Vertebrate Pests

The significance of the three domestic rodents as pests in the United States has increased throughout the past several years. This is evidenced by the increase in calls to pest management professionals, health departments and municipal rodent control officials. In addition, sales of over-the-counter rodent traps and rodenticide baits are also up. Attention by the mass media is another indicator. Several of the largest U.S. city newspapers during the past decade have run front-page stories about local rat and mouse "explosions." There have also been numerous televisions documentaries addressing urban rat populations. Consequently, rodent business for pest management professionals is also up. Of course, on one hand this is welcome. On the other hand, the percentage of callbacks due to rats and mice has risen dramatically, causing pest professionals everywhere some headaches.

Let’s examine the factors that affect urban rodent populations and why from one year to another or from one area to another there may be increases or decreases in rodent activity.

Population Guesstimates. Actually, it is difficult to claim an increase or a decrease in the urban rodent populations when we don’t have accurate baseline data to begin with. This is because it is (for all practical purposes) impossible to estimate and track urban rodent populations with any type of scientific accuracy. No one really knows how many mice or rats live in New York, Chicago, Los Angeles or any other city. Unfortunately, the media often projects estimates of "the number of rats per person" in a particular city.

Articles have been written stating the number of rats as increasing throughout the past 30 years from one to six to nine rats per person. Yet the number of rodents per capita within our major cities and whether the number is increasing or decreasing is unknown. It is probably safe to say there are several million rats and mice co-existing with us. Some areas of every city and town have more rodents than other areas, just as some buildings may have more rodents living in them than other buildings nearby.

Environmental Trends. Many people comment that the current abundance of rodents must be due to the repeated "mild winters" throughout the past decade. It is true that many animals are subject to population declines as a result of repeated harsh winters. Several years of mild winters may contribute to an increase in feral ("wild") populations of house mice, Norway rats and roof rats. Increases in the feral populations may eventually be seen in urban and suburban areas as more feral rodents disperse into developed areas. Still, mild winters play only one part in commensal rodent population dynamics.

Some parts of the United States have been subjected to prolonged droughts. Severe droughts will cause rodents to disperse from fields and other natural cover to areas that may contain more sources of moisture. However, droughts typically cause only occasional peaks in rodent activity in the years the drought occurs.

Construction and Demolition. Rodents take good advantage of our complex urban structural environments. Our largest cities are aging and undergoing many changes on a daily basis. Most older city buildings have hundreds (if not thousands) of various nooks and crannies in which rodents have been hiding from us — especially mice. In some cases, they have had these concealed harborages for up to 150 years — especially the smaller mouse. For rats, deteriorating walls, sidewalks and sewer systems all provide good habitat. Ignored and/or poorly selected landscaped areas (low-lying shrubs and bushes) also provide an important rat habitat. Many people do not realize that the nearest rat family may literally be right beneath their feet on some city sidewalk or nearby where they sit for lunch outside beneath their office building’s landscaping.

Additionally, large office complexes and multi-family housing units are complex structures, containing all types of vertical and horizontal utility chases. Gaining access to rodents within these structures can be difficult and in some cases, impossible. In fact, some architects estimate that humans have access to only 50 percent of a building once the building is completed. The inability to access all rodents plays an important role in professionals not being able to deliver a knockout punch to these pests even when they are equipped with outstanding rodenticides and effective traps.

Large construction and demolition projects also play a significant role in disrupting and dispersing rodent populations. In areas where there hasn’t been any construction for extended periods, rat populations can have activity patterns that go largely unnoticed in these stabilized neighborhoods for long periods of time. Once these areas are disrupted via construction or demolition projects, the established rats are dispersed into new areas.

Without their normal cover and feeding patterns, displaced rats or mice are readily encountered by people in those areas. These sudden encounters in areas that may have been previously perceived to be "rat free" often result in loud and repeated calls into the local board of health, city officials, pest management professionals and occasionally, the media. The result in these areas is the perception that the rat problem is "getting worse" in the city overall, when in fact, the number of rats in the city hasn’t necessarily increased, it has only shifted from one area to another.

Sewers as Rat Restaurants. With the advent of garbage disposal units, human foods are disposed of directly into sewer systems. This results in sewer rat populations having access to perhaps a totally balanced nutritional diet. Therefore, proliferation is simpler.

Urban Sprawl. As we have seen throughout the past two decades, urban sprawl has caused the displacement of many different species of urban wildlife, resulting in a boom in wildlife control programs. Similarly, feral rat and mouse populations are also displaced when we move into their natural areas with new housing projects. It is no longer uncommon for upscale subdivisions to report rats in the neighborhood.

Habitat Fragmentation. Many different types of businesses have found it profitable to offer foods and snacks to the public. Consider bookstores and office buildings with coffee shops and snack bars on multiple levels, gas stations with add-on fast food restaurants, portable snack kiosks within office complexes, airports, shopping malls and other similar areas. As a result, potential food sources for rodents are now readily available and fragmented among many different smaller locations along just one city block as well as in high and low areas. This arrangement allows rodent populations to be established within close proximity of these fragmented food sources in a three-dimensional format.