Mosquitoes are a problematic nuisance in summer in Israel. They reproduce in summer in any standing water source, laying their eggs in or near the water, and their emerging larvae develop in the water. The adult mosquitoes fly and imbibe flower nectar, but the females require a supply of protein-rich blood in order to lay their eggs, and so they bite! In addition to Culex pipiens and other local mosquito species, a little more than a decade ago our region was invaded by a new species – the Asian tiger mosquito (Aedes albopictus). This mosquito is able to reproduce in even the shallowest of standing water, and the adults are partially diurnal, making them a particularly troublesome pest. These mosquitoes, unfortunately, have also not ignored our Garden, which is rich in pools and irrigation pipes.

In the last 18 months we have been making an intense effort to treat the problem. Most of the large pools in the Garden contain fish, which eat the mosquito larvae, but we have found that this in itself is not enough. First, we made a great effort (and still are) to locate and dry up unnecessary water sources. Second, we regularly spray every water source in the Garden with a selective biological control agent called Bti, more commonly known by its commercial name Lossquito (Bactush, Biotush). This material is a toxin produced by the bacteria Bacillus thuringiensis israelensis, which specifically targets only mosquito and fly larvae and breaks down biologically. In contrast to the majority of common mosquito control agents, it does not affect the mosquito larva’s natural enemies (mainly fish, frogs, toads and predatory insects). The bacteria were discovered in Israel in the 1970s by Prof. Yoel Margalit, from Ben-Gurion University, in puddles in HaBesor stream. The material, sprayed on the water, is consumed by the mosquito larvae, which feed on the organic materials in the water. It damages their digestive system and leads to their death. The treatment is continuous and must be repeated weekly, as the material sinks in the water and is then less effective. Unfortunately, there is no magical solution to the problem and the mosquitoes do not disappear overnight. There are currently fewer mosquitoes in the Garden than in previous years and the difference is clearly felt, but the fight goes on and will continue to do so. As long as the temperature remains high enough to enable reproduction, we continue to spray in order to prevent development of the next generation.

And what about mosquito-repelling plants? Many plants produce secondary metabolites that repel insects, and some of these materials are utilised by us humans. We recently received the Plectranthus madagascariensis plant, which features a variety of aromatic leaves of the mint family (Lamiaceae). Its natural habitat is in South Africa, Madagascar and the adjacent islands, and it is a close relative of the Solenostemon genus, whose species are common in Israeli gardens. In the Botanical Garden it grows in the tropical greenhouse. Recently it has become common to claim that Plectranthus repels mosquitoes, and that planting it in the yard and even in a pot on the windowsill will prevent the mosquitoes from entering the house; sadly, however, this is an urban myth. The plant contains volatile secondary metabolites, like many other members of its family (such as sage and rosemary), but no reliable comparative research has been carried out to date that has demonstrated that this plant, or the materials produced from it, is effective as a mosquito repellent. However, the essential oil found and refined from the seeds or leaves of close relatives of the plant – Plectranthus marrubioides and Plectranthus incanus, at a far higher concentration than that existing in the plant itself, has indeed been found effective in repelling mosquitoes under laboratory conditions. The research into these materials is in its early days and they are not yet marketed as mosquito repellents – perhaps in the future. In any case, it is clear that it is not enough to plant this species close to the house in order to repel mosquitoes – the materials are released only when the leaves are injured or by crushing the seeds, and even then their concentration is too low to provide any real defence; but the legends linked to the name of this plant undoubtedly constitute an excellent marketing strategy.

So – what plant materials really work? There are some species of plants with proven effectiveness in repelling mosquitoes. According to the most current recommendations of the U.S. Centre for Disease Control (the CDC) followed by the Israeli Ministry of Health, there is one plant-source essential oil that has a proven long-term effectiveness in repelling mosquitoes, and is therefore recommended for use even in areas plagued by malaria and other mosquito-borne diseases. This essential oil is produced from the lemon eucalyptus (Corymbia citriodora) – a tall tree with a white bark and leaves producing a pleasant lemony scent, found in our Australian plant collection. The main active material in the oil is citronellal (also called citriodiol or OLE). The refined and concentrated eucalyptus oil confers hours of protection, similar to that obtained from protective products containing DEET, a common chemical mosquito repellent but of some toxicity to humans.

An additional plant used as a mosquito repellent is lemon grass (Cymbopogon citratus), of the true grasses family (Poaceae), whose origin is in South-East Asia. It is a familiar spice in cooking and infusions and is grown in beds in our spice garden. From another close relative, Cymbopogon nardus, the essential oil called citronella is produced, which also repels mosquitoes. This material, however, is far less effective than the citrus eucalyptus oil, retaining its potency for only about 30 minutes. Other plants with lower effectivity or whose effectivity has not been proven are the rose geranium (Pelargonium graveolens), rosemary and mint.

It is important to stress that no plant exists (or has been discovered to date), which by placing on a windowsill will prevent mosquitoes from entering the house. So – nonetheless – how can we really stop them? There is actually a very simple solution, effective and non-toxic – install netting on the windows. May we all have a pleasant, mosquito-bite-free summer!

Photo: Moshe Peri

Plectranthus madagascarienis, Photo: Moshe Peri

 Photo: Tal Levanony

Lemon Eucalyptus, Photo: Tal Levanony

Photo: Kineret Manevich

Lemon grass, Photo: Kineret Manevich

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