Here to stay?

California rice industry prepares for what may become annual armyworm infestations.

By Vicky Boyd
Editor

armyworms in California rice

Armyworms in the fourth and fifth instar are voracious feeders and can cause significant defoliation — photos by Dr. Luis Espino, UCCE Colusa County

When Luis Espino, a University of California Cooperative Extension farm adviser, started seeing significant true armyworm populations in numerous rice fields in 2015, he thought it was likely due to the state’s lingering drought. But when sizable numbers returned in 2016 and much worse again in 2017 — a year of record rainfall — and prompted growers to treat about 100,000 acres, Espino says he had to rethink his theory.

“After the 2015 outbreak, I thought we probably wouldn’t see high population levels again for another 30 years because this is a once-in-a-lifetime type of event,” says Espino, also Colusa County UC Cooperative Extension director. “They were here again in 2016, and 2017 was worse. So I’m just going to assume they will be back in 2018.”

As it has since 2015, the California Rice Commission this season planned to seek a Section 18 emergency use registration for Intrepid insecticide from DowDuPont.

Espino also plans to continue an armyworm moth trapping network he started last year as well as fine-tune an accompanying degree-day model designed to help predict when larvae may start appearing in rice fields.

Caught by surprise

Historically, armyworms have caused small, spotty problems, with growers treating an average of about 5 percent of Sacramento Valley rice acreage annually, Espino says. Depending on the field location, true armyworm tends to be the predominant species, with the western yellow-striped armyworm less so.

In 2015, armyworms showed up in force in late June, a week or two earlier than when growers and pest control advisers were used to seeing them.

“It’s not a big difference, but I think that contributed to the fact that people weren’t expecting them and it took everybody by surprise,” he says.

large armyworms in california rice

Pyrethroids are ineffective against larger armyworms, such as these in the fifth instar growth stage found in a Glenn County rice field.

By the time the defoliation was visible, the worms were in the fourth or fifth instar. Lambda-cyhalothrin, a go-to insecticide for armyworms, will control larvae in the first or second instar but not large, older worms.

Although Dimilin, an insect growth regulator from Chemtura Corp., has a 2ee label recommendation for armyworms in rice, it also has a 80-day pre-harvest interval, making it impractical for all but very early applications or late-maturing rice.

A DowDuPont representative suggested the industry look at Intrepid 2F, says Roberta Firoved, California Rice Commission industry affairs manager. After an armyworm ingests Intrepid, it causes them to molt prematurely, which proves fatal. It is effective on all armyworm larval stages.

In 2015, the commission put together a request for a crisis Section 18 for Intrepid 2F.

All Section 18 applications, which must be requested by an entity other than the registrant, are submitted to the California Department of Pesticide Regulation for initial review. If state officials approve it, they forward it to the Environmental Protection Agency. The process takes at least 90 days.

The EPA approved a Section 18 for Intrepid in time for the second flush of armyworms that occurred in August 2015, Firoved says. After the major outbreak, the Rice Commission worked proactively and successfully received Section 18s just in time for early July infestations in 2016 and 2017, she says.

The commission worked this winter with Espino to collect required data and plans to submit a Section 18 request to CDPR in early March. If all goes well, Firoved says the industry could have the emergency exemption by early June – just before the first worms are expected.

“You have to justify the emergency and provide data including that showing significant economic losses and the lack of effectiveness of the alternatives,” she says.

During 2017, rice growers used Intrepid on about 41,400 acres, lambda-cyhalothrin on about 44,500 acres and Dimilin on about 10,560 acres, according to figures from Espino. The total acreage treated for armyworms comprised slightly less than one-fourth of the state’s 453,000 planted acres.

One of the conditions of a Section 18 is the registrant must pursue a full Section 3 registration, which DowDuPont is, Firoved says. The company is currently collecting residue data from additional tests conducted at the Rice Experiment Station to establish pesticide tolerances on rice.

Scouting challenges

armyworm feeding can cause blanking

Armyworm feeding later in the
season can cause blanking.

Although some farmers still were plagued by armyworms in 2016, the season wasn’t nearly as bad as 2015, Espino says.

The pest returned in strong numbers in 2017. One of the challenges with scouting for armyworm is you have to wade out in flooded rice fields, bend over and closely examine leaves for signs of feeding. Even then, early instar larvae may be difficult to see and the damage is minimal because they don’t eat much. But the larvae grow and molt rapidly and in a matter of days can become large worms capable of defoliating rice plants.

“It just happens really quickly,” Espino says. “That’s what seems to get people — they aren’t ready. They don’t see anything during the week. When they come back Monday, half of the foliage is gone and they are in the fourth or fifth instar, so pyrethroids don’t really work at that stage.”

Josh Sheppard, who farms in Butte County, jokes that his family’s farm was “ground zero” for the 2015 armyworm outbreak.

He saw fewer problems in 2016, and armyworms once again were troublesome in 2017 but not to the extent they were in 2015.

“As much as we were looking for them, we wouldn’t actually realize they were there until the plants started to disappear,” Sheppard says about the challenges of scouting for small larvae.

He used Intrepid successfully on about 30 percent of the family’s rice acreage in 2017 and says he was “thankful that the product was available.”

Early warning system

armyworm pheromone traps

Luis Espino, a University of California Cooperative Extension farm adviser, hopes to expand the number of armyworm pheromone traps he had to about 20 this season from seven in 2017.

During 2017, Espino established a network of seven pheromone traps from south Sutter County to Butte County to monitor male armyworm moth activity.

Although trap catches won’t provide an indication of egg laying, they will alert the industry to increases in the overall moth population. At the same time, Espino was monitoring degree-day models to see how they correlated to trap catches and larval development.

“The degree-day model said in 10 days we’d start seeing fourth-instar larvae,” he says. “We forecast it to the day. A day before, I started getting pictures and calls from PCAs saying they were starting to see large armyworms in the field, so I think it’s pretty good in forecasting. I want to see again this year what type of numbers we will get.”

Espino also hopes to add traps to the system this year to take into account possible microclimates and provide a more complete picture of what’s happening in the Sacramento Valley.

What he envisions is a type of early warning system. Once he starts seeing an increase in moth catches, Espino will put out the word electronically to growers and PCAs to start checking their fields more carefully for small armyworms.