Nitrogen fertilization at the rice panicle differentiation stage to compensate for rice water weevil (Coleoptera: Curculionidae) injury

A three-year study was conducted to determine the effect of nitrogen fertilizer rates applied at the panicle initiation stage of rice, Oryza sativa L., and insecticide applications on yield loss caused by rice water weevil, Lissorhoptrus oryzophilus Kushel, damage. Our estimates of yield loss caused by L. oryzophilus injury were, at the recommended nitrogen rate of 191 kg N per ha, 1.7% in 1997, 11.8% in 1998, and 9.9% in 1999. In 1998, doubling the panicle differentiation nitrogen rate from 67 to 134 kg N per ha significantly increased yield by 591 kg per ha, averaged across insecticide treated and untreated plots. In both 1998 and 1999, reducing nitrogen rate below the recommended rate significantly (P<0.05) reduced rice yield, indicating the value of providing the rice plant with sufficient levels of nitrogen. In 1998 and 1999, insecticide treatment increased yield by 570 and 830 kg per ha, respectively. The interaction between insecticide treatment and fertilizer rate was not significant (P>0.05) for yield in any year of our study, suggesting that increasing nitrogen at the panicle differentiation stage does not have a major impact on compensation of the rice plant for L. oryzophilus injury. Our results indicate that farmers should not increase nitrogen fertilizer at the panicle differentiation stage to compensate for L. oryzophilus damage.