The eucalyptus tree, native primarily to Australia, is recognized worldwide for its distinct, pungent aroma. This powerful scent originates from volatile compounds stored within the plant’s leaves, which are released when crushed or distilled. While the question of whether insects are drawn to or repelled by this plant is complex, for the majority of household and biting pests, the answer leans strongly toward deterrence.
The Chemical Basis for Insect Deterrence
The insect-repelling properties of eucalyptus are rooted in the complex chemistry of its essential oil, a mixture of volatile organic compounds. The most prominent of these is 1,8-cineole, also known as eucalyptol, which constitutes a large percentage of the oil in species like Eucalyptus globulus. This monoterpene is the primary agent responsible for the characteristic medicinal scent and strong biological activity against pests.
Certain varieties, like Eucalyptus citriodora (Lemon Eucalyptus), contain high levels of citronellal, which is chemically converted to para-menthane-3,8-diol (PMD) upon processing. The mechanism of action for these compounds is multifaceted, generally targeting the insect’s delicate sensory systems. Volatile molecules interfere with the olfactory receptors of insects, confusing them and effectively masking the scent of a human or animal host.
In higher concentrations, 1,8-cineole acts as a fumigant and contact insecticide, going beyond simple repulsion. Studies indicate that the compound disrupts the nervous system by inhibiting acetylcholinesterase, an enzyme necessary for nerve signal transmission. The oil also shows evidence of causing metabolic disruption by impairing mitochondrial respiration and cellular energy production in insects.
Pests Effectively Repelled by Eucalyptus
Eucalyptus oil is highly effective against a wide range of generalist pests, particularly those that rely on scent to locate their targets. Mosquitoes are one of the most significant insects deterred, with concentrated oil containing PMD showing efficacy comparable to some synthetic repellents against species like the Yellow Fever mosquito, Aedes aegypti. The oil’s active components disrupt the mosquito’s ability to detect primary attractants like carbon dioxide and lactic acid emanating from humans.
The strong aroma also provides measurable protection against common household nuisances like house flies and cockroaches. Research shows that combining eucalyptus oil with other strong essential oils, such as rosemary and oregano, maximizes toxicity and repellency against cockroaches. Furthermore, the oil has been shown to be toxic to sandflies, which are vectors for diseases like leishmaniasis.
Ants are repelled because the volatile oil interferes with their pheromone trails, which are essential for foraging and colony communication. Other pests, including certain species of clothes moths, silverfish, and spiders, are reliably deterred by the concentrated oil or sachets of dried leaves placed in confined spaces.
Insects Unaffected or Attracted to Eucalyptus
While eucalyptus is a broad-spectrum repellent for many general insects, it is the primary food source for specialized Australian herbivores. These native pests have evolved mechanisms to tolerate or even metabolize the plant’s toxic cineole content, making them unaffected by its repellent properties.
Eucalyptus borers, psyllids, and various leaf beetles, such as the Steel Blue Sawfly, specifically target the living plant material. Psyllids feed by sucking sap from the leaves and excrete a crystallized sugary substance known as a lerp, which is a protective cover for their nymphs. These specialized feeders often become invasive pests when eucalyptus is planted in non-native environments, viewing the plant as a nutrient-rich host, not a threat.
Another exception to the repellent rule is certain male Euglossine bees, sometimes called orchid bees, which are attracted to terpenes like 1,8-cineole. These bees collect the volatile compounds for chemical signals, demonstrating that the plant’s chemistry can function as an attractant for select non-pest insects.