Rats - large rodents known for spreading disease, reproducing prolifically, and apparently for making excellent french food by controlling a chef. These structural invaders exploit food sources efficiently and can quickly become a major problem if left unmanaged. There are two species of rats which commonly invade structures across much of the United States, these are the Norway/Brown rat (Rattus norvegicus) and the Roof/Black rat (Rattus rattus). While most people have a general understanding of what a rat is, either from movies or personal experiences, the task of tackling a rat infestation (or understanding ways to prevent it) can be daunting. In this publication we will share with you key aspects of rat surveillance and management, and will discuss environmental and structural conditions that may complicate and contribute to rat infestation. Spoiler alert - we won’t be sharing any rat inspired recipes.

Warning: This publication will contain images of rat control that some readers may find disturbing. Rodents (especially rats) can pose immense health risks to humans, and therefore their control and elimination is essential. Read further only if you are okay with these types of images.

Despite their size, rats are fairly adept at going unnoticed. This is in part due to species specific behaviors, with Roof rats preferring to travel along elevated areas (hence the name) and Norway rats spending large amounts of time underground. Regardless of species, rats are what we call “neophobic” - they fear new things or the unknown - much like many humans. As such, they often wait to move through their environments until they are comfortable with their surroundings and feel hidden. As a result, there are often other indicators (signs of infestation) that professionals and the public can rely on to identify rat infestations. These include:

1. Fecal evidence: Rats are able to produce between 30 and 50 individual droppings per day on average - quickly inundating areas where infestations are left unchecked. These droppings are roughly ½ to ¾ of an inch in length, cylindrical and dark in color when fresh. As they age the pellets wash out in color and can break apart. Norway rat feces are typically more rounded on the ends (Figure 1), while Roof rat feces tend to have pointier ends (Figure 2). Rat feces should be removed with extreme care, as disturbances and contact with the feces, urine, and particulates can potentially lead to infection with rodent-borne disease. Key things to remember when interacting with rat feces:

   1. Ventilate previously enclosed spaces for at least 30 minutes prior to entry

   2. Wear proper PPE (gloves, respirator/mask, goggles)

   3. You can coat droppings with a 10% bleach solution to control dust and help sterilize prior to clean-up

   4. Place all collected droppings in sealed plastic bags and dispose of immediately in outdoor trash receptacles

   5. Once all droppings are removed, thorough clean and sanitize the affected area

Look for rat feces in and around areas where food is readily available (seeds, pet food, livestock feed, trash), or in secluded areas rarely disturbed (attics, crawlspaces, sheds and barns).

2. Chewing evidence: While rats are more cautious about their environment, they are voracious chewers in areas where they are comfortable. They exhibit “gnawing” behaviors, regularly chewing in order to shave down their constantly growing teeth (Figure 3). Potential materials include wood, plastic, wires, insulation, and even some metal. This extreme chewing behavior can result in property damage, and even damage to electrical appliances, machinery, and an increased risk of electrical fires. To obtain stored food, rats will regularly chew through storage containers (cloth, plastic) and can even chew through finer wire mesh (porch screening). The only reliable “rat-proof” storage is a solid metal container with a firm fitting lid, although keep an eye out for rust as this can compromise the container. Look for chewing evidence alongside feces for identification of rats, as mice also chew and can cause damage.

3. Nesting evidence: Rats possess a strong drive for nesting behaviors, and will often build nests in warm cavities and voids, near food. Again, they are neophobic and prefer to live as close as possible to their food source. To construct these nests they will collect nesting material from around the home, using chewed up paper and insulation, boxes, fabrics, or any appealing material they may come across (Figure 4). They may also build nests in open containers and open areas if they are infrequently visited/disturbed (Figure 5).

4. Rubbing evidence: Rats produce an oily substance known as “sebum” naturally, and it coats their fur. Over time, and as a result of their neophobic behaviors and tendency to move along familiar paths, these oils (along with dirt and grime) will be transferred to surfaces they contact. This will result in “rat rubs”, or dark and oily marks along walls, pipes, fences, and other areas (Figure 6). When fresh these rubs can be smeared, but over time the dark and greasy appearance will fade and dry out. These rubs are clear signs not only of rat presence, but of infestation levels and infestation periods long enough to lead to the presence of rub marks.

