The Mouse-Free Warehouse

Warehouses are a standard component of modern business society. Technically, even a small storeroom could be classified as a warehouse; however, when one thinks of a warehouse, a vision of tall metal racks and hundreds of heavily laden pallets comes to mind.

Many types of commercial facilities contain a warehouse of some sort. The back storeroom of a supermarket is a type of warehouse, as are the storage areas behind the display aisles of clothing and other goods in retail stores in shopping malls. Larger warehousing facilities might be found in businesses located in strip mall buildings. In fact, many industrial areas have distribution centers where each building consists of small warehousing units owned by various businesses.

The buildings that meet our preconceived notions of a warehouse are those huge structures totaling thousands to millions of square feet of storage space with ceilings 25 feet high and storage racks that climb to those very heights. Warehouses such as these are often stand-alone distribution centers; however, large warehouses are often incorporated into food processing, manufacturing, and similar facilities. A relatively recent invention is the "shopping warehouse" such as Sam's Club, where consumers take products right off the pallets.

All of these types of warehouses have several characteristics in common: they store items, often on pallets; they have one or more large overhead doors; and they experience problems with mice on a regular basis. This article provides guidelines for establishing an effective preventive mouse control program for a warehouse facility and discusses the procedures for eliminating a mouse infestation that might be found in such a facility.

PREVENTIVE MOUSE CONTROL. The house mouse is a truly cosmopolitan pest that can be found almost anywhere people live around the world. House mice are more important pests inside buildings than rats for a number of reasons:

(1) Mice are more numerous than rats in towns and cities.

(2) The small size of a mouse allows it to utilize more potential openings to enter a building.

(3) The small territories of mice and their high reproduction rate allow larger populations to build more quickly.

(4) Mice are capable of surviving on food alone, without free water, which permits them to exploit more areas in a warehouse.

Warehouse facilities tend to be built in industrial complexes that are often bordered by fields, wooded areas, rivers, creeks, or railroad beds. Each of these environments ensures that plenty of mice will be present outside buildings during most times of the year. The extent of the favorable conditions in these areas determines the risk to a building of mouse invasions.

• Step 1: Attract as Few Mice as Possible. One of the first steps in an effective mouse control program is a survey of the surrounding property to determine the risk of mouse infestation. The presence of piles of debris, tall grass, stacks of pallets, a poorly maintained dumpster, drainage ditches, poor drainage, and spilled food items all increase the likelihood of larger mouse populations.

The more mice that are present near a building, the more likely it is that mice will enter the building. Pest management professionals need to use their trained eyes to identify conditions that contribute to mouse survival and prepare a list of recommendations for the customer. The customer must be "sold" on the idea of eliminating or minimizing these conditions. Managers of warehouses that store food products tend to be more cooperative because of the regulatory pressures placed on them and the fact that their customers are not likely to be happy if a pallet arrives with mice living in it.

• Step 2: Implement Outside Control Measures. No matter how thoroughly contributing conditions are reduced outside, some mice will still be present and could move to the building and then enter. This is true in situations where an adjacent property provides ideal conditions for mouse survival, such as a grain elevator. The goal of placing rodent control devices outside is to further reduce the numbers of mice that reach the building.

It is always best to start a "first line of defense" away from the building at the fence line if one is present. Tamper-resistant bait stations containing rodenticide bait blocks can be attached to the fence or otherwise secured. Bait blocks should be wired inside the station to prevent rodents from dragging the bait out. A good rule of thumb is to space bait stations at 75-foot intervals along a fence line. In areas of high mouse "pressure," or activity for example, the fence bordering the grain elevator next door the stations should be placed closer together.

If a fence line is not present, "bait posts" can be installed near the perimeter of the property. Bait posts are simply fence posts installed in the ground at regularly spaced intervals to which tamper-resistant bait stations are secured.

The purpose of establishing this first line of defense is to provide the mice with a food source away from the building that results in fewer mice possibly moving up to the building. One of the most frequent mistakes made in establishing preventive rodent control programs is ignoring this factor.

The "second line of defense" is the placement of tamper-resistant bait stations along the building's foundation. Again, bait blocks should be secured inside the stations for added safety. Space foundation stations about 50 feet apart and closer together in high mouse pressure areas. Multiple catch traps can also be utilized along building foundations outside, especially in covered areas under loading dock overhangs, near dumpsters, etc.

