Pandan leaves come from the Pandanus amaryllifolius shrub, also known as the dwarf or fragrant screw pine. This sprawling perennial plant is native to southeast Asia and has been introduced in other tropical regions. It produces spirally arranged leaves about 1 to 1 1/2 inches wide and up to 18 inches in length. They have a strong, sweet fragrance and are often used to scent and flavour food. Many people in the pandan's native region use the leaves to repel insect pests.
In southeast Asia, homeowners and business owners use bunches of pandan leaves to repel cockroaches. For instances, according to a 2009 study by researchers at the National University of Singapore, taxi drivers in Malaysia and Singapore hang these leaves in their vehicles to keep pest insects from colonising the warm, dark interior of the car. Fresh pandan leaves are preferred, since dried leaves quickly lose their aromatic qualities. Wintergreen oil may be added to cut pandan leaves to enhance their smell and assumed repellent activity, according to "Plant Resources of South-East Asia."
Pandan leaves contain a number of essential oils and chemicals that cockroaches find unpleasant. Terpenes and sesquiterpene hydrocarbons make up between 6 and 42 per cent of the oils in these leaves, according to the National University of Singapore researchers. Pandan also contains 2-acetyl-1-pyrroline, or 2AP, a substance that repels both American and German cockroach species in test studies.
While pandan leaves contain substances repellent to cockroaches, they actually discourage these insects at a lower rate than other plants from the same geographical region. Pandan leaves are less potent than lemon grass, cinnamon, clove, ginger and turmeric, requiring a greater concentration of their essential oils to have the same effect. Chemically-extracted oils tend to produce a greater repellent effect than the leaves themselves, but the extraction process sometimes degrades the repellent chemicals. The Singapore study recommends using a combination of 2AP and pandan essence for best effect.
Since pandan leaves are a natural product, their concentration of useful aromatic chemicals can vary significantly from one plant to the next. Even when fresh leaves produce an insect-repellent effect, this may not be reliable or easy to duplicate. Pandan essences and extracts produce a more reliable repellent effect, as long as their concentrations of essential oils have been tested and normalised. In general, stronger-smelling leaves offer greater repellent ability.