In addition to these signs, heavy rat infestation can also result in a musty smell due to the presence of urine, and in periodic scratching and rustling noises as a result of rat movement, feeding, and nesting. If you happen to catch or see one of the rats it is easy to tell the two species apart. Roof rats are characterized by having longer snouts, large eyes and ears, a more slender body, and tails often longer than their body. Norway rats are characterized by having small eyes and ears, shorter and hairless tails, and are overall larger and more rotund (Figure 7).

Figure 1. Several droppings collected from a Norway rat infestation.

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Figure 1. Several droppings collected from a Norway rat infestation.

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Figure 2. Roof rat feces found in an attic above a work shed.

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Figure 2. Roof rat feces found in an attic above a work shed.

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Figure 3. A plastic container that has been chewed through by rats.

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Figure 3. A plastic container that has been chewed through by rats.

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Figure 4. A rat nest in a wall void.

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Figure 4. A rat nest in a wall void.

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Figure 5. A rat nest in an open attic space.

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Figure 5. A rat nest in an open attic space.

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Figure 6. Rat rubs along a pipe, created by the sebum which coats the rat's fur.

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Figure 6. Rat rubs along a pipe, created by the sebum which coats the rat's fur.

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Figure 7. A comparison of common identifying characteristics between roof rats and Norway rats.

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Figure 7. A comparison of common identifying characteristics between roof rats and Norway rats.

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With every pest there are a set of environmental factors that can support the establishment and spread of populations - rats are no different. Just as with humans they have two primary needs to remain in an area: food and shelter. There are a vast array of potential sources for these two needs when it comes to rats (and rodents in general). Below is a brief list of several examples, as well as potential solutions, but it is by no means an exhaustive list. Each rat infestation should be thoroughly evaluated for conditions present in, on, and around structure that could exacerbate the situation and conflict with management plans.

1. Food and water: One of the most common conducive conditions to rodent infestation is the presence of improperly stored feed or seed, which quickly can become a food source for the rodents (Figure 8). Examples of this can include dog and cat food, grains and meal to feed livestock and poultry, bird seed, or even grass seed. Many individuals may feel that storing these items in sealed plastic containers is enough, however, rats are avid chewers and frequently chew through plastic to obtain food. The best storage method for these products is in metal containers with firm-fitting lids.
   
   Of note, gravity feeders or bucket-type feeders for pets and livestock can also become food sources for rat populations (and mice as well). These are sometimes avoided by rats due to the animals near the feeders which use them regularly, but access to these feeders should also be limited. Open sources of water can also serve as conducive conditions, including pet bowls, water troughs or containers for livestock and poultry, bird baths, and even leaky pipes and standing water. Address these as feasible, and based on your level of infestation.

2. Shelter: There are many potential nesting locations for rats to exploit. Specific examples of these can include wood piles (milled lumber and logs for burning), piles of hay or soil, crawlspaces with improperly installed (or missing) vents and doors (Figure 9). Other potential nesting sites exist, and each infestation should be thoroughly examined with potential nesting sites removed or addressed in the most feasible manner.

Figure 8. A bag of bird seed that was suspended from a rope to prevent access by rats. Roof rats were easily able to reach this food source by navigating down the rope.

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Figure 8. A bag of bird seed that was suspended from a rope to prevent access by rats. Roof rats were easily able to reach this food source by navigating down the rope.

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Figure 9. A cluttered lean-to which would serve as ideal nesting habitat for rats.

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Figure 9. A cluttered lean-to which would serve as ideal nesting habitat for rats.

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The key to managing any pest issue is proper diagnosis, and despite multiple potential signs of infestation, none of these take the place of visual confirmation and ID. This is especially important as you want to accurately identify the rat, and rule out other structure-invading rodents. Information on common structure-invading mice can be found in the factsheet, Biology and Behavior of Common Structure-Invading Mice, as well as our recommendations for mouse management!

