Mosquito repellents are pesticides whose primary mode of action is to repel
(or prevent attracting) adult female mosquitoes in search of a blood meal.
Pesticides include any substances sold with a claim to kill, repel, or
suppress pests (plant, animal, fungi...). By law, all pesticide products
must be registered with both the US EPA and (in New York State) the
Department of Environmental Conservation. It is quite widely acknowledged
that mosquito repellents are imperfect--their effectiveness diminishes over
time, varies with the intensity of mosquito pressure and with the genes,
metabolism and behavior of the applicant. They can also cause side effects
in sensitive people and when improperly used. Thus it is also pretty widely
agreed that the best way to avoid being bitten by mosquitoes is by
prevention and avoidance.
The most effective way to prevent local mosquito populations is by reducing
and trying to eliminate mosquito breeding sites.
By the time mosquitoes have emerged to adulthood, their populations are
more difficult to reduce by biocontrols or bug-killing devices. They are
not a favored prey of either purple martins nor bats--both of which have
been claimed as effective adult mosquito biocontrols on the basis of
experimental results that were taken out of context. Ultrasonic mosquito
repellents are apparently ineffective and "bug zappers" attract far more
beneficials than nuisance/disease-carrying insects (One study found that
only 0.13% of insects zapped in backyards were female mosquitoes.).
So what about chemical and plant-based mosquito repellents:
(1) As was noted in a previous posting, Consumer Reports, June 2000 issue,
compares effectiveness of several registered mosquito repellents.
(2) New York City's Department of Health developed a 2-pager now available
on the web as a pdf file: "Information for Clinicians: Common Insect
Repellents and Mosquito Control Products." It is a supplement to the June
2000 issue of "City Health Information" that focused on West Nile Virus and
the City's response plan (http://ci.nyc.ny.us/html/doh/pdf/chi/chi19s1.pdf).
This handout covers repellents and other products available to consumers,
and products that are only permitted for use by licensed pesticide
applicators: In the first category (available for consumers) the bulletin
mentions "Mosquito Dunks," a larvicide, and DEET, an insect repellent used
to repel the adult female mosquito in quest of a blood meal.
(3) Information in the handout about repellents draws heavily from a (very
readable) 1998 review from the "Annals of Internal Medicine": "Mosquitoes
and Mosquito Repellents: A Clinician's Guide," which is also available on
the web (http://www.acponline.org/journals/annals/01jun98/mosquito.htm).
Full reference: Mark S. Fradin, MD. 1 June 1998. Mosquitoes and Mosquito
Repellents: A Clinician's Guide. Annals of Internal Medicine, 128:931-940.
This review drew from clinical and analytical data in the scientific
literature, case reports, entomology texts and journals, government and
industry publications, the Extension Toxicology Network database, the
Mosquito-L email discussion group, etc.
Summary from Review Abstract: "DEET (N,N-diethyl-3-methylbenzamide) is the
most effective and best studied insect repellent currently on the market.
This substance has a remarkable safety profile after 40 years of worldwide
use, but toxic reactions can occur (usually when the product is misused).
When DEET-based repellents are applied in combination with
permethrin-treated clothing, protection against bites of nearly 100% can be
achieved. Plant-based repellents are generally less effective that
DEET-based products. Ultrasonic devices, outdoor bug "zapper" and bat
houses are not effective against mosquitoes. Highly sensitive persons may
want to take oral antihistamines to minimize cutaneous reactions to
mosquito bites."
Mosquitoes are apparently attracted to animals by a complex of visual, heat
and smell cues. They see us from afar, then hone in on us (and/or other
animals) by smell (olfactory stimuli), heat and moisture. Carbon dioxide
and lactic acid are the two compounds released by the body that are best
known to be mosquito attractants. Mosquitoes are alerted to the presence
of potential prey by picking up these (and other) odors via chemoreceoptors
on their antennae. DEET repellent apparently works by inhibiting these
chemoreceptors.
DEET (and some other repellent products) are available in different
formulations and concentrations. For the most part, the higher
concentrations are intended to be effective for a longer time. Side
effects are more likely from higher concentrations, so NYS is recommending
concentrations of 30% for adults and no more than 10% for children. DEET
should not be used on infants and children under 2 years. DEET can be
applied to the skin, to clothing (but not to skin under clothing), to
screens and other barriers, etc.
Plant-derived repellents have been far less studied than DEET. Citronella,
originally extracted from a grass, Cymbopogon nardus, has been shown to be
an effective repellent, although studies indicate that a far higher
concentration of active ingredient is needed than is effective using DEET,
and that it is not effective for as long. However, unless one is expecting
to be exposed to mosquitoes for more than several hours, duration of
efficacy may be a moot point. (Ordinary candles were also shown to be a
somewhat effective deterrent, although not as effective as citronella
candles, probably by providing a decoy source of heat.)
Permethrin-based products (e.g., Repel Permanone, Duranon, Cutter
Outdoorsman Gear Guard) are often listed among repellent products--with the
caveat that they are to be used only on clothing and gear, not on the
skin--but they actually work as contact insecticides, not as repellents.
A device called a "Mosquito Magnet" claims to attract mosquitoes using a
combination of octenol and propane (converted to carbon dioxide). Octenol
is registered by the EPA, but is not registered for use as a pesticide in
NYS and thus these products may not be sold locally. I have found few
unbiased efficacy reports (i.e., other than advertisement materials). If
anyone does know more about the efficacy of "Mosquito Magnets," please post
on this listserv or send to me. The EPA report on octenol points out that
while octenol repels some species of mosquito, it attracts others.
Lois Levitan
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Lois Levitan, PhD Program Leader
Environmental Risk Analysis Program
Center for the Environment
213 Rice Hall, Cornell University
Ithaca, New York USA 14853
Phone: (607) 255-4765 Fax: (607) 255-0238
Email: LCL3@cornell.edu
Program Email: envrisk@cornell.edu
http://www.cfe.cornell.edu/risk
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Received on Thu Jun 29 10:25:39 2000
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