Development of defoliation thresholds in rice.
Armyworms are commonly found in rice fields in the mid-southern U.S. and have the potential to cause severe defoliation to the rice crop. The two main armyworm species observed in rice in this region are true armyworms and fall armyworms. It is common to see infestations occur at all growth stages of rice. The current threshold for armyworms in rice is based on the number of larvae per square foot. A defoliation-based threshold would provide growers and consultants with a simple way to make economically sound decisions for controlling armyworms in rice. Studies were conducted in 2019 and 2020 where rice was mechanically defoliated at 0%, 33%, 66%, and 100% with a weed eater at 2-3 leaf, early tiller, late tiller, and green ring growth stages across three planting dates. Large amounts of yield loss were observed when plants were defoliated either 66% or 100% at the green ring growth stage. A delay in heading was also observed when plants were defoliated at 66% or 100% during any growth stage in 2019. Maturity delays were also observed in 2020 but were not as severe as what was observed in 2019. Yield losses were greatest in the May planting date; however, delays in heading were greater for the June planting date. This data has helped form a defoliation-based threshold in rice to help keep rice growers profitable.