They can be.
In a 2003 New York Times article, Elisabeth Bumiller describes how squirrels living on the White House lawn killed an historic tree. It seems the squirrels stripped so much bark off the upper limbs of a buckeye tree planted by Theodore Roosevelt that it died and had to be cut down.
Bark stripping is common practice among many temperate tree squirrels and some ground squirrels. Squirrels strip away the top layer of the bark to get to the sweet, phloem tissue underneath. This occurs most commonly in early summer and can be fatal to trees, either by killing them outright or by making them susceptible to infestation by insects or fungi. Bark stripping, though relatively innocuous in wild forests, can seriously damage managed forests, which tend to have thousands of acres of a single species of similar-aged trees growing together. Squirrels prefer to debark trees of a certain diameter (less than 6 cm or 2.4 inches), and when many trees of this size are located together, the damage can be significant. Besides the risk of killing a tree, bark stripping can stunt a tree’s growth and cause deformations in the wood, both of which can decrease the tree’s usability by timber companies. With estimates of 30–90% damage in certain stands of trees, bark stripping can affect companies negatively. In fruit orchards, woodchucks also strip bark. In this case, they gnaw on the lower bark of trees, and studies show that trees that have been gnawed upon produce smaller and fewer fruits than ungnawed trees.
In tropical plantations tree squirrels are considered unwelcome pests, because they feed extensively on ripe fruit, cocoa, and palm nuts. In central and northern South America, the red-tailed squirrel (Sciurus granatensis) is a common pest on cocoa plantations, and in Southeast Asia, multiple species of Callosciurus commonly invade plantations. In Kenya, the striped ground squirrel (Xerus erythropus) frequently preys on maize seeds and crops, which can account for more than half the damage to all maize crops.
In North America, some ground squirrels also are considered agricultural pests and are known to damage many grain crops through extensive feeding. In 1910, a U.S. Department of Agriculture publication estimated that ground squirrels were responsible for more than $10 million dollars of damage per year to crops. (Today, that would be equal to approximately $200 million in damage.) A 1999 report found that Belding’s ground squirrels feeding on alfalfa crops in northeastern California caused significant damage, costing farmers on average $402 per hectare.
Prairie dogs have been considered pests by ranchers in the western United States for more than a century. Ranchers claim that prairie dogs compete with livestock for food and that prairie dog burrows pose a danger to livestock and machinery. Starting at the turn of the twentieth century, ranchers, with the support of the U.S. government, began systematically killing prairie dogs using any means possible. Prairie dogs were shot, trapped, and poisoned by the hundreds of thousands. This assault, combined with the unintentional introduction of plague from Asia, resulted in an extreme reduction in prairie dog populations and their extirpation from many areas. Today, prairie dogs occupy only a fraction of their historical range (though estimates of the size of their historical range are controversial). In recent studies biologists have found that the supposed damage done by prairie dogs has been overstated, but the view of the prairie dog as pest is deep-rooted and still persists today. While two species of prairie dogs, the Mexican and Utah, have some legal protection, white-tailed and black-tailed prairie dogs do not, and attempts to gain protection for these species have been contentious and highly political.
In suburban areas, humans intentionally and unintentionally provide a plethora of tasty treats for squirrels. Buds, shoots, flowers, bulbs, fruits, and vegetables are all part of a successful suburban squirrel’s diet, much to many a gardener’s dismay. Squirrels readily dig up flower bulbs and will quickly remove every last tomato from a ripe vine. Squirrels frequently show up uninvited at many backyard bird feeders, quickly emptying them, and moving on. Woodchucks, also common in suburban areas, make snacks of fresh garden greens whenever possible.
Municipal parks offer havens for squirrels in the middle of urban areas. The absence of many predators, combined with supplemental feeding by park visitors, allows squirrel populations to grow relatively unchecked. A 1977 study in Lafayette Park in Washington, D.C., documented a squirrel density of approximately 12.5 squirrels per acre—one of the highest densities ever recorded! These large populations do not come without a cost, though. The hard work invested by cities planting flowers, trees, and shrubs can disappear in a few weeks. The abundant Lafayette squirrels, in one season, did $4,500 in damage by digging up 2,000 newly planted geraniums and stripping bark from trees.
Tree squirrels also have an unfortunate habit of venturing into transformers and generators, electrocuting themselves and knocking out power to surrounding communities. In October 1953, squirrel populations were so high in parts of Mississippi that roaming squirrels knocked out 20 transformers in less than a month. In 1979, a single squirrel caused $1 million in damage to an electrical substation in Pennsylvania when it wandered into a generator, triggering a fire and explosion. In 1984, an article titled “The Squirrels Who Can Zap Thanksgiving” appeared in the Washington Post. Reporter Tom Vesey described how eastern gray squirrels in the area were caching their nuts in electrical transformers at the top of poles, causing an increase in the number of blackouts. In the article, the electric company stated that between 1983 and 1984, there were 579 squirrel-caused electrical outages in that region.
