Charle Patrick Garcia, International Rice Research Institute, Los Baños, Philippines
Keng Hong Tan, Tan Hak Heng Sdn. Bhd., Johor Bahru, Malaysia, and
K. L. Heong, International Rice Research Institute, Los Baños, Philippines
Holy basil (Ocimum sanctum Linn. – Lamiaceae) – this purple variety has high contents of methyl eugenol – 66-78% in floral and leaf essential oils (Nurdijati et al. 1996).
Exactly a century ago, Howlett (1912) discovered that citronella grass, Cymbopogon nardus (Poaceae), when used as a mosquito repellant, attracts many Bactrocera fruit flies during the daytime. Subsequently, he showed that the component responsible for the attraction was methyl eugenol (ME) (Howlett 1915). Tan and Nishida (2012) discussed and showed in their review that ME is found in more than 450 plant species, especially in spices, from 80 families spanning across 38 orders; and has many roles in nature. In an article “ask the experts – Why do flowers have scent?” Dudareva (2005) pointed out – “ To date, little is known about how insects respond to the individual chemical components, but it is clear that they are capable of distinguishing among complex aroma mixtures”. In fact, ME of floral fragrances has been shown and known to play an essential and important ecological role as a plant synomone (a semio-/behavior modifying chemical that benefits both parties in an interaction between two species) in the pollination of certain wild orchid species (Tan 1993, 2009; Tan et al. 2002, 2006). Furthermore, it acts as an antifeedant and/or a repellant against certain insects, as well as an antimicrobial agent, especially against some fungi and bacteria (see review by Tan and Nishida 2012). In the review, it was also shown that ME is toxic to the brown planthopper (BPH), Nilaparvata lugens (Stål).
Filter paper treatment: (A) filter paper treated on a glass petri dish; (B) ventilated test cage containing the feeding plants with insects.
Standardized methods developed by the International Rice Research Institute (IRRI) on evaluating methyl eugenol were used. The repellency (as shown by BPH leaving the feeding site) and mortality responses were observed and noted at specified time intervals – 5, 10, 20, 30, and 40 minutes for repellency; and mortality was recorded at 24 and 48 hours. The quantitative data obtained were analyzed using the PoloPlus program. The summary of data analyses is shown in Table 1. The probit lines of the repellency and toxicity responses were plotted as shown in Figures 1 and 2, respectively.
Fig 1. Probit lines of repellency responses in observation periods between 10 min to 3 h.
Based on the probit lines (Fig. 1), repellency responses were already observed during the first 5 min with increasing probit values (3.62 to 4.52) directly proportional to the log ME concentration. The responses increased after 10 min up to 3 hours, showing more than 50% repellency response after 10 min.
Table 1. Summary of probit analyses statistics.
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Fig. 2. Probit lines of mortality responses at 24- and 48-h observation periods.
The effect of repellency action of ME produced significant toxicity to BPH, resulting in significant mortality after 24 hours of exposure (Fig. 2). Table 1 shows the LC50 values of 1.39% and 1.25% after 24 and 48 h, respectively. The difference between LC50s of after 24 and 48 hours was not significant.
Based on the results, ME induced significant repellency (which may prevent immigrant BPH from settling down and feeding on rice plants – hence, it also may help to prevent plant virus transmission and/or spread), and mortality against brown planthoppers. The study points to the possibility of using plant species with high ME e.g. Ocimum sanctum, that are commonly used for culinary and medicinal purposes, has numerous oil glands without openings or pores per leaf (Tan unpublished observations), may be grown in rice fields by increasing biodiversity in the monoculture rice ecosystem as well as enhancing ecological engineering. This natural insect-plant interaction plus the fact that most Ocimum plants are pollinated by small hymenopterans especially stingless bees (Trigona spp.), some herbaceous species, like Ocimum sanctum has three (ME-chemotype) out of seven varieties that have high ME contents (Nurdijati et al. 1996), may also be recommended to promote an environment-friendly pest management technique. This is vital and essential in drastically reducing or avoiding the use and/or misuse of insecticides in the rice ecosystem, thereby, enhancing the sustainability of an area wide integrated pest management program against BPH.
Dudareva, N. (2005) offers an answer to “Why do flowers have scents?”. Sci. Am. 293: pp. 80.
Howlett FM. (1912) VII. The effect of oil of Citronella on two species of Dacus. Trans. Entomol. Soc. Lond. 60:412-418.
Howlett FM. (1915) Chemical reactions of fruit flies. Bull. Entomol. Res. 6:297-305.
Nurdijati, S., Tan, K.H., and Toong, Y.C. (1996) Basil plants (Ocimum spp.) and their prospects in the management of fruit flies. pp.47-51. In: Chua T.H. and Khoo, S.G. (eds.) Problems and management of tropical fruit flies. Kai Wah Press, Kepong, Malaysia.
Tan, K.H. (1993) Ecohormones for the management of fruit fly pests – understanding plant – fruit fly – predator interrelationships. p. 495-503. In: Proceedings of an International Symposium on Management of insect pests: nuclear and related molecular and genetic techniques. IAEA, Vienna (Austria) – ISBN 92-0-000293-5.
Tan, K.H. (2009) Fruit fly pests as pollinators of wild orchids. Orchid Digest 73(3): 180- 187.
Tan KH, Nishida R. 2012. Methyl eugenol – its occurrence, distribution, and role in nature, especially in relation to insect behavior and pollination. J. Insect Sci. 12(56: )1-74. Available online: http://www.insectscience.org/12.56/i1536-2442-12-56.pdf
Tan KH, Nishida R, Toong YC. 2002. Floral synomone of a wild orchid, Bulbophyllum cheiri, lures Bactrocera fruit flies for pollination. J. Chem. Ecol. 28:1161-1172.
Tan KH, Tan LT, Nishida R. 2006. Floral phenylpropanoid cocktail and architecture of Bulbophyllum vinaceum orchid in attracting fruit flies for pollination. J. Chem. Ecol. 32:2429-2441.