Laboratory evaluation of some Sri Lankan plants as post-harvest grain protectants for the control of rice weevil Sitophilus oryzae

 Powdered leaves of thirty seven plant species were screened under laboratory conditions with the temperature of 29±2°C and 84±2% relative humidity for their repellent properties against rice weevil, Sitophilus oryzae (L). The screening was carried out with the view of producing more eco-friendly agents suitable for post-harvest rice protection and as sustainable alternatives to synthetic pesticides in the control of S. oryzae.

Reared weevils were exposed to 15.0g of leaf powders admixed with rice grains in laboratory screening test using a modified cup-bioassay. The results revealed that twenty treatments caused significant repellency of adult S. oryzae than the control. Out of these,  Ocimum gratissimum (Lamiaceae), Cinnamomum verum (Lauraceae), Mentha viridis (Lamiaceae), Plectranthus amboinicus (Lamiaceae) and Citrus reticulate (Rutaceae) showed the highest repellent activity. M. viridis elicited the highest and the strongest repellency (89.0 %) in the weevils when compared with the other three treatments. However, the weevils demonstrated considerably high rates of repellency to C. verum, O.gratissimum, C. reticulate and, P. amboinicus which were 76.0%, 74.0%, 68.0% and 64% respectively.

When weevils were further tested against 5.0, 10.0 and 15.0g of leaf powders, all treatments showed significantly (p<0.05) higher repellency of S. oryzae than the control. M. viridis at 15.0g dose elicited the highest and the strongest repellency (89%) in the weevils when compared with the other treatments and concentrations. The overall results indicated that repellency rate increased proportionately with the increase of the dose of the powder. A similar trend of repellency was also observed with time. However, the effects of all plant powders on the insects were highest within 6 hours post treatment.

The overall results of the research provide scientific corroboration for the use of the four plant powders as effective grain protectants against the rice weevil.

Key words : Sitophilus oryzae; Rice weevil; repellency; plant powders

Vidyodaya Journal of Science Vol.14(2) 69-83