One of the most frequently asked questions we hear is, as the title suggests, “Do eucalyptus repel mosquitos?” (or another biting insect of your choice).
The longer answer? Probably not.
Some confusion appears to have arisen because of the many studies that assess the efficacy of eucalyptus oil as a pesticide. The ability of extracted eucalyptus oils to deter or kill insects is well attested, and a much better alternative to harmful synthetic pesticides. However, highly concentrated eucalyptus oil that has been extracted from leaves and then refined is very different to simply having a tree in your garden in much the same way as lavender oil is rather more potent than a lavender bush.
The concentration and composition of the oils within the leaves of a eucalyptus vary considerably depending upon the species in question. Some species such as E. citriodora have a very strong fragrance indeed, but unfortunately the species that are the richest in oil tend to also be the least hardy. Whilst many of our eucalyptus still have a wonderful fragrance, it is usually somewhat less than their tropical counterparts.
If any eucalyptus were to be capable of deterring mosquitos through the sheer density of the oils in the intact leaves then it would certainly be E. citriodora which we grow periodically at Hardy Eucalyptus as a curiosity for greenhouses. It is, however, not at all cold hardy and it reacts very poorly to being frosted!
The confusion between the effects of concentrated oils and intact trees appears to have permeated much of the internet, with countless articles suggesting that planting a eucalyptus will be sufficient to scare away the invertebrate or arachnid of your choice. I have undertaken a reasonably thorough search for any scientific studies that support this notion, and I have not yet found a single item of research that does so. Quite to the contrary, I did find two studies on mosquitos that describe how they can live and breed upon eucalyptus trees, which is the opposite of what one might expect.
So to summarise. Eucalyptus oil is an effective pesticide and insect deterrent, but there is no obvious scientific research to indicate that eucalyptus trees have the same effect on insects. If any readers know of such research please do get in touch. If you were considering planting a eucalyptus for this purpose then I would recommend choosing one of the more fragrant varieties such as E. neglecta, globulus, or any members of the gunnii family. Even if it doesn’t work you will still have a beautiful, fragrant tree to enliven your garden 🙂
Update: A helpful customer has pointed out that eucalyptus are absolutely amazing at draining wet ground, which happens to also be where mosquitos breed! So planting a thirsty euc to drain your boggy corner may well reduce their numbers.
References (Yes, the formatting is awful and no, I don’t want to fix it!)