Rosemary (Rosmarinus officinalis) is a fragrant, woody herb in the mint family, Lamiaceae, recognized for its culinary uses and natural insect-repelling properties. The herb’s effectiveness as a pest deterrent stems from the high concentration of volatile chemical compounds released from its leaves and stems. This strong, aromatic scent interferes with the sensory systems of many common nuisance insects. Studies confirm that rosemary’s distinct aroma actively works to keep many bugs at a distance.
The Chemical Compounds Responsible for Repulsion
Rosemary’s repelling power comes from a complex mixture of volatile organic compounds (VOCs) concentrated in its essential oil. The primary active ingredients are monoterpenes, most notably 1,8-cineole (eucalyptol), camphor, borneol, and alpha-pinene. These compounds are responsible for the herb’s characteristic sharp, pungent aroma. The mechanism of action involves the compounds disrupting the neurological and sensory systems of insects.
The monoterpenes can overwhelm or confuse the chemoreceptors that pests use to locate hosts or food sources. Studies show that 1,8-cineole and camphor act synergistically, meaning their combined effect is greater than their individual effects. The presence of these compounds can also lead to fumigant toxicity. This toxicity causes mortality in certain pests when they are exposed to high concentrations of the vapor.
Common Pests Rosemary Successfully Deters
The strong aromatic profile of rosemary makes it effective against a range of flying and crawling pests, both in the garden and around the home. Mosquitoes, including species from the Aedes and Anopheles genera, are a primary target of rosemary-based repellents. Rosemary oil, when used at high concentrations, has shown significant protection against the yellow fever mosquito, Aedes aegypti, achieving over 92% effectiveness.
The herb is also a deterrent for various garden pests that prey on vegetable crops. It repels the cabbage looper and cabbage worm, common larvae that damage brassica plants. Rosemary also deters the carrot rust fly, a pest whose larvae feed on the roots of carrots and other root vegetables. Other nuisances kept away by rosemary’s intense scent include house flies, fleas, mites, and aphids. This broad-spectrum repellent action makes it a valuable plant for companion planting and natural home pest management.
Insects That Are Attracted to Rosemary
While rosemary repels many common pests, the plant is not universally disliked in the insect world. Its flowers, which typically bloom in the late winter or early spring, are a source of nectar that attracts beneficial insects. Pollinators such as honey bees and various species of butterflies are drawn to the plant when it is in bloom.
Certain herbivorous insects, however, are adapted to feed on the plant’s foliage. The most notable pest is the Rosemary Beetle (Chrysolina americana). Both the adult beetles and their larvae feed on the leaves of rosemary, as well as related herbs like lavender and thyme. Another occasional pest is the Southern purple mint moth (Pyrausta laticlavia), whose caterpillars damage the stems by feeding and matting them with silken threads.
Methods for Using Rosemary as a Natural Deterrent
One of the simplest ways to use rosemary for pest control is through strategic planting of the live herb. Placing rosemary bushes near outdoor seating areas or entrances creates a natural aromatic barrier against flying insects like mosquitoes and flies. In the garden, positioning rosemary near susceptible crops like cabbage, beans, and carrots helps confuse pests and prevents them from locating their preferred food sources.
A homemade repellent can be created by boiling the fresh leaves to extract the aromatic oils. A simple recipe involves boiling one cup of rosemary leaves in one quart of water for about 30 minutes, then straining the liquid and allowing it to cool. The resulting concentrate can be diluted with an equal amount of cold water and stored in a spray bottle for use on skin or clothing.
For a more potent application, rosemary essential oil is highly effective when used correctly. The oil can be diffused indoors to ward off flying insects or diluted in a carrier oil, such as jojoba or coconut oil, for topical use. For skin application, a proper dilution is necessary, typically aiming for a concentration of 2% to 3% rosemary oil in the carrier oil to ensure safety. The essential oil can also be added to water and a small amount of dish soap to create a strong spray for treating plants or outdoor areas.