• Step 3: Build the Mice Out. No matter how well-planned the exterior control program may be, a few mice will always make it to the building, ignore the foundation bait stations, and attempt to enter the building. Even mice that did feed on rodenticide blocks outside may still try to enter even though they will soon die. For this reason, rodent-proofing is the most important factor in limiting mouse invasions, and provides the "third line of defense" against rodents.

To effectively seal out mice, all openings ¼ inch or larger need to be identified and sealed. This is easier said than done, and for many older structures, numerous overlooked entry points will usually be left unsealed. The alert professional always keeps an eye out, however, to discover new or overlooked openings, and then is prepared to seal small entry points. Weatherstrips at the bottom of doors is the most common entry point for mice; these should be regularly checked for openings that could allow mouse entry.

Most pest control companies rarely consider offering rodent-proofing services to their customers. Enterprising PCOs, however, are reaping the added revenues that rodent-proofing services can bring. Rodent-proofing should be a integral part of every rodent control program because of its obvious importance to the success of the program.

A rodent control program that continuously captures mice inside week after week is really not providing any benefit to the customer. At some point, mouse activity should be at a zero level, and if mice can continuously enter, this will not be achieved. Rodent-proofing left up to the customer to complete may well be the biggest factor leading ultimately to customer cancellations of rodent control services.

• Step 4: Catch Them as They Enter. No matter how effectively holes are sealed in a building's exterior, if doors remain propped open for long periods of time, mice will invariably enter. Many times, this cannot be avoided, and if weather-stripping is damaged or poorly installed, mice will enter under doors.

A good procedure to follow for mouse control is to place a multiple catch trap inside on either side of exterior doorways. Traps are better to use than bait stations beside doorways. When a mouse enters a building, it wants to find some place to hide, quickly. A mouse is likely to enter the hole of a bait station or a trap to hide temporarily. The difference between the two devices is that if a mouse enters the multiple catch trap, it is captured. If the bait station is entered, the mouse can exit any time it wants to without having fed on the rodenticide bait inside, and a mouse is not likely to feed on the bait immediately after entering.

This procedure establishes the "fourth line of defense" against mouse infestations. Although some mice may still ignore traps as they enter, each mouse captured is one that will not be "setting up shop" and starting a family inside the warehouse.

• Step 5: Interior Rodent Control Devices. Because mice may still elude all of the four lines of defense placed in their path and establish themselves in the warehouse, rodent control devices will need to be regularly spaced throughout the warehouse. In food warehouses, the specifications may limit the devices that are permitted to be used to traps only. In nonfood warehouses, both traps and baits can be utilized. Space the devices at about 25- to 30-foot intervals except in areas where mouse activity is detected. In those areas, use more devices and place them closer together.

Select the devices that best fit the situation, but follow one rule: use a variety of devices. Relying on baits only or multiple catch traps only, or glue traps only generally leads to failure. Even though mice are basically curious, some mice do not interact with certain devices or feed on certain types of bait. Versatility is a key to success.

Another key to success is to not depend on the preventive interior control devices as the sole means of dealing with sudden mouse activity. When evidence of activity appears, place more devices in the activity area and utilize some of the procedures that will be discussed later in dealing with active mouse infestations. "Standing pat" with the original rodent control plan is likely to end up with the pest management professional standing outside the door waving "goodbye" to that customer. Always be flexible and involve the customer, if only to educate them.

• Step 6: Sanitation. A good mouse control program includes attention to sanitation, particularly in and around a warehouse. A single mouse needs relatively little food to get along, so the more effective the cleaning program, the fewer numbers of mice that should be able to survive and thrive. The pest management professional always needs to note and report food spillage, especially that which collects in cracks or in the storage racks.

ACTIVE INFESTATIONS. By nature, mice are curious creatures, and readily explore new items in their environment. They are "nibblers" and will feed on a variety of different foods each night. They establish well-used runways and also favorite feeding sites where they carry food to eat. Although mice do not need to drink free water each day, they readily drink water when they encounter it.

All of these habits are important in establishing an effective mouse control program in any structure, especially a warehouse. To eliminate a mouse infestation, each and every mouse in the building will need to interact with and then commit to one of the rodent control devices placed out for them. A common mistake made in mouse control programs is to assume that one is getting "control" of the infestation just because mice are being captured in traps or are feeding on rodenticides.

Purdue University rodent specialist Dr. Robert M. Corrigan has performed numerous studies on the efficacies of various mouse traps. In these studies, numerous mice will be captured night after night for a week or more. Certainly, these are heavily infested buildings; nevertheless you will not capture every mouse in one or two nights, or even a goodly portion of the mice on any given night. The mice that do survive each succeeding night are the "survivors" and are more cautious mice.

