham Services’ technical director position after 44 years of service and is now writing and consulting. Email him at firstname.lastname@example.org.
All photos courtesy of R. Berman