Practical Rodent Inspections

This article emphasizes the importance of quality rodent inspections and provides some inspection techniques for diagnosing rodent signs and estimating the severity of a rodent infestation. Part II, in the October issue of PCT, will present a detailed look at analyzing rodent and other animal droppings during inspections.

VALUABLE INFORMATION. A rodent inspection is important for more than just telling you that rodents are present, and whether or not it’s rats of mice! The information obtained during the inspection will pay dividends over and over again in your rodent control programs. The most successful rodent control programs begin with complete and diligent inspections and should always be conducted before any control work is begun.

The inspection provides you with three valuable pieces of information:

1. The extent and severity of the infestation. Is the problem localized in one area of the building? Are the rodents infesting several floors or wings? How sever is the infestation? By accurately gathering this information prior to any control efforts, much time and money can be saved by you and your client in the months ahead. For example, one of the most common mistakes made in rodent control is to underestimate the severity of an infestation either in numbers of animals or by the area infested. This results in underbaiting which leads to callbacks.
   

2. The location of rodent harborage and activity areas. By inspecting for various rodent signs, clues can be gathered as to the location of nesting areas, as well as where and how the rodents travel from nests to their food and water. By intensifying your control efforts at these locations, greater control can be achieved.
   

3. The safest and most appropriate control strategies. In some situations you will have a wide range of control tools and strategies from which to choose. In other cases, however, your control options may be limited due to food exposure, safety precautions for children and non-target animals, and various environmental factors. All this can be determined through your inspection. Of course, the goal is not only an effective control program, but one that is safe.

INSPECTION TECHNIQUES. PCOs should follow some strict guidelines when inspecting a structure and the surrounding areas for rodents. These include:

Identify the rodent entry areas. For every rodent job, an inspection of both the outside and inside of the building should be conducted. This will aid in identifying possible source areas, as well as pinpointing access to the structure.

Areas to carefully inspect include all utility lines that enter the structure through the walls. For example, all openings around water lines, air conditioning ducts, drain pipes, vents and electrical conduits should be inspected. Of course, the spaces beneath doors are one of the most common entryways for rodents. Doors should be checked to ensure the lower edge is not more than ¼ inch from the floor. Broken basement windows and unscreened vents also are common invasion routes.

In the case of roof rat inspections, roofs also should be checked to see that shingles are down right and sheathing is complete. In addition, check roof ventilators, screen vents and louvered in-wall vents and chimneys.

Locate the rodent zones. The key to effective inspections is to identify the primary harborage, feeding and activity areas (i.e., "the rodent zones") to get the best results from your efforts. In some cases, such as in commercial accounts, the areas to be inspected may be large and complex, making it impractical to inspect every corner and beneath every shelf for possible rodent signs. Typically, rodents build their nests near food sources. Therefore, the rodent zones are those areas between the nest and feeding areas.

When conducting inspections it’s a good idea to think in terms of the rodents requirements first. Survey the building and list all of he areas where food is available. Compared to insects which can get by on extremely small amounts of food and water, rodents are physical giants when it comes to their food, water and space requirements. When you consider an adult rat requires up to 1 ounce of food and water daily to survive, food has to be readily accessible. Keeping this in mind during your inspections, begin by locating all areas within the account that provide food for rodents. This is especially useful when inspecting accounts where there is a minor infestation and the rodent zone is difficult to pinpoint.

Once suspected food sources have been identified, try to locate nearby harborage sites. Inside structures, a few of the more common rodent harborage locations include the following:

• Secluded corners (remember rodents follow wall surfaces and prefer corners because these areas provide them with a sense of security)

• Beneath cabinets

• Beneath and within furniture

• Beneath large appliances

• Floor cabinet voids in kitchens

• Base voids in dresser bureaus

• Wall voids

• Ceiling voids

• Sill plate areas on top of basement and crawl space foundation walls

Analyze the rodent signs. Once you have identified these zones, you can inspect for various rodent signs which will enable you to assess the current infestation and determine what corrective measures need to be taken.

