Aromatic herbs have long been used to deter insects, and rosemary is a popular herb often suggested as a natural way to keep mosquitoes away. Many people believe that burning the dried leaves or sprigs will create a mosquito-free zone due to its pleasant, resinous scent. This belief is rooted in the plant’s natural chemical composition, but the effectiveness requires a closer look at the science of insect repellency and a comparison with its concentrated essential oil.
The Chemical Basis for Repelling Insects
Rosemary’s ability to repel insects stems from the volatile organic compounds (VOCs) concentrated within its leaves and stems. These compounds are a mix of monoterpenes and ketones that the plant produces naturally. The primary active ingredients are 1,8-cineole, also known as eucalyptol, and camphor. These chemicals function by interfering with the highly sensitive olfactory systems of mosquitoes. The strong, pungent aroma overloads the insect’s chemoreceptors, making it difficult for the mosquito to locate human hosts by masking the scents of carbon dioxide and skin odors. Other significant components, such as alpha-pinene, also contribute to the overall repellent effect.
Comparing Efficacy of Burning to Oil Extracts
Burning dried rosemary releases the volatile repellent compounds through smoke and heat, which can create a localized barrier against mosquitoes. This method is a form of fumigation, where the smoke carries the aromatic chemicals into the air around the source. However, the concentration of active compounds released this way is relatively low and highly dependent on air movement. The duration of the repellent effect is typically short, and the smoke rapidly disperses, limiting protection to a small area immediately surrounding the fire or smoldering material.
In contrast, rosemary essential oil represents a highly concentrated form of these same VOCs, extracted through processes like steam distillation. Studies using concentrated rosemary oil have shown promising results, such as a 12.5% oil dilution providing 100% repellency against Aedes aegypti mosquitoes for up to 90 minutes. Applying the essential oil, diluted in a carrier oil, directly to the skin or diffusing it using an electric device offers a much higher, more stable concentration of the active ingredients than burning the raw herb. This controlled delivery method generally provides a more reliable and longer-lasting effect.
Practical Use and Proven Natural Alternatives
For those who wish to use rosemary, one practical application involves boiling the leaves to create a fragrant spray, or simply crushing fresh leaves and rubbing them on the skin to release the oils. If using the burning method, dried rosemary sprigs can be placed directly onto hot coals in a grill or fire pit to maximize the release of smoke and oils. Any method involving smoke should be used in well-ventilated outdoor areas due to inhalation considerations.
When seeking proven, highly effective natural alternatives, oil of lemon eucalyptus (OLE) is the only plant-based repellent recommended by the Centers for Disease Control and Prevention (CDC). Formulated OLE products can provide protection comparable to some synthetic repellents, lasting up to two hours. Other plant-derived oils, such as citronella, peppermint, and thyme, also demonstrate repellent activity and can be used in candles, diffusers, or topical formulations. Using a mechanical fan is another effective, non-chemical option, as the moving air disrupts the flight patterns of mosquitoes.