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

Abstract

Powdered leaves of thirty seven plant species were screened under laboratory conditions withthe temperature of 2920C and 842% relative humidity for their repellent properties against riceweevil, Sitophilus oryzae (L). The screening was carried out with the view of producing moreeco-friendly agents suitable for post-harvest rice protection and as sustainable alternatives tosynthetic pesticides in the control of S. oryzae.

Reared weevils were exposed to 15.0g of leaf powders admixed with rice grains in laboratoryscreening test using a modified cup-bioassay. The results revealed that twenty treatments causedsignificant repellency of adult S. oryzae than the control. Out of these, Ocimum gratissimum(Lamiaceae), Cinnamomum verum (Lauraceae), Mentha viridis (Lamiaceae), Plectranthusamboinicus (Lamiaceae) and Citrus reticulate (Rutaceae) showed the highest repellent activity.M. viridis elicited the highest and the strongest repellency (89.0 %) in the weevils whencompared with the other three treatments. However, the weevils demonstrated considerably highrates of repellency to C. verum, O.gratissimum, C. reticulate and, P. amboinicus which were76.0%, 74.0%, 68.0% and 64% respectively.

When weevils were further tested against 5.0, 10.0 and 15.0g of leaf powders, all treatmentsshowed significantly (p<0.05) higher repellency of S. oryzae than the control. M. viridis at15.0g dose elicited the highest and the strongest repellency (89%) in the weevils when comparedwith the other treatments and concentrations. The overall results indicated that repellencyrate increased proportionately with the increase of the dose of the powder. A similar trend ofrepellency was also observed with time. However, the effects of all plant powders on the insectswere highest within 6 hours post treatment.

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

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