Contrary to common belief, so-called mosquito-repellent plants’ effectiveness is either useless or limited. Photo: 123rf.com
To effectively and safely eliminate mosquitoes, it is essential to destroy their breeding grounds in your garden. This requires a thorough search of habitats or potential breeding sites. There are no shortcuts to this. Key actions include:
1. Drain stagnant water: Empty containers such as plant saucers, toys, and buckets that hold water, as even at 1-2mm depth, they can breed mosquitoes. Flip containers to prevent water accumulation. After rainfall, inspect for water collection in natural cavities, such as tree holes.
2. Declutter the garden: Remove items such as old tyres, broken pots, and discarded containers that can harbour mosquito eggs.
3. Unclog drains and gutters: Ensure that garden ditches and rain gutters are clear to prevent water stagnation.
4. Maintain the lawn and weeds: Short grass and weed removal discourage mosquito sheltering.
5. Keep water moving: For water features, ensure that filters are clean and pumps run continuously to deter egg laying.
6. Treat unavoidable stagnant water: For water bodies without aquatic life, use treatments such as chlorine or larvicides. BTi (Bacillus thuringiensis israelensis) is a safe and natural option. A thin oil or soap layer on water surfaces can also kill larvae.
7. Introduce mosquito predators: Fish such as mosquito fish, goldfish, and guppies eat mosquito larvae. Dragonflies are highly effective predators.
What about mosquito-repellent plants? Contrary to common belief, their effectiveness is either useless or limited. Plants such as citrosa, thyme, and lavender contain essential oils that repel mosquitoes; however, these oils must be extracted and properly prepared to be effective mosquito repellents.
Therefore, merely having these so-called mosquito-repellent plants in the garden is insufficient, and because they create havens for mosquitoes, these plants may ironically attract even more mosquitoes. While fresh plants are ineffective against mosquitoes, several studies have observed that burning the dry leaves of certain plants, such as African basil (Ocimum suave and O. kilimandscharicum), is effective in deterring mosquitoes.
Applying natural plant extracts to our skin can offer us temporary protection, but they are generally less effective and shorter-lasting than synthetic repellents. Some plant extracts, such as Mexican tea or American wormseed (Chenopodium ambrosioides) and pennyroyal (Mentha pulegium or Hedeoma pulegioides), are toxic to humans. Neither of these plant extracts should ever be used.
Synthetic repellents with DEET, picaridin, or IR3535 are recommended, as they are more effective, have a proven safety track record, and have a longer-lasting effect than botanical extracts. However, they are usually more expensive.
Here is a list of common plant extracts with either weak or strong repellency against biting pests and their maximum safe concentration level (%). This list was adapted from “Prevention of bug bites, stings, and disease” by Strickman and his associates (Oxford University Press, 2009).
1. Allium sativum (garlic): Strong repellency, safe level not determined.
2. Apium graveolens (celery): Strong repellency, safe level not determined.
3. Azadirachta indica (Neem): Strong repellency, safe level not determined.
4. Cedrus deodara (Cedar, Deodar): Weak repellency, safe level not determined.
5. Chrysanthemum cinerariifolium (Pyrethrum): Strong repellency, safe level not determined.
6. Cinnamomum cassia (Cassia): Strong repellency, safe level 1, skin irritant.
7. Citrus aurantifolia (Lime): Strong repellency, safe level 0.7, skin irritant.
8. Citrus aurantium bergamia (bergamot): Weak repellency, safe level 0.4, skin irritant.
9. Citrus reticulata (tangerine): Weak repellency, safe level not determined.
10. Cocos nucifera (coconut): Strong repellency, safe level not determined.
11. Curcuma longa (turmeric): Weak repellency, safe level not determined.
12. Cymbopogon citratus (lemongrass): Weak repellency, safe level 0.1 (90% citral), skin irritant.
13. Cymbopogon nardus (citronella): Strong repellency, safe level 0.2 or 9 (methyl eugenol 0.2% or citral 1.3%), carcinogenic and skin irritant.
14. Elaeis guineensis (palm oil): Strong repellency, safe level not determined.
15. Eucalyptus globules (eucalyptus, blue): Weak repellency, safe level not determined.
16. Foeniculum vulgare (fennel): Weak repellency, safe level not determined.
17. Lavandula angustifolia (lavender): Weak repellency, safe level not determined.
18. Melaleuca alternifolia (tea tree): Strong repellency, safe level not determined.
19. Mentha spp. (mint): Weak repellency, safe level 2 (0.1% trans-2-hexenal), skin irritant.
20. Myristica fragrans (nutmeg): Weak repellency, safe level 0.4 (1% methyl eugenol), carcinogen.
21. Nepeta cataria (catnip): Strong repellency, safe level not determined.
22. Ocimum basilicum (basil, sweet): Weak repellency, safe level 0.07 (6% methyl eugenol), carcinogen.
23. Pimpinella anisum (anise): Weak repellency, safe level 3.6 (0.11% methyl eugenol), carcinogen.
24. Piper nigrum (pepper): Weak repellency, safe level not determined.
25. Salvia rosmarinus (rosemary): Weak repellency, safe level 36 (0.011% methyl eugenol), carcinogen.
26. Sesamum indicum (sesame): Weak repellency, safe level not determined.
27. Syzygium aromaticum (clove): Strong repellency, safe level 0.5 (92% eugenol), skin irritant.
28. Tagetes minuta (marigold, wild): Weak repellency, safe level 0.01, skin irritant.
29. Thymus vulgaris (thyme): Strong repellency, safe level 2 (0.1% trans-2-hexenal), skin irritant.
30. Zingiber officinale (ginger): Weak repellency, safe level 12 (0.8% citral), skin irritant.
Assoc Prof Dr Christopher Teh Boon Sung heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation.
