[Vertebrate Pests] Exterior Norway rat infestations, part I

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February 26, 2007

Norway (brown) rat infestations commonly occur outdoors in urban environments. Often exterior rat infestations are associated with rat colonies that have established earthen burrow systems. This should be no surprise because we know the brown rat is, by its nature, a ground-burrowing mammal.

But city rats inhabiting exterior areas also will live above ground by moving into discarded furniture, large equipment and appliances (e.g., abandoned vehicles). They also may occupy various peripheral structural voids, nooks and crannies (e.g., crawl spaces, foundation wall voids, overhead soffits, sign facades, etc.). Such periphery rats may forage exclusively on exterior sources of food and not enter interior occupied building areas unless things change or they find easy entry access and a dependable source of food.

In Part I of this column, we focus on the characteristics of exterior burrow systems of the brown rat. In Part II, next month, we will discuss the professional techniques and strategies for managing these infestations specifically via the different types of burrow treatments and the associated safety practices.


RAT BURROW SYSTEM PROFILE. Exterior rat burrow systems can range from the simple to the complex. In a newly established burrow, the number of rats can range from a single rat to a dozen or more within a single family unit. A new (but completed) rat burrow usually contains two or three holes consisting of a primary entrance and one or two escape or "bolt" holes. Sometimes, the escape holes are beneath vegetation or junk and lightly plugged with earth near the exit point. As the term indicates, the "escape hole" facilitates the rats exiting the burrow should a snake or some other enemy enter, or should a water rush in from one hole or another.

In healthy soil environments, burrows are excavated to depths ranging from 12 to 18 inches/30 to 45 cm, but may be several inches shallower. Others have been found to go as deep as 4 feet/1.2 m to allow rats to enter buildings. Burrow lengths range from about 1½ to 6½ feet /0.45 to 1.0 m. The short burrows, which can be a result of newly arriving rats constructing exploratory burrows, serve to provide rats in established colonies with temporary hiding holes from danger within their home ranges. Or, these types of burrows can serve as a supplemental food cache holes for the rat colony (perhaps analogous to the fast food/drive-thru restaurants in our neighborhoods?).

The diameter of an established burrow measures 2 to 3 inches/50 to 76 mm. This space allows for adult rats to access the system efficiently, but deny easy access to most of the rat’s predators with the exception of snakes (also some ferrets and weasels, but these mammals are of little concern to city rats).

Under good conditions (for the rats), a colony can grow and an infestation can become severe. When this happens, the ground area may become "riddled" with burrows and it is possible to have up to a few dozen burrows in a relatively small area. Such infestations can be comprised of rat colonies with a formal social structure. The burrows in these colonies often become labyrinths of interconnected tunnels. One or two of the holes in these systems may become the "preferred" entrances and exits (i.e., protected, most direct access to food, etc.). Over time and after continued use by many rats, the diameters of these holes can expand to 4 inches/15 cm and larger.

As you already know, rats are prey species for many larger animals (e.g., dogs, cats, coyotes, fox, weasels, skunks, raccoons, snakes, hawks and owls). Thus, in addition to their nocturnal cycles and their limited foraging periods, rats tend to establish their burrows under or nearby cover if it is available.

In urban areas, cover is provided by junk piles and/or by common everyday landscaping. For example, unmanaged shrubs and trees are allowed to form round "caves" draping down to the ground (e.g., Yew bushes, boxwoods, low-hanging pine branches, spruce, etc.) or carpet-like landscaping (e.g., creeping junipers, arborvitae, pachysandras, etc.).

In temperate zones, evergreen plants also help to protect burrows from the snow, rain and wind, as well as provide the cooling effects of sun shade over the burrow systems. Dense shrub plantings also serve to collect wind-blown food trash and paper scraps that can be used by the rats for food and nests.

When an area contains sloping grades, embankments or escarpments, rats will often construct the burrows at the high side of the grade. In this way, rain and melting snow moisture will drain away from their nests and decrease the chances of burrow flooding, potentially killing any unweaned pups.

It should be noted that the brown rat is highly adaptable to dampness and is perfectly at home around watery environments such as stream and river banks, sewers, dank basements and the like. Also, remaining dry — at least during the cold weather months — assists the rat in efficiently maintaining the desired body temperatures.

Finally, rat burrows are not always constructed in protected areas. Occasionally, rats construct them in open areas. Open area burrows, however, usually occur relatively close to a food source that is constantly available (e.g., in the ground a few yards away from a food refuse Dumpster).


