Analyzing Rodent Callbacks

Callbacks in pest management are frustrating and profit-eaters. Too, we must often face an unsatisfied client. With rodents, several factors may come into play as to the reasons for a callback. The purpose of this article is to briefly examine some of the factors responsible for rodent callbacks and present some suggestions for keeping the number of rodent callbacks to a minimum.

The following nine situations that I have found over the years are often responsible for persistent rodent complaints and callbacks. Some of them are attributable to the rodent’s natural behavioral patterns, while others are no more complicated than a technician rushing through an initial job and having to pay the consequences later.

RODENT “HIGH ACTIVITY AREAS” NOT IDENTIFIED.

Rarely do rodents infest buildings evenly throughout all floors and rooms. Instead they establish themselves within reasonable ranges close to food and water (although water is not as critical inside buildings for mice as it is for rats). When food and water are continuously available for prolonged periods, rodent communities will become established nearby. Within these rodent communities, rodents share common harborages, feeding, defecation and food storage areas, as well as various escape burrows and pathways. In turn, the rodent activity within a particular area of a building builds on itself. Pheromone trails are laid down, causing even more activity within the shared areas. Certain pathways, for example, may be used by many rodents in the community for traveling from the nest to the food or water sources (see Figure 1). Or, certain corners within a room may be repeatedly used as a favorite feeding or grooming spot.

When the number of rodents in a community exceeds the available resources, the rodents then begin to move out to other areas within the building which may offer food and space without the competition. In this way, rodents spread not only through our buildings, but also through our cities and towns. Thus, one of the primary goals for effective rodent control is performing thorough initial inspections of all suspected areas within a building that provide mice or rats with abundant food. Then the program must be initiated in these areas.

Too often, perhaps because of time pressures, we sometimes establish traps and baits at only the dosages dictated on the labels (e.g. placing a mouse bait station every 8 to 12 feet along a wall), without any inspection to justify that the wall was within the high activity areas of the rodents. If, unbeknownst to you, the rodent’s community pathway was along the opposite wall, or above in the suspended ceiling, the greatest effect of your efforts and control tools will not be realized. In other words, baiting and trapping are far more effective if we bring the baits as close to the rodents as possible instead of hoping the rodents will intercept or travel to our random placements.

To get into the rodent communities, conduct thorough inspections for abundant droppings, traces of rodent urine and “grease marks,” all of which contain odors (and possibly pheromones) very familiar to other rodents. Control tools placed in these areas will be much more effective than robotic, yardstick placements around walls.

BAIT OR TRAP NUMBERS NOT SUFFICIENT FOR THE INFESTATION.

Underestimating the size of the infestation is another common mistake in rodent control. Often times, we base our “estimates” of the severity of a rodent infestation solely on what we observe during a daytime cursory inspection of a limited area. But rodents are nocturnal, and many of their activity areas may not even be accessible to us. Thus, we may only be seeing a small portion of their evidence.

Unfortunately, we often base our population estimates, and thus our control efforts, on the results of only the evidence that is visible to us. There are no guidelines for establishing accurate population estimates of rodent infestations — at least not to the practicing professional for most everyday accounts. In most cases, we tend to estimate infestation rates as light, medium or severe.

But you should carefully consider the client’s description of the problem, combined with a thorough inspection. In some cases, a night inspection may also be warranted. When populations are underestimated (i.e. categorizing a severe problem as a light or a medium infestation) and not enough traps and baits are installed, we are faced with a frustrating cycle of empty bait stations or good trap catches, but a complaining client and costly callbacks.

Depending on how much the population has been underestimated, this cycle of “harvesting” rodents may continue for months.

When starting out, and for good cost efficiency, the best strategy is too err on the side of overestimating the population, and trap or bait heavily within the high activity areas. Excess baits and traps can be withdrawn as need be with each follow-up visit.

BAIT OR TRAP PLACEMENTS TOO FAR APART.

Careful inspections help to reveal the high activity areas of the rodents. But on-site situation analysis is also important. Consider that the “average” foraging range of the house mouse is stated to be 10 to 30 feet (3 to 10 meters); 50 to 100 feet (15 to 30 meters) for the Norway rat; and 100 to 300 feet (30 to 91 meters) for the roof rat. Keep in mind, however, that these distances are average ranges of these species as determined under the specific conditions of a particular study. On the job, each situation should be interpreted and the ranges considered. In general, rarely are the foraging ranges of the rodents much greater than these, but often they are shorter. This is an important on-the-job guideline.

For instance, when food is abundant and easily accessible, rodents may have foraging areas considerably less than their stated average. Mice living beneath a delicatessen counter in a supermarket containing weeks of spillage may not travel more than 3 to 6 feet for several days or weeks. In this case, if traps or baits are spaced at 10- to 12-foot intervals, it may be several days before some of the mice eventually interact with the traps. Such mice leaving feces or being seen by the deli manager are likely to cause a callback. Thus, you should always conduct a situation analysis at the beginning of any rodent program and strategize accordingly. In general, when clutter and food are abundant, the foraging ranges of rodents tend to be on the shorter end of the scale, and as such, traps or bait stations should be closer together.

This means establishing the minimal distances between trap or bait placements (e.g. every 8 feet for mice and every 15 feet for rats). Spacings can be increased as the population is reduced over time.

STRUCTURAL VOIDS ARE NOT ACCESSED.