Each mouse is an individual. Although in general they tend to be curious and to do a good deal of exploring, some mice are cautious by nature. A mouse that investigates (interacts with) a multiple catch trap will not necessarily enter that trap (commit to it) and be captured. That same mouse might walk right by glue traps, snap traps, and bait stations and ignore them night after night. Then one night, for some reason, it may pick a device to interact with, then eat a lethal dose of bait or be captured in a trap. It is safe to assume that for each type of device used, some mice in the building will ignore that device and never interact with it. The job of the pest management professional, therefore, is to use a variety of devices and use them in such a manner that all the mice will eventually be captured or killed.

Research has demonstrated that if only 96% of a mouse population is killed, the infestation can eventually rebound. If 100 mice are in a warehouse and 96 are captured or killed, the remaining four can continue the infestation. If new mice continue to come in due to poor rodent-proofing, then forget about elimination or even coming close to exceeding 96% control.

Persistence is a virtue when it comes to addressing mouse infestations in any building. The number, type, and placement of mouse control devices may need regular changing to eliminate an infestation. These changes are necessary to eventually entice every mouse to interact with and commit to one of the devices. Only this approach, together with correcting contributing conditions such as cleaning up food spills, reducing harborages, and rodent-proofing, will achieve the results the customer desires.

CONTRIBUTING CONDITIONS. The elimination of potential mouse harborages and food sources and the sealing of rodent entry points are critical in limiting the reinfestation of the building and improving the effectiveness of traps and baits. Proper storage practices are also important to provide inspection lanes along walls and to provide space for placing control devices.

USING BAITS. Bait stations should be interspersed among the traps during the initial week or two of the infestation. As time passes, more bait stations can be used. Experiment with different rodenticides and different formulations. For example, where mice are active in grass or bird seed storage areas, a seed bait may be more effective, but also use pellets and/or blocks.

Mice establish favorite feeding sites that are usually located in corners in protected areas or behind boxes. Mice seem to prefer eating with their backsides protected from potential enemies. Favorite feeding sites are often strewn with mouse droppings and should be a key item to look for during the survey. Placing a bait station (or snap trap) near the sites should maximize success.

In food warehouses, the use of baits is usually restricted or prohibited depending on the facility's own specifications. A warehouse that is part of a food processing plant is not likely to permit rodenticide use indoors. In these situations, the ability to use traps effectively will be more pronounced and important.

The use of liquid bait is often overlooked for mouse control. This belief probably arises from the fact that mice can survive without drinking water every day. Mice do, however, drink whenever water is available. When possible, place liquid bait founts at floor level along walls and between pallets in storage racks. The results can be especially devastating to mouse infestations in warehouses where water sources are scarce. It is important to check these stations regularly to replace the liquid bait as it stagnates.

STORAGE RACKS. Larger warehouses have aisles of storage racks where two pallets back up to each other on each level and down the aisle. The space between the two pallets is often minimal at best and prevents the placement of multiple catch traps and most bait stations. Because pallets may be moved in and out, exposed glue traps and snap traps may be disturbed and affected as pallets are pushed against them or placed on top of them.

Mice can and will live their entire lives among a few pallets of a food product or seeds. It is necessary to take steps to place control devices where the mice can encounter them. Small, sturdy stations such as Bell Laboratories' RTU mouse station can be used in between pallets. Liquid bait founts can be placed at either end of the aisle and in the middle in a couple of spots where storage rack poles can protect the baiter. It is also possible to place snap traps between storage rack poles that back up to each other in the middle of the aisle.

One technique that works well in this situation utilizes PVC pipe and cardboard-backed glue traps. Cut 16-inch lengths of 2- or 3-inch diameter PVC pipe and insert a cardboard glue trap into the center of the pipe. This pipe can now be placed anywhere between pallets without fear that the pipes will be broken as pallets are moved. This technique will surprisingly catch a number of mice, but it should not be relied on as the sole control device used in a given area.

SPECIAL SITUATIONS. When dried pet food, grass seed and bird seed are stored in a warehouse, they become a particular center for mouse activity. The reasons for this are twofold:

• These foods provide all the nourishment and moisture a mouse could want.

• Spillage is always present around and under pallets of these products.

