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Recognizing Rodent Infestations

Here are the signs of a rodent infestation in your home.

Recognizing Rodent Infestations

In the Nick of Time

You can often identify the kind of rodent you're dealing with by the size and shape of their droppings. Mice, being the smaller of the two types, leave behind the smallest droppings, measuring on average 1/8 to 1/4 inch. They're about the size of a grain of rice, thin or rod-shaped with pointed ends. Roof rats are larger than house mice but smaller than Norway rats. Their droppings average about 1/4 inch and are spindle-shaped with pointed ends. Droppings left by Norway rats are the largest, measuring between 1/4 to 1/2 inch in length. They're moon- or crescent-shaped and are typically shiny black in color, but this can vary, depending on what they eat.

In the Nick of Time

Mouse droppings can be easily confused with bat droppings. (For more on bat infestations, see Battling Bats in Your Home If in doubt, try to crush a dropping. Bat droppings easily crush into tiny dry fragments, mouse droppings will not.

In the Nick of Time

To track where rodents are traveling, you can create your own dust by sprinkling baby powder, cornstarch, or flour along suspicious areas. This is also an effective approach for monitoring rodent abatement efforts.

Not sure you're sharing your home with a rodent? One or more of the following are surefire signs:

  • Droppings, typically left behind in kitchen cabinets, pantries, cupboards, drawers, bins, and anywhere else they think they might find food, or where they scurry to avoid predators. Rodents are prolific poopers, so it's pretty easy to spot if you have an infestation. It's also not uncommon to see droppings along walls, on top of wall studs or beams, near nests, and in boxes, bags, old furniture, and other objects.

  • Squeaks and other noises. Rodents aren't what you'd call quiet. If they're in your house, you'll hear squeaks, rustling, and scampering sounds as they move about and nest. Noises are often more apparent at night as you're going to bed and they're waking up.

  • Urine pools or trails. Rodents are notorious for having weak bladders, and they'll dribble all over the place. House mice sometimes make things called "urinating pillars," which are small mounds consisting of grease, dirt, and yes, urine. Sometimes you'll see tiny drops of urine leading to a mound.

  • Nibble marks on food boxes, food, or containers. These telltale signs are often accompanied by nearby droppings.

  • Nests. Rodents build nests from soft, fuzzy, or warm materials, such as fabric, furniture stuffing, quilt batting, shredded paper, grass, and twigs, and will typically stuff them into sheltered, out-of-the-way places like boxes, cabinets and closets, walls, even the subspace between ceilings and floors. Other possible mouse nest sites include dressers, behind and inside appliances, and machinery, even computer cases -- basically, anywhere it's cozy and warm.

  • Grease marks. Mice can wedge through openings as small as a quarter of an inch in size. As they do, they often leave greasy smears — caused by oil and dirt in their coats — behind. The marks left by mice are fainter than those left by rats. If you find large greasy smears, you should suspect a rat infestation instead.

  • Gnaw marks. Gnawing is a defining characteristic of all rodents. They do it to keep their incisor teeth, which grow continually, in check. Wood is a favorite,but they'll pretty much chew on whatever suits them. This includes electrical wire, which, as noted in Electrical Fires, makes them a leading cause of structural fires. On wood, newer gnaws are light colored. They turn darker with age. Sometimes you won't see gnaw marks, but you'll see what looks like fine wood chips or coarse sawdust, especially along baseboards, door and window frames, and cabinets.

  • Holes in food packaging. Rodents will nibble into anything they can smell, including boxes and bags of pasta, rice, beans, and grain products. Dog food bags are also prime-time rodent magnets, and especially so for rats, who like the meaty smell as much as canines do. Another popular nibble, although not a food product: soap.

  • An "off" aroma, or smell. House mice have a distinctive musky odor. It's hard to describe, but once you smell it, you'll never forget it.

  • Tracks. Look for footprints or tail marks in dusty spots. The type of track and tail marks can tell you what kind of rodent you're battling. Mice have the smallest feet, measuring 3/8 inch or less. Rat tracks average between 3/4 to 1 inch. Rats also drag their tails, which leaves a mark between their feet tracks. If tracks are hard to spot, shining a flashlight across a suspicious area can help illuminate them.

  • Pet excitement. If Rover or Miss Kitty is acting a bit nuts (more nuts than usual?), especially around a possible mouse hiding area, chances are good a critter has been there or is still there.

Rodents are nocturnal, so you probably won't see many of them unless you've got a big infestation going on. That said, mice tend to be more active than rats during daylight hours.

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