In his book, Fred Barkalow describes the squirrel as an “energetic nibbler,” and this is an apt depiction. Nothing appears immune from the investigations of the squirrel’s teeth—electrical wires, telephone cables, house siding, gutters, roof shingles, cardboard boxes, lawn chairs, bird feeders, shutters, insulation. You name it, a squirrel will nibble it. This habit can cause significant damage, and homeowners around the world have cursed the bushy-tailed perpetrators. Flying squirrels commonly find their way into attics or behind walls and set up nests, where they nibble through insulation and wire coverings, leaving bare wires that can increase the risk of fire.
If you are truly committed to keeping squirrels away from your homestead, then the best advice is to not put any food out for birds, pets, or other animals. Keep garbage in spill-proof containers and thoroughly clean the outside of barbecue grills. Do not plant vegetables, flowers, or other vegetation that may produce anything squirrels will find tasty. Although these steps may not completely eliminate squirrels from your property, they will probably reduce the number of squirrels.
If these steps seem a bit severe to you, there are many other ways to limit the effect of squirrels on your bird feeder, your garden, and your home.
BIRD FEEDERS. There are hundreds of bird feeders on the market that claim to be “squirrel proof.” And, in all likelihood, there are hundreds, if not thousands, of people who have bought these bird feeders only to find a crafty squirrel blissfully munching away on it one morning. Although there may always be that “one” squirrel who can outwit any squirrel-proof contraption, there are strategies you can use to reduce the chances that a squirrel will gain access to a bird feeder.
Placement of the feeder is key. Squirrels are amazingly adept acrobats. Their hind feet can rotate almost 180 degrees, allowing them to hang off of the most precarious perches to grab food with their fore feet. They also can jump significant distances, especially if there is a tasty reward on the other side. The trick is to eliminate places from which the squirrel can hang, jump, or stretch to reach the feeder. This means that the feeder should be at least 6 feet off the ground and at least 8 feet horizontally (preferably farther) from the nearest launching point (e.g., tree branch, picnic table, roof, dog house).
Even an isolated feeder is not fully protected. Squirrels can shimmy up poles and tiptoe across even very thin wire. Stringing barriers along wire hung feeders is one commonly used trick. Bill Adler Jr., in his book Outwitting Squirrels, suggests hanging a feeder on a horizontal wire. Then, on either side of the feeder, string several record albums, which should be high and wide enough to prevent the squirrel from climbing over. Large aluminum pie plates probably would work just as well. We have used plastic tubing, cut into half-inch lengths, which were then threaded onto the wire. Each piece of tubing rotated freely and easily on the wire, which made it very difficult for squirrels to walk across, until an ice storm cemented them all together.
Baffles are useful additions to pole-mounted bird feeders and come in a variety of shapes—half-spheres, cones, pointed, slanted, among others. They all have the aim of blocking and preventing the squirrel access to the bird seed. Baffles can be good protection, but they are not foolproof as hungry squirrels are not easily deterred and can wiggle, squirm, squeeze, and contort their way around a host of obstacles. Greasing poles is not recommended, because substances such as petroleum jelly can adversely affect the insulating ability of bird feathers and animal fur.
Another option is to fill your bird feeders with foods that squirrels do not like—thistle and safflower seeds are two options. This might limit the types of birds you attract to your feeders, however. Anecdotal evidence suggests that mixing cayenne pepper in with the bird seed will keep squirrels away, because squirrels can taste it but birds cannot.
Finally, you can distract squirrels with a feeding station of their own. Set up an easily accessed feeding station, located a good distance from any bird feeder, and fill it with foods irresistible to a hungry squirrel—sunflower seeds, unsalted peanuts, and acorns. Squirrels typically go for easy to obtain food, and so a supplemental feeding station in combination with the preventative measures above, should limit extraneous raids of your bird feeders.
An eastern fox squirrel raids a bird-feeder in the rain. Keeping squirrels out of their birdfeeders has consumed many a backyard bird lover. Photo © Phil Myers
GARDENS, FLOWERBEDS, AND POTTED PLANTS. In Lafayette Park, across from the White House, squirrels did $4,500 worth of damage one spring by digging up and eating most of the newly planted geraniums. On a much smaller scale, many backyard gardens and flowerbeds suffer similar abuses every spring and summer. Keeping squirrels out of your gardens and flowerbeds is much like keeping them out of your bird feeder—an ongoing battle. You may never be entirely successful, but you can take preventative steps to decrease the damage. One of the most common techniques is to cover newly planted bulbs and seeds with wire mesh, which will prevent squirrels (or other animals) from digging them up. Once the seeds have sprouted, you can cover the entire plant with some sort of netting or cage to prevent unwanted access.
Another trick is to spray plants with a cayenne pepper and water solution, but this wears off quickly and must be reapplied often, and using it on vegetables you plan to eat is not recommended. You can also buy commercial sprays that claim to deter squirrels, but the efficacy of these products is undetermined. Some other tricks that have been claimed to work with varied success include spreading blood meal on your flowerbeds, placing dog hair around your plants, and even using mothballs.
