Delta Armyworm Monitoring
Armyworms have long been a pest of rice, but 2015 was an outbreak year, and high populations have continued in subsequent years. We don’t really know why this is happening, but it could relate to higher minimum temperatures during the winter, and thus better winter survival, as well as migratory patterns from other western states and Canada. UC Cooperative Extension responded to the problem by providing outreach on UC IPM guidelines, including monitoring and treatment thresholds. We also monitor populations using pheromone bucket traps deployed across rice-growing regions. In the Delta, I have been monitoring populations since 2016.
Because small armyworms are hard to scout and large armyworms are hard to treat, we use trap counts and Growing Degree Day modelling (i.e. a temperature measure of time) to determine when the worms are “just right” to treat, knowing that armyworm larvae can grow to full size in three to four weeks. During the season, UCCE farm advisor, Luis Espino, writes a weekly blog to provide real-time information on trap counts to help growers and consultants with scouting and decision-making. Please consider subscribing to the blog.
Our observations over the years lead us to believe that it is the true armyworm (Fig. 1), rather than the western yellowstriped armyworm (Fig. 2), that is most damaging to rice. So, since 2021, we have only monitored true armyworms. In the Delta, I have observed true armyworms in wetland areas and in summer cover crop mixes that included small grains. Therefore, when scouting populations in rice, I suggest also checking nearby fields growing other grasses that might be harboring the pest. Also noteworthy, we observe a lot of variability in populations from year to year. For example, in 2023, the peak population occurred two weeks later than what has been typical, which was likely due to the wet winter, cool spring, and later planting. In 2024, the peak population occurred around July 1st, indicated by the large arrow. I observed that feeding damage was highly variable across the three Delta farms where traps were located but also across fields on the same farm. We have not observed high populations later in the season after heading, even though a second peak has sometimes been observed in the Sacramento Valley.
