As Singapore steps up its fight against the Zika virus, so-called mosquito-repelling plants have been flying off the shelves in nurseries here.
The bestsellers are the citronella-scented geranium (Pelargonium citrosum) and the rose-scented geranium (Pelargonium graveolens) - both dubbed "mosquito plants" as their scents are believed to ward off mosquitoes.
An order of 100 pots of the Pelargonium citrosum that World Farm sells arrived last week and was sold out within a few days. It usually takes the nursery in Bah Soon Pah Road a week to sell the same quantity.
A new order of 300 of these plants that arrived from Cameron Highlands, Malaysia, on Thursday was snapped up within two hours.
At each of Far East Flora's four retail outlets, a sign hangs on a stand listing seven plants: Pelargonium graveolens, lemon balm, rosemary, citronella, lavender, lemongrass and peppermint - all touted to have mosquito-repellent properties.
According to the nursery's sales and marketing director Peter Cheok, the essential oils in these plants are common ingredients in mosquito-repelling sprays.
Far East Flora's weekly order of about 40 to 60 of each of these plants has more than doubled on the back of "swift sales", he says.
Despite this, new batches that arrived earlier this week are running low in stock.
Such plants are also sold out or running low at Candy Floriculture in Thomson Road and Katong Flower Shop in New Upper Changi Road. New shipments have recently arrived or are on the way.
But how effective are these plants?
Botanist Shawn Lum says that while there is some evidence that selected plant essential oils and other compounds have insect-repellent properties, this does not necessarily mean that having the plants in one's compound will repel mosquitoes.
Dr Lum, a senior lecturer from the Asian School of the Environment at Nanyang Technological University (NTU), says: "Some repellent compounds are only released when the leaves are crushed or burnt."
Even then, their effect may not last long.
Organic farmer Alexius Yeo, 31, who grew a mosquito plant and six pots of lemongrass in the garden of his parents' terrace house last year, found that when he crushed the leaves of the mosquito plant and lemongrass and rubbed them all over his skin, the mosquitoes stayed away longer.
The director of CarbonInQ, which runs nature-based education programmes, says: "But the smell went off and, after a while, the mosquitoes came back."
Professor Jorgen Schlundt, a food safety expert from the School of Chemical and Biomedical Engineering at NTU, says that, so far, there has been no scientific evidence that mosquito-repellent plants can bring about a significant drop in a person's risk of getting Zika or dengue.
The former director at the World Health Organisation adds that in most urban areas of South-east Asia, the main breeding source of the Aedes mosquito, which spreads Zika and dengue, is containers storing water for household use.
Assistant Professor Miao Yansong from NTU's School of Biological Sciences points out that the effectiveness of different types of herbs as a mosquito repellent can also vary between mosquito species.
The experts say that the five-step Mozzie Wipeout by the National Environment Agency is a more effective way of reducing mosquito breeding.
Among the steps are removing water in plant pot plates and trays and loosening the hardened soil in plant pots where stagnant water can accumulate.
Dr Lum adds: "That said, there is no harm in growing 'mosquito-repelling plants' at home."
Either way, some businesses are hoping to ride on the current demand.
Candy Floriculture plans to sell a chemical solution called PlantZom to customers at $30 a bottle.
Invented three years ago by two chemists from a home-grown chemical formulation company, it has been offered through a spraying service to hotels, condominiums and companies.
Mr Edwin Chan, 44, one of the chemists who invented it, claims that when sprayed on plants, the chemical develops an invisible web which traps and kills mosquitoes within six hours after they land on the plant.
PlantZom, he claims, lingers around longer - at least 14 days, unlike conventional insecticides which last only about three days.
He says: "I am confident that it will sell well."
Crush and repel
MOSQUITO PLANT (PELARGONIUM CITROSUM OR GRAVEOLENS)
Crushed leaves of the Pelargonium citrosum seem to offer short-term relief from mosquitoes. Some studies show that it has 30 to 40 per cent of the repelling power of Deet. Deet (N, N-Diethyl-meta-toluamide) is a synthetic chemical that has been shown to be effective in repelling mosquitoes. Another species of the same family, Pelargonium graveolens (pictured), is also sold here for its repellent properties. Place a few pots together or occasionally rub the leaves to trigger the release of essential oils. Before rubbing crushed leaves on your skin, test them on a small spot first. Essential oils may cause some people to break out in rashes.
Available at: World Farm ($5 to $6 a pot), Far East Flora ($8) and Candy Floriculture ($8)
BASIL (OCIMUM BASILICUM)
Basil plants are among the most pungent herbs. They are one of the few herbs which give off a scent without their leaves being crushed. For temporary protection in the garden, rub some leaves on your skin to release the essential oils.
Available at: World Farm ($5 a pot), Katong Flower Shop ($7 a pot) and Candy Floriculture ($8 a pot)
LEMON BALM (MELISSA OFFICINALIS)
Crushing the leaves of this herb and rubbing them on your skin supposedly provides a measure of protection against mosquito bites.
Available at: World Farm and Candy Floriculture (both $8 a pot)
•Sources: BugOfff.com, National Gardening Association in the United States