Incorporating neonicotinoid and anthranilic diamide insecticides into integrated pest management of rice water weevil, Lissorhoptrus oryzophilus

Abstract

Seed treatments with chlorantraniliprole and thiamethoxam have been used for the management of rice water weevil, Lissorhoptrus oryzophilus in the United States. Both insecticides being plant-systemic could potentially target multiple life stages of L. oryzophilus. The rational use of these chemicals as seed treatments in weevil management requires thorough understanding on targeted life stages of L.oryzophilus and, the relative persistence and potency of both chemicals. A series of greenhouse and laboratory experiments were conducted on weevils by culturing rice plants treated as seeds with different rates of chlorantraniliprole and thiamethoxam. Adult feeding assays on foliage from treated plants revealed divergent effects: treatments with thiamethoxam but not chlorantraniliprole affected adult mortality and foliar consumption. Dose-mortality relationships for thiamethoxam were determined by combining the estimates of leaf biomass consumed by weevils in conjunction with foliar insecticide residues estimated by LC/MS/MS. The LD50 s obtained were the first LD50 s in leaf feeding insects feeding on foliage of thiamethoxam-treated plants. Changes in adulticidal activity with plant-growth were related to the residues analyzed by ELISA method. Adult exposure to treated rice (6-7 leaf stage) led to reduced egg numbers and first instar emergence. The low egg numbers by adults in chlorantraniliprole was confirmed as a sub-lethal effect: survival was not impacted on foliage but the number of eggs by adults was reduced when released on untreated plants. Furthermore, a comparison of first instar emergence from chlorantraniliprole-treated plants and from untreated plants infested with weevils previously exposed to this chemical suggested that chlorantraniliprole was also reducing egg or first instar survival. Sub-lethal effects of thiamethoxam on weevils that survived feeding exposure were manifested in reduced egg numbers. Finally, differential activities of both seed treatments on weevil life stages were related to residue patterns in plant portions. In chlorantraniliprole, the greatest reduction occurred in late instars feeding on roots, whereas in thiamethoxam, reduction was largely due to disrupted adult survival and egg-laying. High above ground concentrations of thiamethoxam and high below ground concentrations of chlorantraniliprole in rice plants were consistent with differential activities. The implications of such divergent distribution patterns on L.oryzophilus management were discussed.