VERTEBRATE PESTS: The Fight Against Pallet Mice

Mouse infestations inside warehouses and distribution centers are occasionally among the most challenging of all types of pest management operations for pest professionals. This is especially true of warehouses that store and distribute various seeds (i.e., bird, grass, crop, etc.), bulk storage warehouses of cereals, candies, dry pet foods and reclaim warehouses that process and redistribute foods for various charity organizations.

Mouse infestations inside these warehouses may be from two sources. First, as with most structures, mice enter these warehouses from exterior areas such as grassy fields, weed patches, woodlots and drainage ditches surrounding the warehouse. These mice are traditionally controlled and eliminated via the standard perimeter defense systems of exterior bait stations and interior perimeter wall traps.

The second source of mice inside these warehouses are those that arrive within incoming products as stowaways or those mice that enter from outside and then immediately enter into one of the food pallets. These mice often "hole-up" inside the pallets and feed within (and on) the pallets’ contents. These mice can be appropriately termed "pallet mice." The standardized interior perimeter wall defense program using repeating traps or baits does not significantly impact most pallet mice since they do not leave their pallets. Or, if they do, they typically do not venture far from the pallet bases. Thus, wall traps or baits may only affect some of the mice living in those pallets that are stored nearby the walls.

Once mice have arrived inside pallets and are now inside the warehouse, they must be carefully dealt with to avoid "scattering" as the infested pallet is moved, reorganized or disassembled while food is distributed.

PALLET MICE TIPS. The following strategies are recommended for eliminating or managing pallet mice infestations:

1. Once infested pallets are identified, they can be installed into designated "reefer trailers" or light rail cars and the trailer/car can be fumigated to kill all the mice. For some warehouses, a designated trailer is kept on the property for this periodical treatment. The fumigation route will be most cost effective for those operations where either: a) many pallets are infested or b) infested pallets are a common and periodical occurrence. Some of the food distribution warehouses that handle damaged goods donated for charity purposes tend to be prone to ongoing pallet mouse infestations.

2. It is important for warehouse managers to carefully record those products that mice have been found in and the vendors they come from. It is likely these vendors or product distribution centers have mice infestations themselves. A close eye needs to be kept on all deliveries from these vendors. In some cases, such as food reclaim centers or warehouses that accept donated food materials scheduled for charity purposes, product rejection may not be an option.

3. Some warehouses cannot (or may not want to) fumigate their products. For these situations, the mice can be controlled, but there will be a trade-off in time on the part of the warehouse staff. The non-fumigation route requires disassembling and capturing of the mice infesting some pallets. For these operations, a staging/disinfesting area should be established in a designated corner inside (or outside) the warehouse. This can be done as follows:

a. The size of the corner area can range in size, but allowing about 15 to 20 feet along each of two walls will be required to allow for forklifts to enter the corner area, drop off the pallet and allow employees the space to hand-disassemble the pallet. Larger corner areas can be used if several pallets will be done at the same time or if more employees will be working in the areas.

b. The disinfesting corner must be temporarily enclosed. Installing a cardboard barrier (or Masonite, plywood, etc.) that connects to both ends of the two corner walls (i.e., forming an isosceles or right-angle triangle) easily does this. A height of 24 inches for the barrier wall will provide good protection in preventing any mice scurrying out of the pallets from jumping over the wall.

c. Along the two warehouse walls, rat-size glue tray traps can be installed end to end, lining the walls until the traps meet the cardboard barrier. Or, several multiple- catch "curiosity" mousetraps can also be used with 5-foot spacings between the traps. For severe infestations, traps can be installed along the barrier wall as well. If glue traps are used, it is important to note that the mouse-size glue traps may not be large enough to accommodate the number of mice and that inexpensive glue cardboard traps may not be strong enough to hold some of the scurrying adult mice.

d. Infested (or suspected) pallets can be deposited into the disinfestation corner where the pallet can be disassembled by hand. As each item is taken off the pallet, any pallet mice will scurry (or can be herded) towards the wall and stumble onto the glue "moat" or seek safety by entering the curiosity traps.

e. For any boxes or bags that are noted to contain many mice, these items should also be carefully checked for mouse nests and unweaned mice.

f. Once the disinfesting activity is completed and the mice collected and removed, the floor and wall area should be thoroughly swept and washed with disinfectants to remove any urine or excreta from the scurrying mice.

g. Employees involved in the dis-infesting activities should tape their pant bottoms closed to prevent against the occasional mouse scurrying up someone’s leg. Additionally, musophobic (fearful of mice) employees should not be assigned to these operations. (Author’s note: Musophobia may affect both men and women of all types, backgrounds and occupations. Even professional male athletes may have a strong fear of tiny harmless mice.)

h. The disinfestation corner can be easily disassembled when not in use.

EXTERIOR DISINFESTING EFFORTS. Should a corner of the warehouse not be available for disinfestation practices or if this is not desirable to the warehouse management, disinfesting practices can also be done outside. Designated empty trailers can be used for this purpose, although obviously less space is available for disassembling pallets. Still, glue traps can line the front and the appropriate length of sidewalls of the trailer, forming a disinfesting rectangle instead of the corner triangle.

If the disassembling process is conducted completely outdoors, a dissembling box or triangular area can be established using Masonite panels or strips of sheet metal cut and fitted to construct the appropriate area needed. If constructing walls out of Masonite or sheet metal is not practical or desirable, or if this is done only on an occasional basis, suspected pallets can be surrounded by a ring (moat) of glueboards. For these operations, the glueboard moat should be made two boards wide to capture these mice that will attempt to jump over the boards. In cold climates during the winter, this activity may not be possible due to the temperature. Glue trays are not effective in low temperatures. It is also important to keep in mind that any mice escaping outdoors will likely end up inside the warehouse eventually.

CONCLUSION. Managing and preventing serious cases of pallet mice is possible in commercial food-storage facilities. But warehousing personnel must take a proactive role in inspecting incoming products and directly accessing pallet mice via hand disassembling of pallets or via fumigation efforts. There are no magic wands once pallet mice are inside. In fact, pallet mice are "the untouchables" relative to standard interior rodent control using perimeter wall traps.

Editor’s note: More discussion regarding managing mice inside seed warehouses, distribution centers and food reclamation facilities is covered in Bobby Corrigan’s upcoming book: Rodent Pest Management: A Practical Guide for Pest Management Professionals, which will be available this fall from PCT.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com.