Odor Animal Repellents: Are They Effective?

October 1, 1998

Oftentimes pest management professionals and wildlife control professionals are called upon to capture or “chase” squirrels, rodents, snakes, bats, skunks or other urban wild animals out of buildings. When done on a professional level, most times, this is accomplished via live trapping and removal, followed by exclusion efforts. Many homeowners also inquire as to the usefulness of having a chemical repellent applied in or around their home or building to repel future pest activity. But do the animal repellents labeled for this purpose really work? Let’s examine this issue closer.

TYPES OF ODOR REPELLENTS. Many different types of animal repellents are available, including over-the-counter products, which attempt to change the behavior of wild mammals and birds (tactile, visual, gustatory, olfactory and auditory, for example). This article focuses only on those repellents currently sold with a premise of action as to odor repellents (also known as “area repellents”) for the most common structural urban animal pests. A representative selection is listed in Table 1.

Having a putative action, the odor repellents sold to discourage animals from harboring in or around homes are presumed to offend the olfactory senses of the animal, or cause respiratory and general discomfort. Some may also serve as irritants, should the animal pick up some of the repellent on its body.

Naphthalene (i.e. “moth flakes”) has been and remains one of the most universally known insect “repellents.” Specifically, it has been used to protect woolen clothing against fabric pests. Interestingly, naphthalene is not an effective repellent against fabric or other pests (1). If it is used in high concentrations and kept in tight containers for prolonged periods, naphthalene works as a fumigant, actually killing insects. How naphthalene came to be known and used as an animal repellent is not exactly clear, but in part, the minor fumigants naphthalene and paradichlorobenzene (PDB) may have been assumed to be general pest repellents due to their volatility and resulting disagreeable odor. Obviously, any mammal, including man, within an enclosed area containing high levels of naphthalene (as well as other chemical compounds) will seek to exit.

Sulfur is also used as an animal repellent. One of the oldest pesticides, sulfur was used throughout the first half of the century for all types of “pest cleansing operations.” Eventually, burning sulfur candles became commonplace for people in attempts to repel mosquitoes, gnats and other nuisance flying insects. Some area animal repellents are made up of 28% sulfur and 7% naphthalene and are registered as repellents against snakes, rats, mice and other animals.

DO ODOR REPELLENTS WORK? Rodent Repellents. For about 20 years at the turn of the century, U.S. government biologists such as D. Lance and James Silver were publishing various papers and government pamphlets regarding the control of rats around buildings (10). These publications formed the foundation on which many other rodent control publications later based their recommendations.

Silver, in his early publications, may have been among those to start the ball rolling. He recommended “deterrent odors such as flake naphthalene in quantity, as well as sulfur can be used to repel rodents.” Mills and Munch (8), in the magazine Pests state that naphthalene was the best odorous rodent repellent, while powdered sulfur, cayenne pepper, lye and several others were good non-odorous rodent repellents. Whether Mills and Munch based their recommendations on the previous recommendations of Silver or some previous government bulletins is not known. Still, none of this began with any solid field research demonstrating that these products were efficacious. Later, extensive research was carried out worldwide for more than 20 years in search of an effective rodent repellent. Hundreds of compounds were tested, but none were of practical value for the purposes intended. Most of the work is summarized and referenced by Meehan (7). The bottom line is that no efficacy data exists for sulfur or naphthalene as a rodent repellent. All this is not to say, however, that effective rodent repellents do not exist or can’t be developed. Work is continuing along this line with investigations of natural and synthetic compounds and new rodent repellents may be forthcoming (3).

Snake Repellents. Coincidentally, the same combination of naphthalene and sulfur which is now registered for rats, mice and other nuisance mammals, is also purported to be effective against snakes. But as with the rodents, formal field research proving efficacy of sulfur and naphthalene against snakes is lacking. In fact, two research papers, Ferraro (2), and San Julian and Woodward (9), as well various technical sources (e.g. References 4, 11) demonstrate and support the ineffectiveness of sulfur-naphthalene as a snake repellent. The Ferraro study in particular is a strong case against this repellent, as his study was conducted in the field at 24 locations against the plains garter snake, a common visitor around homes and gardens. Moreover, the sulfur-naphthalene repellent has been denied registration since 1991 in California due to a lack of efficacy data, and the U.S. military (which of course would have a keen interest in snake repellents), does not use sulfur-naphthalene repellents for the same reason.

Bat, Squirrel and Bird Repellents. Naphthalene has not been shown to be of value as an avian repellent (6). But bats can be evicted from building spaces using 100% naphthalene if the area is confined, and thus the concentrations of naphthalene can be elevated high enough (4). Recommended dosages are at 2.5 pounds per 1000 cubic feet.

At twice this dosage, bats can be dislodged during the day. This author has used naphthalene successfully to evict bats out of exterior confined soffit voids and wall voids of a storage building. But he has also seen mothballs installed into many types of attics with unsuccessful results. Often, the attics are too large or too leaky to allow for proper repellent dosages. In one case, I witnessed 100 pounds of naphthalene installed into a church attic attempting to discourage a large bat colony (See figure 2). Even with this outrageous dosage, the bats remained. Considering naphthalene is 4.4 times heavier than air, coupled with the fact that prolonged inhalation of naphthalene vapors can be hazardous to humans, this particular application was risky business to say the least.

SUCCESS STORIES AND FALSE POSITIVES. Formal efficacy data aside, when talking about repellents to homeowners and professionals alike, results range from zero to 100% satisfaction. And, in the literature on vertebrate pest management, you often see catch phrases for the use of repellents such as “sometimes they may work,” or “may be worth a try.” After 15 years at Purdue University, I received annual calls from different individuals who swore that spinning daisies, chewing gum, moth balls, etc., controlled their moles. Others claimed Osage orange fruits kept rodents and snakes away from their farms. What accounts for the occasional positive feedback from people who try commercial and homeowner repellents? Well, there are many complex biological and environmental variables involved relative to the relationship between animals and their behavior of exploring and “selecting” harborages and food. But to a large degree it comes down to timing and resource availability.

