Traditionally, aromatic plants have been used as a way to drive away insects in general, and mosquitoes in particular. These hematophagous diptera —which feed on blood— can be very annoying and transmit serious diseases in certain places, so an effective prevention method is to repel them.
Although there are chemically synthesized repellents, they cannot always be used, and they are not available to everyone. Using plants or plant extracts seems like a good alternative. But for it to be a real alternative it is necessary to prove its effectiveness ; verify that these plants have the properties expected of them.
The plant or the active ingredient?
In general, when talking about certain properties of plants, such as their medicinal effects or as insect repellents, reference is made to the properties of some active ingredients that the plant has in some of its parts, whether they are molecules present in its tissues or volatile substances they emit. Among them, we can highlight phytohormones such as ethylene, key in some communication processes between plants, or aromatic substances, such as essential oils.
Sometimes, the concentration of these active ingredients is sufficient for the plant, by itself, to be effective in its intended purpose. For example, the concentration of caffeine present in the coffee bean is sufficient to cause the exciting effect expected from its infusion.
But the amount of active ingredients produced by a plant does not always remain constant. These molecules are secondary metabolites , substances that the plant produces to fulfill various purposes, such as protection against stress factors or attraction of pollinators. The environment can modulate or alter the amount of substances it produces; a plant exposed to constant attack by insects or herbivores will produce more tannins —molecules that are involved in protecting the plant against aggression— than a plant that does not suffer such attacks.
Hence, the concentration of active principle can fluctuate . Therefore, to obtain the effective dose of a substance, a greater or lesser amount of plant matter may be required. For example, to obtain enough salicylin to make an aspirin, you need between 100 and 300 grams of white willow bark. This also means that, very often, the amount of plant needed to achieve the desired effect is so high that it is unaffordable.
Anti-mosquito essential oils
When we talk about plants with a possible anti-mosquito effect, it is rare to find one that, by itself, produces a sufficient concentration of essential oils to be effective. However, extracting these essential oils and using them in concentrate , as used in perfumes, can be a good option.
There are many plants that have traditionally been used as repellents against mosquitoes. Some, such as marcela ( Achyrocline satureioides ) or serrano anise ( Tagetes pusilla ), after scientific analysis, have not shown sufficient efficacy. However, essential oils from other plant species of the Labiatae family, such as lemon verbena ( Aloysia citriodora ) or rosemary ( Rosmarinus officinalis ) successfully repel mosquitoes for more than an hour, even diluted in a ratio of 1:8. Another very effective product, especially for closed or semi-closed spaces, is citronella extract ( Cymbopogon nardus ), which can be found on the market in a wide variety of presentations.
In general, the active ingredients responsible for the repellent effect of essential oils are limonene , camphor and citronellal .
Plant extracts against synthetic products
We must forget about the false premise that “everything natural is good, and everything artificial is bad”. There are chemically synthesized mosquito repellents and plant extracts with proven efficacy. And both have advantages and disadvantages.
Chemical synthesis products , such as dimethyl phthalate or diethyl toluamide, are much more effective, however, they are relatively often irritating to some people, who react with hives or skin rashes, and even encephalopathy in children. In addition, used massively, they can become environmental contaminants.
But the active principles of plants are not always harmless. Although less frequently, they can also cause irritation, especially those that contain limonene in their composition, and those that contain linalool can cause allergic reactions. Certain essential oils in their pure state are, in fact, caustic. That is why they must always be used diluted.
From a social perspective, not everyone has access to synthetic repellents. and especially where mosquitoes can be carriers of dangerous endemic diseases. The essential oil extraction process is simple, does not require large industrial facilities, and is relatively cheap as long as the raw material is available: aromatic plants that are easy to grow.
References:
Gillij, Y. G. et al. 2008. Mosquito repellent activity of essential oils of aromatic plants growing in Argentina. Bioresource Technology, 99(7), 2507-2515. DOI: 10.1016/j.biortech.2007.04.066
Kim, J.-K. et al. 2005. Evaluation of Repellency Effect of Two Natural Aroma Mosquito Repellent Compounds, Citronella and Citronellal*. Entomological Research, 35(2), 117-120. DOI: 10.1111/j.1748-5967.2005.tb00146.x
Tisgratog, R. et al. 2016. Plants traditionally used as mosquito repellents and the implication for their use in vector control. Acta Tropica, 157, 136-144. DOI: 10.1016/j.actatropica.2016.01.024