In terms of rats, especially in settings where infestation could occur quickly (areas with a history of rats, densely populated urban areas), the use of monitoring tools is an essential component of a comprehensive management plan. There are a variety of tools used to monitor rat activity, and in fact many of these monitoring tools also serve as forms of mechanical management. These include products designed to trap and kill rats passively in the environment. All of the potential monitoring devices have different strengths, weaknesses, and risks and should be used by professionals and with a full understanding of the risks involved. Key examples of these include:

1. Snap-traps: These are the traditional spring loaded traps, made specifically for rats, using higher caliber springs and larger bars. They function by providing immediate kill (typically) via a metal bar that “snaps” down onto the neck from above. They are loaded with bait AND THEN set (trust me, this will save your fingers). Once prepared these traps should be placed along walls and in areas where rat evidence is present (Figure 10). When first introduced rats may avoid these traps, as they are new fixtures in the environment, but as part of a long-term surveillance plan they can and do yield results.

   1. Strengths: this device will often provide simultaneous immediate control of the rat that stumbles across it.

   2. Weaknesses: Single-use then requires reset, long-term use requires bait replacement

   3. Risks: Injury to fingers, especially to small curious hands of children, and to pets. Note: Mouse snap-traps are typically not an effective tool for rat surveillance or management (Figure 11). Rat snap-traps are very powerful; great care should be taken to avoid the traps snapping on someone’s finger. In addition, these traps should be placed in areas inaccessible to pets or other non targets, as accidental tripping of the trap can result in broken fingers.

2. Glue boards: These are paper or plastic trays lined with a layer of super glue. This glue can be scented, or bait can be placed in the center of the trap to lure in potential rats. These should be placed in areas of rat evidence or likely contact (along walls in the crawlspace or basement), folded or placed away from potential contact with residents and animals. As with snap-traps, mice glue boards should not be used for rats. Glue boards made specifically for rats will be more effective, typically incorporating stronger glue to prevent the stronger rats pulling themselves free. Of unique note, some evidence exists demonstrating opportunistic predatory behaviors of rats on glue board-trapped mice. As such, mice left for long periods of time on glue boards could become food for rats present in and around the structure.

   1. Strengths: provide longer-term control, as long as the glue remains sticky

   2. Weaknesses: prolonged time to death for caught rats (if this is a concern for you or your client), regularly collects unintended animals or insects (Figure 12).

   3. Risks: contact with hands or paws can lead to entrapment in the glue. This can be resolved by using oil of some kind poured over the glue and trapped area, and gently and repeatedly working the oil and glue until the individual is freed.

3. Live traps: As the name suggests, these are traps designed to catch rats alive, and allow for release into the environment outside of the structure (Figure 13).

   1. Strengths: Can provide mess-free collections of rats from homes where killing the rats is not desired.

   2. Weaknesses: Live traps do not provide food or water to caught rats , and must be checked regularly or trapped individuals will die. If trapped rats are able to escape the traps they enter back into the structure.

   3. Risks: Contact with live rats can occur during release, potentially exposing individuals to disease or disease transmitting arthropods.

Figure 10: A rat captured in a snap trap.

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Figure 10: A rat captured in a snap trap.

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Figure 11: A series of mouse traps placed in an attic in an attempt to catch rats.

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Figure 11: A series of mouse traps placed in an attic in an attempt to catch rats.

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Figure 12: A glue board containing accidental catch.

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Figure 12: A glue board containing accidental catch.

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Figure 13: Live traps deployed into a yard to catch rats.

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Figure 13: Live traps deployed into a yard to catch rats.

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Exclusion is the process of sealing off all potential entry points to prevent rats from entering a building, and while some aspects of exclusion may fall under mechanical management, exclusion should be performed regularly even without evidence of rats. Exclusion is necessary to prevent and manage all structure-invading rodents. Most exclusion can be accomplished with simple tools and construction materials, but some professional pest management companies offer exclusion services for a fee. Ask your company, or the companies you are considering, if this is an option.

Building exterior

Start by identifying potential access points. Gaps of ¼” in size or larger allow for the juvenile rats to enter a structure, while gaps of ½” in size or larger allow for adult rats to enter a structure. Gaps and holes around electrical lines, plumbing and drain pipes, small holes and cracks in foundations, soffits, exhaust vents, roofs, and other areas should be sealed off to prevent the ingress of rodents.

