As pest populations are expected to increase soon, the University of California Cooperative Extension is monitoring the presence of rice-eating insects called armyworms throughout the north state. 

Luis Espino, a rice farming systems advisor for UCCE, said the most recent numbers were low but are expected to climb in the next few weeks.

Larval armyworms grow from caterpillars into moths. The insects are considered agricultural pests because they can defoliate rice fields, and the bigger they get the more they eat. Espino said in severe cases, armyworms could eat all of a field’s foliage down to water level. While the rice crop can recover, the pest’s presence can lead to yield reduction.

“The defoliation happens really quickly,” Espino said. “I’ve heard numerous stories where growers were used to some defoliation and would go on vacation, and after the weekend they’d come back and see full defoliation.” 

The region had an outbreak of armyworms in 2015. To help control pest populations, the industry turned to an insecticide called Intrepid that has worked well since, Espino said. 

The biggest problem areas in the north state where yield reduction has occurred have been in Butte, Glenn and Sutter counties. Espino and his team set traps at 15 locations across the valley to monitor pest populations weekly. The pheromone traps are small buckets that attract male moths. They take the information collected from the traps and notify farmers on whether or not they should begin examining their own fields for the presence of armyworms. 

“We usually start to see numbers increase around mid-June and peak in late June or early July,” Espino said. “Right now, numbers are very low but they will come up. Over the years, we’ve learned that once we see the moth population peak, we see a peak of the worm population a week later. Once we start seeing those go up, we begin letting growers know to check their fields.”

Both Yuba and Sutter counties have one monitoring site each. Espino’s team will visit each site once a week and count the number of moths in each trap. They divide that number with the number of days since the last check to determine the number of true armyworm moths per night.

On May 25, Sutter County had 1.1 armyworm moths per night, while Yuba County had 0.1 moths per night. Espino said a high number would be over 30, and the biggest peaks he’s seen are as high as 80-90 moths per night. 

“We will continue to monitor and update growers. We also have some fields where we take samples to assess how larva are growing, and we will have some trials for insecticide products to see if we can get more options for control,” Espino said. “When talking about pest management, we want people to make sure you don’t put a pesticide down if you don’t have to, and our monitoring helps with that.”

To view the latest numbers, visit

Recommended for you