A significant infestation of mice can literally be introduced into a warehouse on pallets of seeds or dried pet food. A colony of mice may never leave a pallet of these products, and only sends its overabundant offspring to infest other areas of the warehouse. When mice are found to be infesting pallets containing seeds or pet food, two control options are possible.

(1) The quickest control method is to move the infested pallets onto a trailer in the parking lot and fumigate them. Follow the label directions for fumigating rodents on any particular food-based product. While the pallets are being fumigated, food spillage should be completely removed from floors, cracks and under neighboring pallets. Grass seed should not be fumigated, as the fumigation can "kill" the seed and prevent it from germinating after it is planted.

After the fumigation, the pallets should be unstacked onto new pallets so dead mice and their evidence can be removed. The warehouse will likely not want to ship evidence of a mouse infestation to its customers, if this can be avoided.

(2) If fumigation is out of the question or if grass seed is involved, infested pallets should be moved to a loading dock or an open area. The pallet should be surrounded with a continual line of glue traps and multiple catch traps maintained along walls nearby. Unstack the infested pallet onto a new pallet. Hopefully, as mice exit the pallet to escape, they will be captured by the surrounding traps. In any event, the displaced mice will be more likely to encounter and commit to control devices placed elsewhere in the warehouse if they are not trapped immediately.

It is a good idea to advise the warehouse to store such items off the floor level to minimize mice getting into them in the future.

FOLLOW-UP VISITS. Pest management professionals often do not follow up mouse control efforts as quickly or as often as they should. Traps need to be maintained, replaced, rebaited, and moved. Mouse carcasses need to be discovered and removed. Rodenticide baits need to be refreshed or changed.

The larger the initial mouse population, the longer it will take to achieve total control. It will take time to entice every mouse to interact with and commit to one of the control devices. Visit daily for the first week and then several times a week thereafter until success has been achieved.

CONCLUSION. If done thoroughly, mouse control is not difficult. Persistence and creativity will be your masters; serve them well and you will be very successful. Remember, take all necessary steps to limit and prevent mouse invasions and use a variety of numerous control devices placed close together in all control programs. Follow-up! Follow up! Follow up!

Elimination of a warehouse mouse infestation will take more than a day or a week. Daily follow-up visits are the best course of action where practicable or where the situation dictates the quickest results possible.

Stoy A. Hedges, a contributing author to Pest Control Technology magazine, is manager of technical services for Terminix International, Memphis, Tenn.

Sidebar: The Survey

Before placing a single trap or bait station, a survey inside and outside the warehouse must be made to determine the following:

• Areas of mouse activity.

• Potential harborages.

• Rodent entry points.

• Rodent food sources.

• Other contributing conditions.

The survey may include the placement of tracking patches of talc to determine mouse activity and to gauge the general size of the mouse population. The patches are placed during the initial survey and then checked over the next several nights. Tracking patches can also be utilized during the course of the control program to evaluate results.

A graph of the facility will assist in noting the areas of activity and contributing conditions noted on the graph. The graph will also be used to record the location of rodent control devices.

The results of the survey are used to plan the overall program to eliminate the activity inside and to prevent new infestations. — Stoy Hedges

Sidebar: The Importance Of Traps

Snap traps, multiple catch traps, and (to a lesser degree) glue traps should be the primary devices used initially to control an active mouse infestation. Quick reduction in the mouse population is important to the customer, and traps can be an efficient means for accomplishing this goal.

When using mouse traps, one simple rule holds true: "You can never use too many traps!" A common mistake a PCO can make in controlling mice using traps is to place them a certain number of feet apart, e.g. every 15 feet, thereby limiting himself to the particular number of traps that fits that interval. Remember, every mouse has to interact with and commit to a control device. The more traps that are placed, the greater the likelihood any particular mouse will at least interact with one or more traps. The more interaction, the more likely the mouse will commit and be captured.

A secondary consideration comes into play in heavy infestations. As a mouse population grows, each mouse's territory shrinks in size. The larger the population, the closer traps need to be placed to each other. To get any particular mouse to interact with a trap, it is often effective to use several different traps within that mouse's territory. If that mouse's territory shrinks from 20 feet to 10 feet, the best results will be achieved by placing at least two different types of traps within that 10-foot span. An effective approach is to use all three types of traps every 8 to 10 feet; or to use two snap traps with two different bait attractants; or to use a Tin Cat and a wind-up trap. The important factor is to use lots of traps in areas where mice are active and to vary the type of traps used and the presentation of the traps. Be creative and change strategies when success is not being realized. — Stoy Hedges