THE DEN AND NEST. Within the burrow, the rat hollows out a den chamber and constructs a nest. Usually it is located near the mid-section of the burrow, where it is deep and far enough from the burrow holes to allow for protection from danger and from temperature and/or wetness extremes.

A nest is constructed to keep the adults and any pups warm and dry. Nests usually are constructed of natural materials collected from nearby vegetation such as leaves, grasses, weed stems, twigs and the like. In city environments, human litter materials can serve as substitutes for natural materials such as paper wrappers, Styrofoam, cardboard, cigarette butts and a myriad of other litter items. Even paper money has been found in rat nests.


COLONY SIZE. The number of rats occupying these ground colonies is difficult to accurately determine without excavating the burrows. (This actually has been researched.) Studies have shown that colony numbers fluctuate and vary depending on the season, food abundance, disturbance, social stress within the colony and other factors.

However, as a general baseline, a "textbook" family unit of rats is comprised of a dominant male, a breeding female and upwards of 12 juvenile rats ("textbook" descriptions are useful in providing us general scenarios when we are out in the field trying to understand what we are looking at). As the juveniles mature, they eventually disperse from the birth burrow and begin new families by excavating burrows extending out from, or close by, their birth burrow. Or, these rats may disperse some distance and start with the new exploratory burrow cycle. They also may re-occupy burrows that recently have been vacated by rats that have succumbed to age, extermination or predation.


ANALYZING EXTERIOR GROUND INFESTATIONS. On the job, it is pointless to be concerned about the precise numbers of rats occupying ground burrows — such information is of little value. But it is important to be able to at least estimate the level of severity and consequently estimate the amount of work and materials that will be necessary to achieve elimination of the colony. From past publications on urban rat infestations, exterior infestations occurring in typical urban spaces (such as an empty lot, the backyard of an apartment complex, the rats behind the restaurant) are usually categorized as minor, moderate or severe.

Admittedly, such terms can be subjective according to a specific area (e.g., school grounds vs. an empty city lot vs. a dump, etc.). Still, a reasonable label of "minor" could be used to describe up to six burrow holes within an active system; "moderate" up to 12 holes; and "severe" for those infestations containing more than 12 burrow holes.

Again, such categories merely can serve as roadmaps for the pest professional’s purpose; not for a town meeting discussing a rat problem and arguing over whether the rat infestation is officially "minor" or "severe."

The most practical method to estimate the severity of an exterior ground colony of rats is to inspect as many of the burrow holes as possible and note how many openings appear, or can be confirmed to be active. From a simple visual perspective, active burrows have a smooth, well-worn appearance at the entrances. Inactive entrances are usually covered with vegetation or spider webs.

Two techniques can be used to help assess the activity status, category and scope of an exterior rat colony. The simplest is to stuff paper wads into the burrows and re-check a day or two later. In those cases where burrows are scattered over large areas, or areas with abundant cover and junk, flags (pesticide treatment alert flags) can be used to help visually relocate closed burrows. Flags also are valuable if the job needs to be delegated to a different technician the next day or several days later.

Another way to evaluate the size and extent of a burrow infestation is to conduct an evening inspection during the rats’ peak foraging period that occurs in the specific area. Most brown rats become active within an hour after dark, but the peak foraging periods vary according to the situation and sometimes the weather.

Although neither of these will not give you accurate colony numbers, they are helpful in categorizing an infestation as to being minor, moderate or severe.


TIME IS MONEY. Even if it is your first time to an area where rats have been reported, you often can proceed directly to the rats. (When a layperson is with you, he or she will think you’re Sherlock Holmes or some kind of magician.)

Instead of walking all about the area looking for rats, walk about looking for the environment that supports rats. On the job you don’t have time to be the end-all rodentologist or pest detective. Here are four tips that often pay off if there are complaints about rats being repeatedly seen around yards, lots, parks, food shop properties and so forth:

1. Visually scan the complaint area for the human food trash locales. Obviously, these include Dumpsters, trash cans or litter areas.

2. Visually scan within a 100-foot radius of the trash locales for ground cover (e.g., lines of thick shrubbery, carpeting ground cover, junk piles, equipment, cement slabs of one type or another [walkways, statues, perimeter walls, etc.]).

3. If there are sloping embankments, pay particular attention to the "high side" of the areas for the reasons discussed earlier.

4. If the area is a public city park, scan for where the park visitors congregate and eat, feed the pigeons and use the trash cans. Then head for the areas that offer the rats cover.

Next month, we’ll discuss treatment strategies. See you then.


The author is president of RMC Consulting, Richmond, Ind. and can be reached via e-mail at rcorrigan@giemedia.com.