Illustration by Britt Spencer

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. Warehouses associated with food production facilities are structures totaling thousands of square feet of storage space with ceilings 20 to 25 feet high and storage racks that climb to 15 or more feet above the floor.

All warehouses have several characteristics in common — they store items, usually on pallets; they have one or more large overhead doors; they experience problems with mice at one time or another.

That last characteristic makes establishing an effective preventive mouse control program for a warehouse facility and eliminating a mouse infestation all the more important. Here are some tips, tricks and more.

Preventive Mouse Control.

The house mouse is a truly cosmopolitan pest that can be found almost anywhere people live in most parts of the world. House mice are worse pests inside buildings than rats for a number of reasons.

  1. Mice are simply 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. Their small territories and high reproduction rate allow larger populations to build more quickly.
  4. Mice are capable of surviving on food alone — without available water — which lets them exploit more areas in a warehouse.

Warehouse facilities tend to be built in industrial complexes that may be 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. The first step 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 present near a building, the more likely it is that mice will enter the building. The inspector needs to use his or her trained eye to identify conditions possibly contributing to mouse survival and prepare a list of recommendations.

Step 2: Implement outside control measures. No matter how thoroughly contributing conditions are reduced outside, some mice will still be present and could move toward the building and eventually enter. This is especially true in situations where an adjacent property provides ideal conditions for mouse survival, such as a grain elevator or a field. The goal of placing rodent control devices outside is to monitor for activity and to reduce the numbers of mice managing to reach the building.

It is always best to start your first line of defense away from the building — at the fence line if one is present. The purpose of establishing this first line of defense is to intercept mice possibly moving up to the building.

Increasingly, nontoxic bait blocks are being placed at fence lines (or next to buildings) in tamper-resistant bait stations to first monitor for mouse activity before enacting rodent control measures. Once activity is detected, the nontoxic bait is replaced with rodenticide bait or snap traps to kill or capture mice. Where high levels of activity are noted, additional stations may be temporarily placed.

No matter how well planned exterior control measures are, a few mice will always make it to the building and attempt to enter.

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. Often, bait stations are spaced 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 — stations should be placed closer together.

If a fence line is not available, it is possible to install bait posts near the perimeter of the property. A bait post is simply the installation of a fence post in the ground at regularly spaced intervals to which a tamper-resistant bait station can be secured.

Step 3: Analyze past captures. Individual buildings demonstrate patterns of pest activity over time. The more past mouse capture/activity data a facility has to analyze, the better one can plan to keep mice out of the facility.

Third-party audit agencies often dictate the spacing of rodent devices along the exterior foundation of the building. For any particular warehouse, most devices may never show any pest activity and in reality, take valuable time away from the pest professional that might be spent elsewhere in inspection and pest service.

The house mouse is a truly cosmopolitan pest that can be found almost anywhere people live in most parts of the world.
© Szasz-Fabian Jozsef | Adobe Stock

Analysis of past mouse activity data can pinpoint those areas of a particular building that have the highest risk of mouse activity and demonstrate to an auditing agency the need to target such sites with greater mouse control efforts while minimizing checks in areas where activity may be minimal or nonexistent.

In general, the most important areas along the foundation to focus efforts are around doors leading into the building and walls that face situations with high mouse pressure, such as a field, grain storage or rail lines.

Step 4: Build the mice out. No matter how well planned exterior control measures are outside a warehouse, a few mice will always make it to the building, ignore the exterior control devices and attempt to enter. Even mice that did feed on any rodenticide in stations outside may still try to enter even though they will soon die. For this reason, rodent proofing is probably the most important factor in limiting mouse invasions. In general, the older the building, the more difficult it may be to effectively seal out mice, but this is not always the case.

To effectively seal out mice, all openings one-quarter 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. Weatherstrips at the bottom of doors is the most common entry point for mice; these should be regularly checked for spaces allowing possible mouse entry.

A mouse control program that continuously captures mice week after week is not providing any real benefit. At some point, mouse activity should cease to be present, and if mice can continuously enter, the mouse-free warehouse is just a dream. Rodent proofing may well be the largest factor in continued interior mouse activity.

Step 5: 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 weatherstripping is damaged or poorly installed, mice will enter under doors anyway.

Placement of multiple-catch mouse traps inside on either side of exterior doorways is a good practice for capturing mice as they enter. Where space allows, such traps should be placed several feet from a doorway. A trap right next to the door opening may be bypassed by mice as they enter.

Step 6: Use interior rodent control devices. Because mice may still elude all the lines of defense placed in their path and establish themselves inside the warehouse, rodent control devices may need to be placed elsewhere inside the warehouse in sites where mouse activity has been noted.

Select the devices best fitting the situation but follow one rule: Use a variety of devices when live activity is present. Relying on just multiple-catch traps, snap traps or glue traps often leads to failure. Even though mice are basically curious, some mice simply will not interact with a particular type of device. 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 out more devices in the activity area and utilize some of the procedures that will be discussed later in dealing with active mouse infestations.

Step 7: Make sure sanitation is up to par. A good mouse control program includes attention to sanitation, particularly in 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 professional always needs to watch out for and report food spillage, especially that which collects in cracks or within rails of storage racks.

Step 8: Keep an eye out for pallet mice. On occasion, mice colonies living within a pallet of food can be delivered into a warehouse, bypassing all mouse prevention and control measures. Pallets of seed products (grass, bird, whole grains) are commonly the culprit. Warehouse employees should be trained to look for signs of mice on pallets and, if found, the facility should have a plan for isolating infested pallets and eliminating these pallet mice.

Active Infestations.

By nature, mice are curious creatures and readily explore new items in their environment. To eliminate a mouse infestation, each mouse in the building will need to interact with and then commit to one of the control devices. 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.

Research has demonstrated that if only 96% of a mouse population is killed, the infestation will persist and 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 enter due to inadequate rodent proofing, forget about elimination or even coming close to exceeding 96% control.

Mouse control is not a difficult science. Persistence and creativity will be your masters; serve them well and you will be successful.

Persistence is a virtue when it comes to addressing mouse infestations in any building. The number, type and placement of rodent control devices must change over each week of the effort 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 desired results.

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 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.

Mouse control is not a difficult science. Persistence and creativity will be your masters; serve them well and you will be 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. Elimination of a 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.

The author is the owner of Stoy Pest Consulting, one of the country’s leading urban entomologists and an author of several books.