Science: Essential Oils for Insect Repellent
Scents can affect insect behavior. That part is not simply herbal tradition or marketing. It has been demonstrated repeatedly in laboratory studies. This leads to the concept of using essential oils for insect repellent.
The more complicated question is what repellent actually means, how long that effect lasts, and whether an essential oil that works against one insect will work equally well against another.
The research is much more interesting when we leave room for those complications.
A useful place to start is this review of plant-based insect repellents:
Plant-based insect repellents: a review of their efficacy, development and testing
Researchers have investigated essential oils from dozens of aromatic plants against mosquitoes, ticks, flies and other arthropods. Many show measurable repellent activity. The challenge is that essential oils are made largely of volatile compounds. Volatile is exactly what makes them smell so strongly: their aromatic molecules readily evaporate into the surrounding air.
That is useful when scent itself is influencing insect behavior, but it also means the effect often fades relatively quickly.
In other words, an essential oil can be genuinely repellent without being an all-day repellent.
Bugs experience scent very differently than we do
When we smell lavender or eucalyptus, we experience an aroma. An insect is receiving chemical information.
Mosquitoes in particular use chemical cues to locate potential hosts. Their sensory systems respond to compounds associated with breath, skin, sweat and the surrounding environment. Aromatic plant compounds can interfere with that process in several ways. Some appear unpleasant or aversive to the insect, while others may interfere with its ability to orient toward the cues it normally follows.
That helps explain why researchers do not evaluate essential oils simply by asking whether an insect “likes the smell.” They measure behaviors such as landing, feeding, movement toward a host, or willingness to cross an area containing a particular substance.
A 2018 review of essential oils as arthropod repellents summarizes a large body of this work:
Essential Oils as Repellents against Arthropods
The results make one thing clear: plants produce aromatic compounds that can meaningfully change insect behavior.
But the particular plant, chemical composition, concentration, insect species and formulation all matter.
Lavender
Lavender has one of the more interesting research histories among familiar garden herbs.
In a laboratory study published in the Journal of the American Mosquito Control Association, researchers tested essential oils from lavender, eucalyptus, rosemary and thyme against Culex pipiens pallens mosquitoes. All four oils demonstrated repellent activity under the study conditions.
Repellent activities of essential oils and monoterpenes against Culex pipiens pallens
Lavender’s chemistry may help explain some of that activity. Lavender essential oil commonly contains compounds such as linalool and linalyl acetate, although the exact proportions vary depending on species, growing conditions and distillation.
Linalool itself has also been examined for mosquito repellency. In one field study comparing aromatic compounds released from diffusers, linalool significantly reduced the number of female mosquitoes collected near the treated area.
Efficacy of the botanical repellents geraniol, linalool, and citronella against mosquitoes
Lavender has also been investigated against ticks, although results vary considerably with concentration and the particular preparation being tested.
The interesting point is not that lavender creates an invisible force field around a person. It is that some of the aromatic chemicals lavender produces appear capable of changing how certain arthropods behave.
Eucalyptus
Eucalyptus is where terminology becomes especially important.
“Eucalyptus oil” and “oil of lemon eucalyptus” are often talked about as though they are the same thing. They are not.
Oil of lemon eucalyptus, commonly associated with the compound PMD, comes from Corymbia citriodora and has its own substantial body of repellent research.
Regular eucalyptus essential oil, such as Eucalyptus globulus, is chemically different.
Fortunately, Eucalyptus globulus has also been studied.
The same mosquito study that examined lavender found Eucalyptus globulus essential oil to have repellent activity against Culex mosquitoes:
Repellent activities of essential oils and monoterpenes against Culex pipiens pallens
A later study evaluated Eucalyptus globulus against Aedes aegypti, the mosquito species associated with diseases including dengue and yellow fever. Researchers found that eucalyptus oil, along with several other plant oils, showed mosquito-repellent potential, and that a blend of the oils performed particularly well under the study conditions.
Mosquito repellent activity of volatile oils from selected aromatic plants
Another study tested Eucalyptus globulus against Anopheles stephensi. Eucalyptus oil again demonstrated repellent activity, with combinations of eucalyptus and clove oil performing particularly well.
