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Essential oil was isolated from leaves of Ruta graveolens (Arudha in Sinhala) by
hydro-distillation to evaluate its repellent and toxic activities against the rice weevil,
Sitophilus oryzae. Essential oil at concentrations of 50, 100, 150 and 200 µl/ 10 ml acetone
were tested against one week old adults using un-infested white raw rice as the food medium.
Repellency rate of insects increased with the increase in dose and 100% adult repellent
activity was observed at the highest dose (200 µl) after 30 minutes. Relatively high level of
repellency was also recorded at the lowest dose (50 µl) amounting to 89%. In contact and
fumigation toxicity bioassays, highest doses of the oil tested (150 and 200 µl) were extremely
effective in inducing 100% mortality of weevils within an hour and over 90% within half an
hour compared to that of the control which gave no mortality. The efficacy in respect to
contact and fumigation toxicity after one hour was found to be 93% and 91% respectively at
the lowest dose. Present study reveals that essential oil present in leaves of R. graveolens has
very high potential as a repellent, contact toxicant and a potent fumigant in controlling S.
oryzae infestations in pest management programmes.
