Flies do not like rosemary; the herb acts as a natural repellent against them. The strong, pungent aroma released by the plant deters these insects. This repellent effect is a direct result of the plant’s natural chemical defenses. Rosemary’s potent scent creates an unwelcoming environment for flies, causing them to actively avoid the source.
The Volatile Compounds Responsible for Repulsion
The repellent action of Rosmarinus officinalis stems from its volatile organic compounds (VOCs), which are aromatic molecules that easily evaporate into the air. These VOCs are primarily monoterpenes, a class of natural chemicals that give rosemary its characteristic strong scent. The most abundant and studied compounds are 1,8-cineole (eucalyptol) and camphor.
Rosemary essential oil often contains high concentrations of 1,8-cineole, sometimes making up over 40% of the total composition, with camphor accounting for another significant portion. Other repellent compounds present include borneol and verbenone. These chemicals serve as the plant’s internal defense mechanism against insect pests. The combination and concentration of these multiple volatile compounds create a powerful deterrent effect against flies.
How Rosemary Disrupts Fly Olfaction
The strong scent molecules from rosemary interfere with the fly’s highly sensitive olfactory system, which is crucial for finding food and mates. Flies use specialized antennae to detect airborne chemical signals. The high concentration of compounds like 1,8-cineole and camphor overloads these receptors, making it difficult for the fly to process attractive scents of food or decaying matter. The repellent compounds effectively mask the odors that would normally draw the insects in.
Beyond masking odors, these monoterpenes also have a direct physiological effect on the insect’s nervous system. The compounds disrupt the octopamine receptor system in insects, which leads to an irritant effect and neurotoxicity. This disruption causes the fly to perceive the area as highly unpleasant, triggering avoidance behavior. The fly leaves the area because the rosemary compounds are irritating and potentially toxic to its internal biology.
Effective Ways to Use Rosemary for Fly Control
Fresh Bundles
Using fresh bundles of rosemary is a simple, non-toxic method for short-term fly deterrence in small areas. Crush the leaves slightly to release the volatile oils and place the bundles near doorways or on outdoor tables. The effectiveness of fresh cuttings is limited by the quick evaporation of the oils, often lasting only a few hours before needing to be refreshed.
Homemade Spray
A homemade repellent spray offers a more concentrated solution. Create this by boiling one cup of fresh rosemary leaves in one quart of water for 30 minutes, then straining and cooling the liquid. The resulting decoction, which contains the extracted oils, can be stored in a spray bottle and applied to surfaces or plants. This method is effective for a few hours at a time and the spray should be kept refrigerated to maintain potency.
Potted Plants and Essential Oil
For long-term, passive control, strategically placed potted rosemary plants work well on patios or near windows. While the live plant releases a lower concentration of oils, it provides a continuous, low-level deterrent barrier. To increase the release of repellent compounds, gently brush or trim the foliage to bruise the leaves, which releases a stronger burst of volatile oils. Using rosemary essential oil diluted in a carrier, such as witch hazel and water, can also be sprayed or diffused for a more potent effect.