**Field Mice** (a term often used for several species, including the **Deer Mouse** (*Peromyscus maniculatus*) and the European **Wood Mouse** (*Apodemus sylvaticus*)) are small rodents that primarily inhabit outdoor environments like fields, gardens, and wooded areas. They are highly skilled foragers, feeding on seeds, nuts, grains, and insects. While they prefer the outdoors, they frequently become nuisance pests by entering homes, garages, and sheds, particularly in the fall and winter, seeking warmth, shelter, and a site to cache food reserves.
Taxonomy and Classification
Field Mice belong to the order Rodentia. They are distinct from the common House Mouse (*Mus musculus*). Key differences are that Field Mice generally have **larger ears and eyes**, and a tail that is longer than their body. Field mice are also the main reservoir for the potentially deadly **Hantavirus** in North America, posing a significant public health risk when they infest structures.
Physical Description and Signs
Field mice are typically $3$ to $5$ inches long (excluding the tail). They have soft, sandy brown or gray fur on their back, often with a clear demarcation to a white or lighter colored belly and feet. They are very agile climbers.
Signs of an indoor infestation include:
- **Droppings:** Small, dark, pellet-shaped droppings, slightly larger than those of a House Mouse, often found along walls, in food cupboards, or in attics/basements.
- **Gnaw Marks:** Small marks on wood, plastics, food packaging, and wires, as they constantly gnaw to keep their incisors worn down.
- **Nests:** Cup-shaped nests made of shredded materials like paper, insulation, and soft plant fibers, typically found in secluded, undisturbed areas.
- **Caching:** Evidence of stored food (seeds, nuts) in hidden spots.
Distribution and Habitat
Field mice are globally distributed. They prefer outdoor habitats near cover, such as woodpiles, overgrown shrubs, tall grasses, and in ground burrows. When they enter buildings, they seek quiet, dark, undisturbed areas like wall voids, cabinets, attics, and stored boxes. They are expert explorers and can squeeze through openings as small as $1/4$ inch in diameter (the size of a pencil eraser).
Behavior and Dangers
Field mice are primarily nocturnal and forage short distances (10–25 feet) from their nest. They are highly driven to find and cache food in the fall to survive the winter.
The dangers associated with field mice include:
- **Disease Transmission:** Feces, urine, and saliva can transmit pathogens, most importantly **Hantavirus Pulmonary Syndrome (HPS)**, which can be contracted by humans when cleaning up droppings or nesting materials. They also spread *Salmonella*.
- **Property Damage:** Gnawing can damage pipes, structural wood, and, critically, electrical wiring, posing a fire risk.
- **Food Contamination:** They contaminate stored food and surfaces with waste and hair as they forage.
Management and Prevention
Effective control requires a three-part Integrated Pest Management (IPM) approach: **Exclusion, Sanitation, and Population Reduction.**
1. Exclusion (Mouse-Proofing)
- **Seal Entry Points:** Inspect the exterior of the home for any openings larger than $1/4$ inch. Seal cracks and holes around utility lines, vents, and foundations using durable materials like **steel wool mixed with caulk** or cement.
- **Doors and Windows:** Ensure all doors and windows fit tightly, installing door sweeps or weather stripping to close gaps at the bottom.
2. Sanitation and Habitat Modification
- **Eliminate Outdoor Cover:** Clear clutter, woodpiles, and debris near the foundation of the home. Keep grass cut short and trim tree branches and shrubs away from the siding.
- **Secure Food Sources:** Store human food, pet food, and bird seed in sealed metal or glass containers. Ensure trash cans have tight-fitting lids. Avoid leaving pet food bowls out overnight.
3. Population Reduction
- **Trapping (Preferred Method):** Snap traps are highly effective, inexpensive, and pose less risk to pets and children than poisons. Bait them with peanut butter, dried fruit, or a small piece of bacon tied securely to the trigger. Place traps along walls, perpendicular to the baseboard, and in concealed areas where droppings are found. Use multiple traps spaced 5–10 feet apart.
- **Rodenticides (Poison Baits):** Should be used with extreme caution and only placed in tamper-resistant bait stations that are secured and clearly labeled, and positioned out of reach of children, pets, and wildlife.
**Safety Note:** When cleaning areas affected by field mice, wear gloves and a respirator. Do not sweep or vacuum droppings, as this can aerosolize viral particles. Instead, wet the droppings and nesting materials with a bleach or disinfectant solution before carefully wiping them up and disposing of them.