Tobacco leaf extracts against rice weevil Sitophilus oryzae L.
Tobacco leaf extracts against rice weevil Sitophilus oryzae L.
Rice weevil, Sitophilus oryzae is one of the serious storage pests in rice drastically affect the food availability for a large number of people. The toxicity of tobacco (Nicotiana tabacum) leaf extracts as an alternative to synthetic insecticides in the management of rice weevils. The crude solvent leaf extracts of N. tabacum were analyzed for its phytochemical compounds via GC-MS. The chloroform and acetone extracts of N. tabacum leaves exhibited the highest adulticidal activity against S. oryzae. Their LD50 values were 1.62, 0.64 and 0.48; 1.54, 0.83 and 0.48 mg/L after 24, 48 and 72 hr of extracts exposure respectively to the rice weevil. The use of tobacco leaf extracts with its excellent insecticidal activity owing to the action of nicotine would be economically feasible to alleviate the rice weevil problem.
Methods
Mature and healthy leaves of N. tabacum were washed in dechlorinated water, shade dried, and powdered with the aid of an electric blender. The powdered leaves (1 kg) were extracted with different solvents (3 L) each viz., hexane, petroleum ether, dichloromethane, chloroform, ethyl acetate, acetone, methanol and distilled water in a Soxhlet apparatus. The crude solvent leaf extracts thus obtained were then stored in air tight amber-colored bottles at 4 °C. The extracting yields was estimated and subjected to the phytochemical evaluation and bioassay
Results
The yields of different solvent extracts in the order of polarity reported elsewhere are 3.66, 3.85, 9.88, 6.33, 2.08, 8.48, 12.40 and 11.36 % w/w. The phytochemical components of N. tabacum leaves extracted by different solvents were analyzed and identified as multiple chemical components. The extracts of tobacco leaves tested positive for alkaloids, cardiac glycosides, flavonoids, phenolic compounds, quinones, saponins, steroids, tannins, and terpenoids. The constituent compounds in the chloroform and acetone revealed 13 compounds represented by benzene carboxylic acid ester, alkaloids, and steroids. The major compounds found in GC-MS analysis are nicotine, nicotinonitrile, nornicotine, nicotinic acid, neonicotine, cotinine, indole, farnesol, sclareol, 9,12-octadecadienoic acid, squalene, palmitic acid and 15-tetracosenoic acid methyl ester. The crude solvent leaf extracts of N. tabacumshowed adulticidal activity against S. oryzae. No adult mortality was observed in treated and untreated control. After 24 h of exposure, 100% adult mortality was observed in chloroform extract at 5.00 mg/L. The highest percent adult mortality at the lowest concentration was revealed by the crude chloroform extract with LD50 and LD90 values of 1.62 and 2.85 mg/L, respectively. The same trend followed after 48 and 72 h of exposure, except for 100% adult mortality at 2.5 and 1.25 mg/L in the chloroform extract, respectively and its LD50 and LD90 values were 0.64 and 0.99; 0.48 and 0.80 mg/L, respectively. Overall assessment indicated the chloroform and acetone extracts at 24, 48, and 72 h exhibited the highest adulticidal activity against S. oryzae.
This study revealed that the chloroform and acetonic extracts of tobacco leaves exhibited more pronounced activity than the others. An alkaloid nicotine present in tobacco leaf which was responsible for the weevil mortality. Nicotine kills the insects rapidly within an hour causing intensive tremors, convulsions, and then paralysis. This alkaloid acts by mimicking acetylcholine binding to acetylcholine receptors in the nervous system causing synaptic blocking and continuous firing of affected nerves which presents such symptoms as ganglionic stimulation which types into central nervous failure and death. Interestingly, the insecticidal activity of alkaloids acts acutely by causing knockdown in beetles and weevils which drew the attention of researchers for its application in the management of coleopteran pests.
Conclusion
The use of tobacco leaf extracts with its excellent insecticidal activity would be economically feasible to alleviate the rice weevil problem. This is due to the fact that these products are highly promising, relatively available, cheap, and easy to prepare and apply on rice grains when compared to synthetic insecticides. Though the present contact toxicity bioassay exposed that the tobacco leaf extracts were moderate to strong toxicants to the adult weevils, it is to be noted that they were depended on the duration of exposure and concentration and route of application. A constituent’s level study along with structure activity relationships of natural products is to be required for wider insecticidal performance and efficacy not only against S. oryzae but also against all other stored grain insect pests.
Reference:
Kanmani, S., Kumar, L., Raveen, R., Tennyson, S., Arivoli, S. and Jayakumar, M., 2021. Toxicity of tobacco Nicotiana tabacum Linnaeus (Solanaceae) leaf extracts to the rice weevil Sitophilus oryzae Linnaeus 1763 (Coleoptera: Curculionidae). The Journal of Basic and Applied Zoology, 82(1), pp.1-12.