Consider a few examples. If an animal has several suitable harborages nearby food and all in close proximity, it obviously has alternatives. Should one of the harborages contain a more uncomfortable atmosphere or environment, then the animal may indeed select, and re-select, the harborage that provides maximum protection and comfort.

However, if only one harborage is available, uncomfortable or not, the animal will adapt as best as it can. From a biological perspective, bats might be easily evicted from a small attic containing an irritating but non-lethal atmosphere of naphthalene shortly after their arrival from hibernacula and prior to the maternity season. But this same level of naphthalene several weeks after their young are born, during the critical maternity period, will not have impact, as the bats are in a crucial survival period and have a strong roost attachment.

Success stories sometimes come from people who have installed mothballs (and ultrasonics) into bat-infested attics during September and October and claim “outstanding results” after just a few weeks. But unbeknownst to them the bats were naturally beginning to disperse from their summer roosts anyway. The coincidental time of applying a chemical repellent or ultrasonic resulted in a false positive.

Disturbance alone may cause wild animals to leave an area or harborage. Many wild animals, if alternate harborage is available, and prior to harborage attachment behavior, will evacuate a given harborage after being disturbed. Most field biologists and wildlife researchers appreciate the importance of minimal disturbance to wild animals being studied in the field.

Finally, the unpredictability of animal encounters must be considered. There are thousands of people/wild animal encounters that occur only occasionally. Snakes slither through homes on their way somewhere else and are never seen again. Woodpeckers attack a home, and after drilling two or three holes, disappear. Raccoons or skunks use a crawlspace for a couple of days but then wander off. Panicked individuals, upon their first encounter with the pest, apply a repellent. But in these cases the problem has solved itself. Of course, the repellent is credited.

SAFETY CONSIDERATIONS. In this era of chemophobic individuals, installing odor area repellents into homes and other buildings is perhaps best done by the client or by the professional, but only after the client has clearly been informed to read the labels. Although the compound naphthalene is often listed as having “low mammalian toxicity,” there are safety concerns with this mild fumigant. Some individuals are highly sensitive to naphthalene.

Descriptions on the pharmacology of naphthalene (12), state “prolonged inhalation may cause headaches, nausea, vomiting, and sweating, followed by anemia, haematuria (i.e. blood in the urine), and optic neuritis. Some individuals may have a severe hemolytic crisis which may be delayed for several days after exposure.” With these descriptions in mind, it is understandable that the labels of the 100% naphthalene products registered for use indoors against squirrels, birds and bats clearly state the following: “Avoid breathing vapors. Rooms should be well-ventilated before occupancy.” Yet these same labels recommended indoor use of the material to be applied on floors, or between walls at dosages of 8 ounces for every 200 cubic feet. And associated technical sheets state that when applied indoors, 100% naphthalene has a four- to six-month residual.

Whether or not you feel the precautionary statements are overkill is a moot point when considering whether or not you wish to be the professional responsible for installing 100% naphthalene inside (or around?) an occupied building. This is especially crucial when it comes to treating commercial buildings, where many people with different chemical sensitivities may work or visit on a daily basis.


1. Abbott, W.S., and S.C. Billings. 1935. Further work showing that paradichorobenzene, naphthalene and cedar oils are ineffective as repellents against clothes moths. J. Econ. Entomol. 28:493-495.

2. Ferraro, D.M. 1994. The efficacy of naphthalene and sulfur repellents to cause avoidance behavior in the plains garter snake. Pages 116-20. In: R.E. Masters and J. G. Huggins, eds. Twelfth Great Plains Wildl. Damage Control Workshop Proc. Published by Noble Foundation, Ardmore, OK.

3. Gurney, J.E., R.W. Watkins, E.L. Gill, and D.P. Cowan. 1996. Non-lethal mouse repellents: evaluation of cinnamamide as a repellent against commensal and field rodents. Applied Animal Behaviour Science. 49:(4): 353-63.

4. Hygnstrom, S., R.M.Timm and G. Larson (Eds.). 1994. Prevention and Control of Wildlife Damage. Vols 1 and 2. Coop. Extension Service. Univ. of Nebraska and USDA/APHIS/ADC.

5. Lance D.E. 1910. The rat and its relation to the public health. Pub. Health Bull. No. 30. G.P.O.

6. Mason, J.R. and L. Clark. 1992. Non lethal repellents: The development of cost-effective, practical solutions to agricultural and industrial problems. Pgs. 115-29. In: Proc. 15th Vertebrate Pest Conf. (J. E. Borrecco and R.E. Marsh, Eds.). Univ. of Calif.-Davis.

7. Meehan, A.P. 1988. Chemical Repellents. Pages 399-406. In: Rodent Pest Management. I. Prakash, Ed. CRC Press. Boca Raton.

8. Mills, E.M., and D. Munch. 1942. Repellents in rat control. Pests. 10(2):20-21.

9. San Julian, G.J. and D.K. Woodward. 1985. What you wanted to know about all you ever heard concerning snake repellents. Pages 243-48. In: The Proc. Second Eastern Wildlife Damage Control Conference. (P.T. Bromley, Ed). N.C. St. Univ. Raleigh.

10. Silver, J. 1927. Rat Control. Farmers Bull. No. 1533.

11. Story, K. 1987. Snakes: Separating fact from fantasy. PCT. Vol.15(11):54-60. 12. Sweet, D.V. ed. 1993. Registry of toxic effects of chemical substances. January 1993. NIOSH Publ. No. 93-101-2.