UPSC Prelims 2021 Q21: Natural Mosquito Repellent Plants — Explained
Which one of the following is used in preparing a natural mosquito repellent?
- Congress grass
- Elephant grass
- Lemongrass
- Nut grass
Answer: (c) Lemongrass
Statement-wise explanation
- a: incorrect — Congress grass is Parthenium hysterophorus. Despite its name, it is not a true grass but a member of the sunflower family (Asteraceae). It is an invasive alien weed from tropical America. It is thought to have reached India in imported wheat in the 1950s. It causes skin allergy and asthma in people, and its toxin parthenin harms livestock. It is a weed to be controlled, not a source of repellent.
- b: incorrect — Elephant grass, also called Napier grass (Pennisetum purpureum, now Cenchrus purpureus), is a tall C4 perennial grass native to Africa. It is valued as a high-yield green fodder for dairy cattle and is also used for erosion control. It has no recognised use as a mosquito repellent.
- c: correct — Lemongrass (Cymbopogon citratus and C. flexuosus) is an aromatic grass. Its leaves are steam-distilled to give an essential oil rich in citral. This oil, and the oil of its close relative citronella grass (Cymbopogon winterianus or nardus), are used in herbal mosquito-repellent creams, sprays, incense sticks and vaporisers. Lemongrass is one of the main crops promoted under CSIR's Aroma Mission.
- d: incorrect — Nut grass (Cyperus rotundus) is a sedge (family Cyperaceae), not a true grass. It is counted among the world's worst agricultural weeds because its underground tubers spread fast. The tubers ('musta') are used in Ayurvedic medicine, but the plant is not a source of natural mosquito repellent.
How to eliminate
You can work this out from everyday knowledge. Natural repellents work through strong-smelling essential oils, so look for the aromatic plant. Lemongrass is familiar from herbal tea, cooking and 'citronella/lemongrass' mosquito products, and it is the only aromatic plant among the options. The others can be ruled out as well. Congress grass is a well-known harmful invasive weed. Elephant grass is a fodder grass. Nut grass is a stubborn field weed. Fact to remember: the Cymbopogon genus (lemongrass, citronella, palmarosa) supplies the essential oils used in herbal mosquito repellents.
Know more
Aromatic grasses and the 'grass' names UPSC likes to test
Several common names contain 'grass', but they belong to very different groups. Lemongrass and its relatives in the genus Cymbopogon store essential oils in their leaves. Steam distillation extracts these oils. Compounds such as citral, citronellal and geraniol repel mosquitoes, so the oils are used in herbal creams, coils and vaporisers.
| Common name | Scientific name | Family | Main relevance |
|---|---|---|---|
| Lemongrass | Cymbopogon citratus / flexuosus | Poaceae (true grass) | Citral-rich oil; perfumery, flavour, herbal repellent |
| Citronella grass | Cymbopogon winterianus / nardus | Poaceae | Citronella oil; classic mosquito repellent |
| Palmarosa | Cymbopogon martinii | Poaceae | Geraniol-rich oil; perfumery |
| Vetiver (khus) | Chrysopogon zizanioides | Poaceae | Root oil; soil conservation |
| Elephant/Napier grass | Cenchrus purpureus | Poaceae | Green fodder |
| Congress grass | Parthenium hysterophorus | Asteraceae (not a grass) | Invasive weed; allergies |
| Nut grass | Cyperus rotundus | Cyperaceae (sedge) | Noxious weed; Ayurvedic tuber |
CSIR Aroma Mission. The mission began in 2017 with CSIR-CIMAP, Lucknow, as the nodal lab. It promotes aromatic crops such as lemongrass, lavender, vetiver, palmarosa and mint, mainly on marginal and rain-fed land, to raise farmers' incomes. The lavender part of the mission in Jammu & Kashmir is called the Purple Revolution. Aromatic grasses grow on degraded land and need little water, and animals rarely graze them. These traits make them useful in drylands and in the North-East.
Invasive weeds to remember: Parthenium, Lantana camara and water hyacinth (Eichhornia) are the standard NCERT examples of alien species that reduce biodiversity.
NCERT Concepts to Revise
- Invasive alien species: Congress grass (Parthenium) is a textbook invasive alien species. Knowing this helps you place one option as a weed rather than a useful plant.
- Purple Revolution: The Purple Revolution is part of CSIR's Aroma Mission. The mission also promotes aromatic grasses grown for their essential oils.