Most small holes and cracks in building siding can be filled with the appropriate type of filler of a suitable color or painted after the material dries. Filler materials that may be used include:

1. Lath Screen or Lath Metal: This is a galvanized, light-gauge metal mesh that is installed over wooden walls before the plaster finish is applied. Lath screen does not rust or bleed through caulk and can be easily compressed to completely fill odd-shaped openings. After forcing the lath screen into holes, slightly expand its sharp edges with a screwdriver to better fill the cracks and force the metal into the edges of the hole (Figure 14).

2. Steel Wool: Only 00-sized steel wool can deter rodents and is an “easy to use” filler for small holes. However, steel wool is flammable, rusts, and rodents often work their way between the steel wool and edges of the opening. Always caulk over steel wool to seal it in place.

3. Copper Mesh: Copper mesh material, such as the commercially available Stuf-Fit®, is a compressible product that does not rust. As is the case with steel wool, it is best to caulk over openings filled with copper mesh as rodents can push the mesh aside to gain entry through an opening.

Larger holes may require a wire mesh before caulking is applied. Holes with large open spaces behind them (voids) that prevent the use of caulks may be covered with thin, galvanized sheet metal and then painted. When using sheet metal, ensure that all edges are securely attached to the structure (Figure 15). Expanding foam is not recommended as rats can easily chew through the material .

Install metal kick plates, metal thresholds, rodent-proof door sweeps or on all exterior doors that allow no more than 1⁄4-inch clearance between the door and floor (Figure 16). Rodent-proof door sweeps are not the same as weather stripping. A rodent-proof door sweep forms a tight seal and is filled with rodent-proof metal mesh. Ensure windows close properly and have intact screens, preferably constructed of metal. Seal all cracks and gaps around windows and door frames with caulk or repair the facings. Attic and crawl space openings should have tight doors, with commercially available screened vents over attic and crawl space openings. Alternatively, they may be screened with 1⁄4-inch-mesh hardware cloth.

Building Interior

All holes, gaps, and cracks in walls, floors, ceilings, and cupboard interiors, and around bathtubs or chimneys, etc., should be sealed to keep rodents from entering rooms. Similarly, all open chases and gaps must be sealed around conduits, electrical wires, water pipes, exhaust vents and ducts, air conditioners, and other elements that penetrate walls, floors, and ceilings. The use of caulk, plaster, and/or paint may be needed to provide an acceptable finish.

Figure 14. An exterior pipe sealed using metal wire mesh and expanding foam. Foam alone is not effective to exclude rats.

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Figure 14. An exterior pipe sealed using metal wire mesh and expanding foam. Foam alone is not effective to exclude rats.

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Figure 15. Full exclusion via a metal panel.

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Figure 15. Full exclusion via a metal panel.

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Figure 16. A door with a gap providing access to exterior rodents.

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Figure 16. A door with a gap providing access to exterior rodents.

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As mentioned above, many of the common monitoring devices can also serve as tools for mechanical management of structure-invading rats. Briefly from the perspective of management:

1. Snap-traps: Snap traps can be baited with various attractants, including food items. Peanut butter or honey can be used to stick other foods to the trigger. Snap traps can also be placed in cardboard or plastic boxes designed to hold snap traps. Snap traps should be avoided in areas accessible to children, pets or other non-targets. Traps should be set along rat runways,or areas where signs of rat activity have been observed, to be most effective (Figure 17). As stated above, only use snap-traps made for rats; snap-traps made for mice are generally ineffective for controlling rats.

2. Glue boards: When rats try to cross the board they become trapped on the surface and eventually die. Glue boards may be scented or can be baited (such as with peanut butter). Place glue boards along rat runs, or in areas where rat evidence is observed. “Boxing” of the glue board can allow them to fit well into narrow areas, but may limit the effectiveness of the traps. Do not use glue boards made for mice when attempting to trap rats, as these are often light weight and able to be moved. Use only glue boards made specifically for rats.

Additionally, as mentioned above, if trapping mice and rats simultaneously, be aware that trapped or killed mice could become a food source for foraging rats.

3. Live traps: These traps regularly contain areas designed for use with glue boards (Figure 18). This can allow for trapping and killing of invading rats, while keeping the sight of the process contained. Alternatively, rats may be trapped and released before they die. Of important note, in some areas, it’s illegal to release captured rodents on someone else’s property, so you should check local regulations before using this method.