There are 10 rodent signs you should look, smell or listen for during a rodent inspection: 1) droppings (feces); 2)gnawing damage; 3) burrows; 4) runways; 5) tracks; 6) grease marks; 7) urine stains; 8) visual sightings of live or dead rodents; 9) rodent sounds; and 10) rodent odors (especially for mice). A brief review of several of these follows:

Droppings. Droppings are the most frequently encountered, and therefore, the most relied upon rodent sign. Part II of this article, "Analyzing Droppings During Inspections," will be published in next month’s issue of PCT and will present a detailed approach to identifying the different types of animal droppings found during on-the-job inspections, as well as how to analyze the droppings to evaluate an infestation.

Gnawing damage. Evidence of recent gnawing and/or damage as a result of gnawing is an excellent sign for determining the presence of rodents. In fact, the word rodent means "to gnaw". Rodents possess a pair of incisors which grow constantly at the rate of .3 to .4 millimeters each day. Rodents constantly gnaw with these incisors on many different objects within their activity range. (Contrary to popular belief, rodents do not have to gnaw on objects to keep their incisors filed down. They accomplish this by grinding the upper and lower incisors against each other).

By examining the size of the two shallow, parallel grooves left by the incisors on the gnawed object, you can determine whether it is rat or mouse damage. The incisors of mice leave a mark 1 to 2 mm wide, while the Norway rat’s mark measures about 4 mm in width.

Rodents gnaw on many items including aluminum, mortar, plastic, lead and any other items softer than the enamel of their incisors. Rodents seem especially fond of gnawing on electrical wires found throughout buildings. For this reason alone rodents can be extremely destructive pests in computer facilities.

Rats commonly gnaw on wooden structural members such as door corners, floor and ceiling joists, and wall studs. Recently gnawed wood will have a light colored, rough and splintered appearance. Wood shavings are usually evident below the damaged area. Over time, the gnawed area darkens, and the rodents will continue to gnaw until the wood is smooth with rounded edges.

When rats gnaw holes in cartons and boxes, the holes typically measure about 2 inches or more in diameter, and often contain rough, torn edges.

In addition to wires, mice gnaw on plastic items, cardboard, waxy substances and wood. In homes, mice shred paper and gnaw holes in the corners of food boxes and bags Mouse damage holes are typically small, measuring about 1-1/2 inches in diameter, and clean cut. In bathrooms mice often gnaw on bar soap stored in cabinets.

Runways, tracks and grease marks. Runways are usually evident in rodent infestations because rodents repeatedly use the same pathways between their nests and food sources.

Runways are easier to detect with Norway rats than for mice or roof rats. Outdoors, fresh rat runways are smooth, well packed and free of vegetation. Indoors, runways along the floors or rafters are identified by clean paths through dust or dirt. Grease marks (or rub marks) from the oil and dirt of rats and mice often appear along wall areas next to runways. Grease marks may also occur around the bottoms of joists where rodents have been traveling along beams or sill plates, on stairways or around burrow opening in walls, floors or ceilings.

Track marks, like incisor marks, indicate whether you are dealing with rats or mice. The key identifying character of a rodent track is its size. In most cases, you will be looking at the prints of the hind foot because this is where most of the weight is placed as the animals travel about. Rat tracks are relatively large measuring between ¾-inch to 1.5 inches. The mouse’s hind foot track is considerable smaller measuring only about 3/8-inch or less.

Both rats and mice have the same number of toes on their front and hind feet; five toes on the back, four on the front. Both rats and mice may also leave "tail drag" marks which will appear as a thin line between their footprints. In many cases, it is the older members of a colony which tend to drag their tails.

Burrows/eating spots. Burrows are an obvious sign when dealing with the Norway rat in outdoor infestations. Rat burrows measure about 3 inches in diameter and may be found next to walls, along foundations, or beneath debris or shrubbery. Around the outside of residences, two of the most common areas to inspect for rat burrows outdoors is adjacent to bird feeders and/or dog houses. Pay special attention to woodpiles and storage sheds located in these areas.