House mice and roof rats are adept at accessing hard-to-reach voids within our buildings and equipment. Such voids include suspended ceilings and floors, telecommunication channels, wireways, equipment chases, gondola voids and the like. Rodents use these areas for nesting or as hidden highways which will lead them quickly to food (see Figure 2). In some of our older, larger, inner-city buildings or our major cities, mouse populations may be “deep” within these buildings and very difficult to access and treat. Recently, I inspected an old nursing home which had been remodeled several times over the past 50 years. The building contained two ceiling voids — one within the other. The chronic mouse problem could be traced to the mice nesting and traveling within the older suspended ceiling void. But because this space was tight and time-consuming to access (ladders and lights were needed), the technician was only trapping at those areas where mice were emerging to grab a piece of food, and being seen by the clients. Although some mice were being caught in this manner, the amount of time to get control was significantly extended, and many callbacks resulted in a costly account for the pest management company. For the most efficient control strategy, getting into the inner ceiling void was key. Structural and equipment voids, when they are being used by rodents, must be accessed.

EXTERIOR PRESSURE IS NOT CONSIDERED.

Sometimes, persistent rodent problems originate on the outside of a building. Heavy vegetation (weeds, dense and low-lying ornamental plantings) may support reservoir populations of rodents. Or, a property near your account may be infested with rodents. As the rodents become overpopulated, they disperse to your account. This is a common occurrence in inner-city areas where many buildings are connected or adjoin each other. Utility lines, cellars or roofs may be common among several different properties. Apartments above commercial food stores, restaurants and cellars, for example, are well known for persistent pest problems. Although people install fences, dividing walls and locking doors, rodents do not respect property lines. When exterior areas or adjoining buildings are suspected to be contributing to chronic problems, this should be discussed with the client on the front-end of the service and put in writing. In addition to remedial and maintenance rodent control programs, pest-proofing services and/or recommendations are the key to long-term success. If the client fails to understand this, you will lose big-time in callbacks due to incoming rodents beyond your control.

NEOPHOBIC RATS ARE PRESENT.

It is common on a large rat control program to be faced with callbacks to get the last couple of rats (especially at the tail-end of the program — excuse the pun). Often these are neophobic rats (also referred to as “smart rats” or “super rats”). Neophobic rats are those individuals within the colony which have an extra sense of “caution,” and as such, tend not to readily interact with our traps or bait stations. It is common for neophobic rats to cause weeks’ worth of work and callbacks. The best strategy is to first remove as much of the rats’ suspected food source as possible and replace it with baits and/or pre-baited and disguised rat traps. For further discussion on tricks to capture neophobic rats, see “Vertebrate Pests” in the November 1997 issue of PCT. When pricing initial programs, it is wise to build in the assumption that a few additional visits will be necessary to deal with the last few “smart rats.”

BAIT DOES NOT MATCH THE RODENTS’ PREFERRED FOOD.

Most rodents in urban environments tend to be opportunistic in their feeding behaviors and generally respond well to our grain-based rodenticide baits. Occasionally, however, some rodent colonies or individuals within a colony develop specific food preferences (based on specific environmental factors). These rodents may reject a rodenticide bait, or perhaps a specific formulation. Mice in office complexes, for example, are known to bypass rodent baits in favor of the various types of snack foods found in desks and cafeterias. In these cases, a trapping program can be employed matching the suspected “preferred” foods as the trigger bait (here again, pre-baiting is a wise strategy). Another trick is to place out pellet, meal, seed and block-style baits all together in small amounts at a few locations at the high activity areas. By checking several days later, you will be able to determine if it is grain foods (i.e. baits) they are ignoring, or only a specific formulation of the grain foods. If the rodents select one of the rodenticide formulations, switch all bait placements over to that formulation.

BAITS ARE OLD OR CONTAMINATED.

Sometimes, callbacks are the result of a technician simply not paying attention to the basics of good rodent baiting practices. Remember, any baits installed into an account must compete with the food already available and being consumed by the rodents. In many cases, these foods are quite good (bakery items, restaurant scraps, kitchen foods, snacks, etc.).

Under these circumstances, rodents may not be enticed by moldy baits, or baits contaminated with insecticide, or some other chemical odors or residues. When baits are installed, ensure they are fresh and uncontaminated by any other chemical. Over time, if the bait’s freshness becomes questionable, change the bait.

RESISTANT RODENTS?

Finally, many times when callbacks occur despite seemingly thorough baiting programs, rodenticide bait resistance is suspected by both the client and the pest management professional. Currently, the level (if any) of rodenticide resistance to the second-generation anticoagulants (e.g. bromadiolone, brodifacoum, difethialone) in the U.S. is unknown. Thus far, none has been documented — at least on a formal level — and to any serious scale. Currently, however, it is likely that control failures occurring with rodenticide baits are likely due to one of the above other reasons, and not as a result of true physiological resistance. Nevertheless, in cases of repeated “bait failures,” switching to a non-anticoagulant bait such as bromethalin (e.g. Vengeance), or one of the more potent anticoagulants (e.g. brodifacoum) may offer better control.

Summary. Rodent callbacks can be frustrating and costly. But the bottom-line to avoiding callbacks is to start with careful inspections to identify exterior rodent pressures and interior high activity areas. Then use the correct amount of traps or baits and space them according to the particular situation.

But prior to all of this, it is also important to sell the account to ensure that enough time is allocated for a thorough program which incorporates all of these procedures. Good luck.