California rice producers may be resting a little easier this year following last year’s epic armyworm infestation.

“Last year was the worst I’ve seen it for armyworm in 25 years,” said University of California Cooperative Extension Entomologist Larry Godfrey.

Godfrey discussed armyworm at the annual Rice Day in Biggs.

“There were some rice fields that were completely decimated. Plants were eaten down to the waterline,” Godfrey continued.

Though armyworm numbers this year fell to more manageable levels, UCCE Farm Advisor Luis Espino says growers still need to monitor and manage the pest.

Espino does not know exactly how or why armyworm numbers exploded last year – perhaps it was tied to the drought and warm winter. Nevertheless it did cause problems in a year when planted rice acreage was significantly lower across northern California because of water availability and drought.

According to the UC Integrated Pest Management Program website, there are two species of armyworms. Espino refers to the one simply named “armyworm,” Mythimna (= Pseudaletia) unipuncta, which gave growers problems in 2015.

The second is the western yellowstriped armyworm, Spodoptera praefica.

The first armyworm is more prevalent in northern California rice systems, Espino says. The western yellowstriped armyworm can be more easily controlled.

In typical seasons, growers may see two peaks of armyworm activity, Espino says. The first tends to be in mid-to-late June and the second in mid-August. The first period tends to result in defoliation while the second period tends to injure the panicle  causing blank grains.

Espino says rice can tolerate some defoliation and still not affect yield. The recommended threshold for defoliation is 25 percent. Beyond that Espino says yield loss can occur.

Under typical conditions, this threshold can result in 10 percent of California rice treated for armyworm.

Evidence of the armyworm can be easier to detect than the pest itself, Espino says. The pest will defoliate rice and can feed on developing rice kernels. Both can lead to reduced yield.

Armyworms tend to remain low in the rice canopy during the day, rising to the top of the canopy at night to feed.

Last year growers saw the first peak of armyworm in early June, a couple weeks ahead of normal.

“The numbers at that time were very high,” he said.

Growers turned to pyrethroids to treat armyworm populations, but those were simply not effective against the large populations of armyworm, Espino said.

“Pyrethroids are just not an excellent product for armyworms,” he said. “They can do an okay job, but not when you have the high populations like we had last year.”

Espino continued: “Armyworms tend to have a natural resistance because they feed on so many things and they have evolved mechanisms to detoxify.”

As a result, the rice industry turned to the California Rice Commission to seek a Section 18 exemption for Intrepid 2F, a Dow AgroSciences insect growth regulator, labeled for numerous crops, but not rice.

Roberta Firoved, industry affairs manager for the California Rice Commission (CRC), said they obtained the Section 18 in August 2015, after the second peak of armyworm in rice fields.

The current Section 18 expires this Sept. 30. She said the industry may resubmit an application for an extension. Along with its EPA registration, the product is currently labeled for other crops in California and Arizona.

Firoved says rice growers can help the commission, and themselves, to secure another Section 18 exemption while the CRC works with Dow to secure a permanent label for rice in the U.S. 

Rice growers should document yield losses and provide their findings to the CRC. The Environmental Protection Agency will consider the losses in making its decision on whether to grant an exemption.