Active rat burrows are usually clear of vegetation, and the burrow entrance appears compacted and smooth. In some instances fresh soil will have been recently kicked out of the burrow. To verify an active burrow, wads of paper can be stuffed into the opening or the burrows can be caved in with dirt, then recheck these closed burrows the following day for signs of activity.

House mice living near buildings typically do not burrow unless conditions are right and a suitable food source is nearby. Mouse burrows measure about 1 inch in diameter. Occasionally homeowners inquire about small burrows in and around ornamental plantings and beneath decorative fruit trees. In most cases, these burrows are made by deer mice or moles and not by house mice.

Secluded spots near runways or tunnels are often used for eating places, and piles of debris accumulated in corners are indications of rodent infestations.

Urine stains and odors. Rodent urine is often deposited on their runways and other frequented areas. Under ultra-violet light, rodent urine will fluoresce yellow on burlap bags and pale, blue-white on various packaging paper. Thus, PCOs and food industry personnel often use black lights to inspect for rodent urine contamination on packages. However, some skill is required using black lights because other items such as food starch, glues, dyes and some cleaning agents also fluoresce. The key to an accurate diagnosis when using black lights is to first notice the pattern of the area fluorescing. As rodents often urinate while moving, their patterns appear as a line of fine drops or streaks, whereas chemical spills or food items tend to be large patchy areas, or uniformly spread out. There are also test kits available to chemically verify urine stains.

Rodent odors are often detectable in well established infestations as both rats and mice produce characteristic odors from their urine and various body glands. Rodent odors may be particularly pronounced in large mouse infestations and many persist for prolonged periods.

Rodent sightings and sounds. Obviously, seeing live rodents during an inspection is a sure confirmation of an infestation. But most rodents are nocturnal and/or tend to be very secretive animals, especially if there is a lot of human activity in their area. Generally, if rodents are seen regularly during the day in different locations, there is a heavy infestation. If a quick confirmation of a rodent infestation is desired, then a night inspection will be most valuable.

Various sounds produced by rodents and their young such as high-pitched squeaks, gnawing and scratching sounds, and sounds of rodents fighting can be heard if you conduct the inspection quietly, and listen carefully. With rats, the best to hear these sounds is during an evening inspection.

ESTIMATING RODENT NUMBERS. Clients frequently inquire, "How many rodents do you think are present?" But other than estimating in general terms (i.e., "a few," "not too many," or "a lot"), it is difficult or impractical for the PCO to attempt to estimate rodent numbers with any degree of accuracy. Generally, when rodent signs are fresh and found in abundance, you can assume there is a heavy population present. When the signs are old and few in number, the infestation may consist of only one or a few rodents. But in the cases where there are clearly more than just a couple of rodents, it is better to overestimate the infestation. Underestimating a rodent infestation often results in not placing out enough traps or baits, which then may lead to a series of expensive callbacks, delayed results and unhappy customers. To get a reasonably good - yet practical - estimate of a rodent infestation, try one of the specialized techniques listed in the sidebar accompanying this article.

 

Robert M. Corrigan is a staff specialist in rodent control at the Center for Urban and Industrial Pest Management at Purdue University. Prior to his career with Purdue, he spent four years full time in the pest control industry running a pest control route in the New York City area.

 

REFERENCES & ACKNOWLEDGEMENTS

Thanks to Dr. John Klotz, Purdue University. Some of the best information and photographs published on rodent signs and inspections have been provided by Dr. Harry Pratt and others from the Public Health Service CDC. Their work (1949 and 1982)is listed below.

Meehan, A.P. 1984. Rats and Mice. Their biology and control. Rentokill Limited. East Grinstead. 383

pp.

Pratt, H.D. and R.Z. Brown. 1982. Biological Factors in Domestic Rodent Control. U.S. Dept. of Health

and Human Services. PHS. Centers for Disease Control. HHS Publ No. (CDC) 82-8144. 30 pp.

(This publication was also excerpted in the Sept. 1988 issue of PCT magazine.)

U.S. Public Health Service. 1949. Rat-borne disease prevention and control. Federal Security Agency.

Public Health Service. Communicable Disease Center. Atlanta. 292 pp.