That does not mean every eucalyptus product will perform the same way. Different eucalyptus species contain different proportions of compounds such as 1,8-cineole, and concentration and formulation can change the outcome substantially.
But eucalyptus has earned its place in the scientific conversation about botanical repellents.
Cedarwood is a little more complicated
Cedarwood is a good example of why I like looking at the studies rather than repeating lists of “bug-repelling essential oils.”
Cedarwood really does interact with arthropods.
Virginia cedarwood oil, Juniperus virginiana, and one of its components, cedrol, have been studied against several pests. Researchers found cedarwood oil could act as a repellent barrier to red imported fire ants, while cedrol demonstrated toxicity against black-legged tick nymphs.
Bioactivity of cedarwood oil and cedrol against arthropod pests
Later research found cedarwood oil both repellent and toxic to nymphs of several hard tick species.
Repellency and toxicity of a CO2-derived cedarwood oil on hard tick species
Mosquitoes are another story.
In a study comparing several essential oils against mosquitoes, cedarwood did not provide meaningful mosquito repellency under the conditions tested.
Repellency of essential oils to mosquitoes
A National Toxicology Program review of cedarwood oil reaches a similarly nuanced conclusion: cedarwood demonstrates biological activity against several arthropods, but research has not established it as a reliable topical mosquito repellent.
That distinction matters.
Cedarwood does not need to be promoted as something the evidence does not support in order to be an interesting ingredient. Its aromatic compounds clearly interact with arthropods. We simply have better evidence for some insects, particularly ticks and certain other pests, than we do for mosquitoes.
Concentration matters
One of the patterns that appears again and again in essential-oil research is that dose matters.
An oil that produces little repellent activity at one concentration may produce a much stronger response at another. Different researchers also use different carriers, application methods and testing environments.
That makes statements such as “lavender repels mosquitoes for X hours” much less useful than they initially appear.
Which lavender species?
At what concentration?
Diluted in what?
Against which mosquito?
Measured in a laboratory cage or outdoors?
Those details change the answer.
A systematic review examining plant repellents against Anopheles mosquitoes looked at 62 trials and found considerable evidence of repellent effects from plant preparations, while also finding substantial variation among plants, formulations and protection times.
Effectiveness of plant-based repellents against different Anopheles species
This variation is not evidence that essential oils “do not work.” It means biological systems are complicated.
The biggest challenge may be evaporation
Using essential oils for insect repellent has a built-in limitation: the molecules we want floating around us are the same molecules that eventually float away.
Studies comparing botanical repellents repeatedly find that many essential oils provide their strongest protection soon after application and then decline as their volatile constituents evaporate.
Researchers have tried to solve this through different carriers, fixatives, emulsions, encapsulation techniques and combinations of oils.
This is also why reapplication makes intuitive sense for a light botanical body spray. Rather than expecting one application of volatile plant oils to remain unchanged for many hours, the aromatic environment can be refreshed periodically.
That is very different from claiming that an essential-oil spray provides the same duration or reliability as a registered long-duration insect repellent.
What does this mean for lavender, eucalyptus and cedarwood together?
There is no study showing that my exact blend of essential oils for insect repellent (bug off spray) of lavender, eucalyptus and cedarwood essential oils has a particular percentage of effectiveness or provides a specific number of hours of protection.
And I think that is worth saying plainly.
What the research does show is that using essential oils for insect repellent is not an idea that came out of nowhere.
Lavender and Eucalyptus globulus essential oils have demonstrated mosquito-repellent activity in laboratory research. Cedarwood has demonstrated repellent or pesticidal activity against several other arthropods, particularly ticks, although its evidence as a mosquito repellent is considerably weaker.
Together they create an aromatic blend containing a variety of volatile plant compounds that insects can perceive.
Whether that translates into a particular level of protection from a finished product would require testing the finished product itself.
That is the line between evidence-informed formulation and evidence-proven product, and it is an important one.
Essential oils are fascinating precisely because the science does not require us to pretend they are magic.
Plants manufacture complex aromatic chemicals for their own purposes. Insects detect those chemicals and sometimes change their behavior in response. Researchers can observe that behavior, measure it and begin figuring out which compounds are responsible.
The evidence is real. It is also wonderfully messy. And sometimes that is where the most interesting herbal science begins.