[Vertebrate Pests] Rodents and Weeds

March 20, 2008

As pest management professionals know, the house mouse is the No. 1 rodent invader to homes, apartments, offices and commercial structures of all types. The pest management industry is hired to perform millions of dollars of service work each year to control this mouse. Similarly, millions of dollars are spent each year by homeowners on retail mouse traps and baits.

One of the most frequently asked questions to servicing pest technicians from property owners experiencing invading mice is: “From where did my mice come?” The stock generic answer is usually along the lines of: “They came in from the outside.” And, unless the mice are among a resident interior population, exterior invading mice in the temperate zones of the world usually do come in from the outside, especially with the onset of the cold-weather months.

But specifically where “outside” do invading mice originate from, and what factors might determine whether or not a mouse or multiples of mice will dwell nearby? For many urban areas, the answer is relatively simple: outside mice originate from those areas that contain exterior vegetation. And even more specifically, many times, it is simply from weedy areas.

OVERLOOKED WEEDS. In the summer and fall when we walk about outside, we are likely to walk by dozens and perhaps even hundreds of weedy plants of one species or another on any given day. At the level of our ankles, most of us have little perspective, and thus little awareness of these plants. And because weeds are essentially all around us all the time, we, for the most part, may no longer even notice them. And, we pay even less attention to the tiny seeds weeds produce.

But it’s quite different for the mice. The same weeds and seeds we humans overlook play crucial roles in the lives of mice. So, if we subscribe to the maxim that “pest professionals should train themselves to see what others overlook,” let’s take a detailed look at everyday ubiquitous urban weeds and weed seeds and their role in the overall management of urban mice.

HISTORY. Before the advent of modern day human developments, our commensal rodent species existed in the grassy steppes of Western and Central Asia. Here, they fed on those foods associated with the ecology of grasslands — namely all kinds of seeds. In fact, part of the reason the mouse’s foraging strategy is characterized as a “nervous, erratic nibbler” is due to the manner in which the mouse was forever chasing the scattered windblown seeds dispersed hither and yon in all different directions by Mother Nature on the grassy steppes and open fields.

The seeds mice consume are supplemented with the different animals that depend on the grasses for food. These include a long list of invertebrates such as insects, millipedes, centipedes, worms, slugs, etc., as well as various grass-nesting birds and their eggs. Well, today, within fields and weedy lots, even in our largest, most developed cities and towns, the mouse of the field and its foraging strategy is essentially the same. Perhaps it’s fair to say that from the mouse’s perspective, weeds, in fact, are wonderful.

To illustrate this point, let’s take an everyday example that you could find on any of your pest management routes. Let’s consider a small typical weedy lot (e.g., a 0.25 acre lot) nearby one of your residential or commercial client’s properties. How might this weed patch benefit mice? Well, in five essential ways: 1) food; 2) water; 3) cover; 4) nesting materials; and 5) communication.

FOOD AND WATER. The various reproductive seeds of plants provide essential nutrition for a large portion of life on Earth. Consider our own consumption of plant seeds. Many of these we refer to as “grains” such as rice, wheat, oats, corn, sunflowers, etc. In fact, rice and wheat alone have been an essential factor for the survival and dispersal of mankind around the globe. So too is it for rodents and the seeds they encounter in their world. If you Google “seeds, nutritional value,” you’ll quickly see that many of the seeds produced by Mother Nature, including ordinary weed seeds (e.g., foxtail weeds, Setaria spp.), contain essential proteins, carbohydrates, fats, water, vitamins and nutrients. (To me, when I look at a weed plant with seeds, I visualize a mouse-sized “Nutrition Facts Table” being stamped somewhere on the weed’s leaves.)

Moreover, grassy/weedy fields are particular ecosystems. Along with the mice and other mammals, many insects, birds and reptiles live and interact within this system — all of which and more serve as important additional food sources for mice and rats. Insect larvae from beetles and moths, for example, and bird eggs are excellent sources of both protein and water.

John Whittaker, the well-known mammalogist from Indiana State University, in one of his classic studies on the natural diets of wild mammals, analyzed outdoor populations of the house mouse, Mus musculus, in field environments. In the stomachs of the mice, Whittaker found approximately 40 different items. But about 50 percent of the mouse’s food comprised different weed seeds. Considering the nutritional value of these seeds, this should come as no surprise. Other significant portions of their diet involved caterpillars and other insect larvae. (See, mice eat meat and grains, just like you and me.)

In Whittaker’s study, common foxtail seeds (Setaria spp.) were taken with about a 25 percent frequency. The mice also were eating pigweed, johnsongrass, crabgrass, witch grass, ragweed, knotweed, fleabane and others. Do these weeds sound familiar? They should, because they are the common weeds found in many towns and the ones you pass everyday outside office buildings, schools, homes shopping malls and the like (or the ones you step on in your own lawn).

Water is perhaps the more obvious element rodents can obtain from weeds. The house mouse and the rat can obtain water directly off of a weed’s leaves from rain or condensation (e.g., dew drops). Or, water can be obtained when the rodent cleaves open a weed’s stem, leaves or roots with its incisors. The seeds themselves also contain water. And because the house mouse has a kidney system similar to that of a desert rat, even the tiny amounts of water in what is seemingly “dry” seeds may be enough to sustain the mouse through times of drought.
COVER. Weeds (and vegetation in general) provide small mammal prey species with one of the most essential elements needed for survival: protection from their enemies. Of course, mice (and voles, shrews, rats and  rabbits) essentially comprise the breakfast, lunch and dinner for hawks, owls, snakes, coyotes, cats, skunks, foxes and other predators. Under the protective cover of weeds, it is much more difficult for predators to spot and kill small prey. More protection means more individuals survive long enough to reproduce.

NESTING MATERIALS. Outside our buildings, mice must nest in ground burrows, within nooks and crannies of logs, inside tree hollows and virtually any hidey-hole they can find. Or, mice will collect plant leaves and stems and “weave” comfy globular nests amid the branches of dense bushes. The stems and leaves of plants can provide the essential insulation needed to protect rodents from the cold ground, or to allow for water and moisture to drain off, or be wicked off of the nest, thereby enabling mice to conserve body heat. These absorbent materials also help mice avoid their own contamination.

COMMUNICATION. Rodents use urine, feces and various uro-genital secretions to mark items and trails for “communicating messages” to other members of their family or colony. Information about dominance, estrus, species and tribe identification and other information can be communicated. Certain pheromones are used for identifying certain trails and burrow systems. Plants with vertical stems or broad leaves are ideal “marking posts” for chemical cues. A weed stem, for example, may be marked so as to guide a mouse to its own burrow, or to one of its secondary hiding spots. No doubt, just the physical arrangement of the vegetation in a mouse’s territory assists in daily orientation and movements.

WEED CONTROL SERVICES? Around the exteriors of our buildings, weeds and weedy patches are natural habitats for supporting mice, or depending on the size of the grassy area: mouse populations of varying sizes. As pest professionals, we should always educate our clients, especially our commercial clients, as to the importance of controlling weeds and overgrown vegetation. Better still, weed management should always be offered as a comprehensive aspect of rodent IPM services.

Incidentally, the next time you’re thinking about which bagel to order, pass over the plain; go for the bagel with sesame seeds. It’s a healthier foraging strategy.

The author is president of RMC Consulting, Richmond, Ind.