Ground squirrels, particularly woodchucks, can be very difficult to keep out of a vegetable garden, because they easily dig under a fence. If you take flashing or some other hard barrier and sink it into the ground a foot or so beneath a fence, you may be able to defeat them.
Most species of squirrels are active during the day, so they are easily spotted on their raids and therefore most commonly blamed for damage. There are many nocturnal animals that also may be eating your plants, including raccoons, possums, mice, rats, rabbits, skunks, bears, foxes, and deer.
ATTICS, WALLS, AND PORCHES. Squirrels cause damage to homes in two main ways: by gnawing on siding, roof tiles, or wiring or by nesting inside an attic or crawl space. The gnawing habit of squirrels is hard to prevent, as squirrels have ever-growing incisors and therefore must gnaw on hard objects (usually nuts) to keep them well worn and sharp. It just so happens that siding, roof tiles, porch railings, and wooden steps all provide hard surfaces ideal for chewing.
The squirrel gnawing habit becomes most worrisome if they strip the insulation off of electrical wires.
The warm, dry protection offered by an attic will attract female squirrels looking for safe nesting sites. In North America, flying squirrels are the most common attic inhabitants, but eastern gray and eastern fox squirrels also will take advantage of them from time to time. Squirrels commonly gain entrance to an attic or crawl space through openings between the roof and the walls, through gutter spouts, and through ventilation fans. Squirrels also can gain entrance to houses through chimneys, but once inside the chimney they rarely can get out. The best way to keep squirrels out of a house is to block all of their entrances. You should walk around the house and take stock of any opening they may be able to use. Even openings that seem too small should be noted, because squirrels can get through very small openings. Any holes or openings should be patched, making sure that (1) the squirrels are no longer in the house and (2) that no babies are left inside the house. Also, all chimneys and vents should be capped.
Finally, another complaint is about squirrels chewing on pillows and rugs left out on porches. Most likely, these squirrels are gathering soft material to use in lining their nests. Leaving out some old pillow batting or loose wool for the squirrel to use instead may help to keep cushions intact.
TRAPPING SQUIRRELS. In the United States, wildlife is the property of the state, and many states consider that squirrels are game animals to be managed. They therefore have strict regulations on hunting and trapping, which may or may not pertain to your situation. It is wise to check the regulations before trapping or killing squirrels.
Live trapping and relocating nuisance squirrels, though common, is not a good idea. Although relocating squirrels to a “nice home in the country” seems humane, relocated squirrels rarely, if ever, survive successfully in new territory. A three-year study by squirrel expert Van Flyger and colleagues found that 97% of suburban squirrels that were relocated to a park either died or “disappeared” within the year. Squirrels that are relocated to new, unfamiliar areas are at high risk for predation because they do not have established nests and hiding places. Depending on the season, they are at high risk for starvation, because they do not have established food stores to exploit. Finally, they are also at high risk of aggressive encounters with other squirrels. Relocation is exceedingly stressful to the animal and, as the study above has shown, rarely successful. In addition, when you remove squirrels from your neighborhood, new squirrels soon enter the area to fill the recent vacancy. Laws in many states and counties prohibit the trapping, transportation, and release of live animals without a license.
Finally, and this cannot be said enough, NEVER USE POISON! It is almost impossible to ensure that poison will be ingested only by the intended pest animal. In most cases other wildlife will ingest the poison, and there is a very high risk that your neighborhood or family pets will ingest the poison as well. Also at high risk are the predators that feed on the animals that ingest the poisoned bait. Eagles, hawks, falcons, owls, snakes, carrion feeders (i.e., buzzards, crows), foxes, and mountain lions are just a few of the many predators that inadvertently die each year because they ingested poisoned animals.
Squirrels are not considered important vectors (or carriers) of any human diseases. Rabies is a case in point. Squirrels can get rabies, but it is not very commonly documented. According to the Centers for Disease Control and Prevention (CDC) squirrels are almost never found to be infected and no case of squirrel to human transmission of rabies has been documented in the United States. An upsurge in reports occurred between 1980 and 1984 in the mid-Atlantic states, where there were more than 50 cases of rabies reported in woodchucks (Marmota monax), in association with an epizootic among raccoons (Procyon lotor). During the next decade there were almost 300 additional cases countrywide, mostly in the East. In Maryland 15 cases of human exposure to rabies due to woodchucks were reported by the Health Department, 1981–1986. To put this in perspective, we note that woodchucks accounted for less then 1% of the animals found rabid in Maryland during this period.
Although some other diseases squirrels carry can be transmitted to humans, the transmission does not commonly occur directly from squirrel to human. Two examples of this are the West Nile virus and Lyme disease. In both cases, even though the squirrel carries the pathogen, humans can only get infected through the bite of the actual vector—the mosquito and tick, respectively.