All traps should be checked daily, if possible, to remove any dead/live rats. Longer periods of time between checking traps could result in unpleasant odors or experience for customers, residents, or the public. Of additional note, it is important to promptly remove dead rats from traps if rodent bait is also being used to prevent off-target bait exposure among predator species. With all deployed traps, it could take up to a week to begin catching your rats, as they are wary of new objects in their environment. Don’t be discouraged if you don’t catch anything the first few nights - be patient.

Figure 17: A rat run through a crawlspace, with their footprints captured in the dust.

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Figure 17: A rat run through a crawlspace, with their footprints captured in the dust.

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Figure 18: A live trap with an optional glue board installed.

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Figure 18: A live trap with an optional glue board installed.

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Baits

The primary option for chemical-based rat management is through the use of rodenticides - chemicals specifically formulated to kill rodents. The primary mode of action for most rodenticides is as an anticoagulant. In other words, they disrupt the mouse’s ability to clot blood - often by preventing the activation of vitamin K. Ingestion of large amounts of these anticoagulant products lead to internal and external bleeding, resulting in management of exposed mice. Other non-anticoagulant rodenticides are also available and vary in their modes of action.

Typically rodenticides are deployed in the form of a bait, designed to encourage feeding and rodenticide ingestion, and should always be deployed in secure packaging (Fig. 19). This is especially true in rat management, as oftentimes larger amounts of bait are required, and the strength of the rats could easily result in bait relocation. Further, as many rats travel outdoors, external bait stations may be incorporated into a management plan but should be done so safely. Never use exposed rodenticide baits for rodent management, especially not in outdoor settings (Figure 20). These products could easily be carried off and find their way into the digestive tract of many animals.

You may be reading this section and thinking ; “Wait, don’t I need to be able to clot my blood?” - and the answer is yes. Rodenticides are formulated to kill rats and other rodents, which are mammals - and you, your children, pets, and many wild animals (mammals and others) are highly susceptible to poisoning via rodenticide exposure. Secondary or accidental primary exposure of non-target animals to deployed rodenticides can be fatal, and so the utmost care should always be taken when involving rodenticides into your rat management plan. Rodenticides are best handled by pest management professionals. Our recommendation is to rely on professional pest management services if you feel bait is required to manage your level of rat infestation.

If you decide to purchase and use consumer-grade rodenticide baits make sure you follow all label instructions, and deploy the minimum amount necessary for the observed level of infestation. Baits should always be deployed in secure and discrete areas away from pets and children, in secure containers. Certain bait stations also incorporate mechanical control measures, including snap-traps (Figure 21). Rodenticide consumption can often be seen in rat fecal pellets - taking on a hue of the colorful bait they consume. This can help to determine if bait placement and bait selection is appealing to the rats being targeted. Remember - rats are neophobic and so deployed baits and stations may not immediately appeal to them, it could take up to a week for bait consumption to begin. Give the bait some time before assuming the rats you’re dealing with don’t like it.

Repellents

This is a brief statement on repellents as a means of rat management. These “repellents” can include various oils (peppermint), moth balls, ammonia, vinegar, and commercial products that use various mixtures of these. There is some evidence that certain odors can repel rodents ( Jokic et al. 2018), however repellents are not a reliable form of rat management or prevention. Highly volatile oils disappear quickly once introduced into the environment, and may compromise the efficacy of deployed baits and traps.

Disclaimer: Any commercial products mentioned in this publication serve as examples only, and do not represent endorsements of the products by any entities associated with this publication.

Figure 19. A rodent bait station with correctly installed bait blocks.

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Figure 19. A rodent bait station with correctly installed bait blocks.

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Figure 20. A live-catch bucket beside a rat bait box where the rodenticide has been placed on top of the station. Rodenticides should only be used in secure and provided containers.

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Figure 20. A live-catch bucket beside a rat bait box where the rodenticide has been placed on top of the station. Rodenticides should only be used in secure and provided containers.

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Figure 21. A bait box with an included snap trap.

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Figure 21. A bait box with an included snap trap.

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