GROWTH AND YIELD RESPONSE OF RICE TO RICE WATER WEEVIL INJURY

Analysis of larval and pupal age-structured data indicates that the so il core sampler-soil separation method underestimates the abundance of early instar rice water weevil, Lissorhoptrus oryzophilus Kuschel. Th e estimates for bias were used to derive seasonal larval age-structure d population estimates for 4 yr of field data from 2 experiments repre senting untreated and carbofuran treated plots. These estimates were u sed as input to a physiologically based rice population model to simul ate the response of rice to rice water weevil injury. An iterative hal ving-grid size optimization approach enabled least squares estimation of the larval feeding rate as a function of larval mass, the effect of larval crowding on root mass consumption by larvae, and the effect of larval feeding on root nitrogen uptake efficiency. A statistically ba sed parameterization-verification-validation procedure was used to qua ntify the robustness of the model at simulating rice straw mass, grain yield, and total above ground mass. The model accurately simulated ea ch of these variables for the range of rice water weevil seasonal age- structured population patterns, and explained 95% of the yield variabi lity in the observed data. In contrast, a multiple linear regression o f grain yield as a function of cumulative larval-density degree-days a nd the timing of peak density explained only 58% of the variability. A n analysis of the benefits and costs accrued from the carbofuran treat ments suggests that the current rice water weevil action threshold may be too high when infestations begin during early stages of crop growt h. Results from sensitivity analyses indicate that the stage of crop g rowth during which rice water weevil injury occurs greatly affects cro p tolerance for root injury, suggesting that optimal timing of rice wa ter weevil controls should take into account both larval density and s tage of crop growth.