Essential oils are aromatic liquids derived from plants; their name comes from the fact that the oil gives off the characteristic aroma, or essence, of the plant from which it is derived. Some sources ascribe insecticidal and repellent qualities to essential oils such as tea tree oil, eucalyptus oil, citronella and neem oil, and although reports of their effectiveness is often based on anecdotal evidence, scientific studies have confirmed the ability of some essential oils to kill and repel cockroaches.
A study published in 1999 in Pest Management Science tested nine different components of essential oils to determine their effectiveness at both killing and repelling insects. Of the nine compounds tested, the most effective at knocking down insects was methyl-eugenol, a compound found in the essential oils of hundreds of plant species, including parsley, wild carrot, fennel and mango. The compound most effective at killing insects on contact was eugenol, which is found in plants such as clove, basil, nutmeg, oregano and marjoram.
A study published in the Journal of Agricultural and Urban Entomology tested the effectiveness of mint oil as an insecticide against German and American cockroaches. The study found that the oil was toxic to both species when applied topically, and solutions with an oil concentration of 3 percent or more killed roaches through continuous exposure. The oil also proved to be an effective repellent.
Some essential oils, such as mint oil, have a relatively low toxicity for mammals and leave behind little residue when they dry, so they may be suitable for use where more toxic chemical insecticides or repellents are not appropriate. Oils containing eugenol, on the other hand, are toxic to humans, as are many other essential oils, and they must be used with care.
Commercial insecticides that contain essential oils as an active ingredient are available. To make a homemade essential-oil based insecticide and repellent, add 1 to 2 ounces of oil in a spray bottle filled with 12 to 16 ounces of water and mix well. Although essential oil sprays may have a repellent effect, the oils have little residual toxic effect and may work as an insecticide only when sprayed directly on insects.