Mechanisms of insecticidal action of chlorantraniliprole and thiamethoxam as seed treatments against the rice water weevil, Lissorhoptrus oryzophilus Kuschel

Activities of chlorantraniliprole and thiamethoxam were tested as seed treatments in rice against Rice Water Weevil, for 2 years under greenhouse conditions. Infestations with weevils at 6-7 leaf stage for all rates revealed significantly lower egg densities and mean larval numbers than no seed treatments. The leaf material from thiamethoxam seed treatments in insect feeding bioassays revealed high mortality in adult weevils whereas, no lethality was recorded in feeding assays on leaves from chlorantraniliprole treatments. These observations instigated studies on sublethal effects of seed treatments on rice water weevils. Conditioning of weevil pairs on leaf material from different treatments and no seed treatments in petridishes followed by infestations on plants revealed carry-over effects of adult feeding manifested on eggs and larvae. Lower levels of first instar larval emergence from treated plants infested with weevils formerly fed upon control leaf material revealed possible existence of ovo-larvicidal activity of chlorantraniliprole as systemic insecticide. Results indicated threshold activity levels of insecticides operating at different rates.