VERTEBRATE PESTS: Mouse Control In Portable Structures

Most people think of a portable style buildings in the form of a mobile or (trailer) home. But portable style buildings (“portables”) are now a popular form of supplemental space for many commercial operations such as office complexes, school classrooms, adjunct day-care centers, field laboratories, construction headquarters and a host of others.

But as most pest professionals can attest, “portables” tend to be highly vulnerable to mouse invasions. This is because portables provide mice with readily accessible harborage beneath and/or within the portable. Mice find the crawlspaces beneath the portable attractive because it is dark, cool in summer, protected in winter and shields them from predators. And because portables are typically not tightly sealed, mice often gain easy access to the portable interior via some improperly sealed exterior electrical, telephone or gas line or beneath a door threshold gap. Most times, an exploring mouse merely needs to follow its nose towards the food odors or warm air currents that escape from these same gaps.

Once inside, mice typically establish themselves inside closets, cabinet bases or they shimmy up wall utility lines for electrical or plumbing and nest with the suspended ceiling spaces of the portable. In order to reduce the threat of Hantavirus, mouse allergens and other possible health threats from mice, it is important to make every reasonable effort to prevent mice from becoming established inside portables. This is especially important when the portable buildings are used for school classrooms or to house employees.

EXCLUDE THE MICE. Although portables are not as tight as permanent buildings, good mouse exclusion can be achieved without too much difficulty. The following are essential tips for excluding mice from these structures.
1. Most mice enter portables from below. If skirting is used to enclose the crawlspace, it is not possible to mouse-proof the skirting. However, the skirting should be kept as tight as possible and in good contact with the ground to prevent other pests such as raccoons, feral cats, skunks and rats from gaining easy access to the crawl.
2. All doors of the portable should be mouse proofed using high-quality pest brushes that seal the gap between the threshold and the door base.
3. All ventilation screens, louvers used in attic spaces, furnace closets and so forth, should be kept in good repair. All gaps around the frames of screens and louvers should also be kept tightly sealed.
4. Inside, all floor and wall areas through which utility lines penetrate (e.g., gas, plumbing and electrical) should be properly sealed. Any gap of a 6 mm (¼ inch) or more should be properly sealed using a quality sealant (stuffing steel wool rags provides only temporary results).

DON’T ATTRACT THE MICE. All trash along the exterior skirting and beneath the portable must be kept clean on a daily basis. If food trash collects around the base or beneath the portable, mice will be attracted to the portable’s perimeter.

IDENTIFY MOUSE VULNERABLE AREAS. Inside portables, mice most commonly nest and/or forage about in four areas (listed below). These mouse vulnerable areas are the target zones for pest management professionals for installing mousetraps and/or maintenance baiting programs.
1. The crawlspace beneath the portable. Invading mice will often construct platform nests up on the various structural ledges made up of grasses, leaves, feathers or the building’s batting insulation. The mice will also carry in and store relatively large amounts of seeds, nuts and insect carcasses in floor, wall and ceiling nooks.
2. Furnace closets (especially if the closet’s ventilation louver is not in good repair).
3. Beneath kitchenette and bath cabinets where utility lines come up through the floor.
4. Within the suspended ceilings during cold weather months.

ELIMINATING ESTABLISHED MICE. Mice inside portable buildings can be eliminated via the use of rodenticide baits and/or snap traps.

SNAP TRAPPING PROGRAMS. Snap trapping programs offer the fastest killing of mice. Traps can be set out in pairs inside each of the mouse vulnerable areas mentioned above. For light infestations and monitoring purposes, two pairs of traps are recommended for furnace closets and cabinet voids. Other areas can contain as many traps as necessary to effectively monitor and control the specific infestation.

Set traps should be checked frequently. Plastic-style snap traps are sometimes preferred over the common wooden based snap trap because the plastic traps are more durable and can be cleaned with disinfectants more easily. Traps can be baited with small smudges of peanut butter or a few drops of vanilla or orange extract oils or virtually any other food item that facilitates quick bait installment.

MOUSE BAITS. If the proper rodenticide formulation is used and installed correctly in the mouse vulnerable areas and within the new tamper-resistant mouse bait containers, poison mouse baits offer a practical, safe approach to eliminating mice for portable structures. Thus, mouse baits, like cockroach baits, fit well into school IPM programs. The use of baits also significantly decreases the time teachers, custodians and pest professionals need to handle dead or dying mice. In the case of deer mice, minimizing mouse to human contact is obviously desirable.

The installation of mouse baits as a maintenance program is also an obvious choice for those cases when mouse activity in the portables is intermittent. Snap traps are not likely to be checked daily for those buildings that only sustain occasional mouse invasions. What’s more, should snap traps become unset over time due to unsuccessful captures, building vibrations or incidental contact from employees, no control tool exists for any future incoming mice. Rodenticide baits, on the other hand, are available 24 hours a day/7 days a week/365 days a year (providing they are kept fresh).

Bait stations should be inspected every two weeks in those buildings with chronic mouse history and during the onset of the autumn months when many mice disperse from fields to buildings. For other times of year and for those buildings with low mouse activity, the stations need only be inspected and serviced on a monthly basis. Block baits should be replenished on an eight- to 12-week basis or as necessary due to consumption or spoilage of the blocks.

It must be stressed that only block formulation of bait should be used and that the blocks should be installed into mouse size tamper-resistant bait stations in which the blocks will not fall out should the stations be picked up and shaken. One station can be installed indoors into the following areas: 1) within the furnace closet, in the back area of the closet, preferably behind the furnace; 2) beneath any kitchenette sink; 3) beneath any bath cabinet; 4) four stations in the suspended ceiling; one station positioned directly above the kitchenette, bath and nearby the furnace closet. Baits should never be tossed in packet style bait or as blocks into “inaccessible” ceiling or floor spaces.

To control mice in the undersides of the portables, four rat-sized tamper-resistant bait stations should be installed beneath the portable. One station is installed along the middle of each of the four sides of the portable. To accomplish this, each portable must have an access door that it easily opened and closes tightly.

Finally, one of the concerns by homeowners, teachers, office administrators and other persons regarding the use of rodenticide baits inside buildings is that rodents will die in inaccessible spaces and create an odor problem. Although this is a concern with rats (i.e., larger amounts of body tissues) mice poisoned with baits usually do not pose decomposing odor issues inside buildings.

Editor’s note: Watch for next month’s column addressing the myths associated with dead rodent odors inside buildings.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.