The true armyworm is a common pest of flooded rice in California. In 2015, an armyworm outbreak caused yield losses in several areas of the Sacramento Valley. Since then, high armyworm densities have caused problems some years. Armyworm economic thresholds were developed in the late 1970s using rice varieties no longer in use. To improve management of armyworms in rice, trials were conducted in 2019 and 2020 to determine the effect of defoliation during the tillering stage on rice development and yield. In both years, rice plots (10x10 ft) were defoliated 0, 25, 50, or 100% of their height to the water level 45 days after seeding. Rice was defoliated in increments of 25 or 50% to mimic armyworm feeding. In 2020, a nitrogen application was added after defoliation to determine if yield losses could be reduced. Additionally, three varieties were used in 2020 to determine their response to defoliation. Plant heights were measured weekly, percentage panicle emergence estimated, and grain harvested. Results in 2019 showed that when plants were defoliated to the water level, plants were significantly shorter, panicle emergence delayed, and yields reduced by 25%.
