Still a perplexing problem

California researchers continue work to unravel armyworm dynamics and trapping results.

• By Vicky Boyd,
Editor •

california armyworms

True armyworms can go from first- or second-instar to inch-long worms in seven to 10 days — photos by Dr. Luis Espino, UCCE

Dr. Luis Espino, a University of California Cooperative Extension rice systems adviser, had hoped pheromone traps could provide an early warning system for true armyworm infestations as they have worm pests in other crops.

But after two years of running a network of 15 traps throughout California’s rice belt, he says he hasn’t been able to correlate the number of moths caught to the potential severity of an infestation in nearby fields.

“Unfortunately, there’s no direct correlation that if there’s X number of moths in the traps, then you’re going to have problems,” says Espino, who is also the Butte County Cooperative Extension director. “We’re not finding that.

“But if you have high numbers of moths in the traps, it’s something you need to be aware of and something you should be out there monitoring more frequently, because it means the risk of having a problem is higher.”

Yet during the past two years, some traps picked up large numbers of moths with very few worms found in nearby fields.

“We don’t know what the differences are,” he told attendees of the recent Colusa County Farm Supply annual meeting in Maxwell, California.

Trapping network

Two armyworm species occur in California rice — Western yellowstriped armyworm and true armyworm — with the latter being more problematic.

During the 2018 and 2019 seasons, Espino had 15 pheromone traps scattered throughout the Northern Sacramento Valley rice belt. The attractant targeted male true armyworm moths.

Every week, student assistant Marcus Rehrman checked the traps and recorded the armyworm moths caught. During the 2019 season, Espino also tested a few automated traps that transmitted images of trapped moths. Unfortunately, he says, birds stole moths from the traps, so they didn’t provide accurate numbers. Nevertheless, Espino says he plans to continue testing them.

During both years, Espino charted the moths caught to determine when peak flights occurred. The moths typically have two peak flights per season, with the first one being larger and occurring in late June or early July. The second, smaller flight is in mid-August.

automated traps

During the 2019 season, Espino also tested a few automated traps that transmitted images of trapped moths.

One of the challenges is you don’t know when the peak occurs until about a week afterward when you see trap numbers begin to drop, he says. If farmers have large acreage — particularly if they routinely have armyworm problems — and want to put up their own traps, they could check them more frequently and would know sooner when the peak has occurred.

Although Espino says they have a fairly good understanding of the feeding done by the worms early in the season, they are still trying to figure out what happens around heading. When are the worms likely to bite panicle branches and when are they not interested in them anymore?

Another challenge with true armyworms is once you start seeing small first- or second-instar worms, they can reach inch-long larvae in seven to 10 days.

“They’re very quick to develop — that’s where the problem comes,” Espino says. “You don’t see defoliation and you come back seven to 10 days later and they’re all over the place chewing up the rice.”

When to treat

How much armyworm defoliation can occur before growers see yield reductions? Espino says artificial defoliation trials were conducted to answer that question. Students hand-defoliated rice plants with hedge shears 40 days after seeding. The treatments in the 10-by-10-foot plots were an untreated check and 25%, 50% and 100% defoliation of foliage above the water.

The plants in the completely defoliated plots never totally recovered and were stunted.

Maturity also was delayed, with the plants reaching 100% heading 12 days later than the untreated check. In addition, yield was reduced by 25% compared to the untreated plots.

Plants in plots receiving 50% defoliation were slightly shorter and were a couple days later in reaching 100% heading.

There were no differences in plant height or maturity between the untreated check and the 25% defoliation treatments. In addition, there was no significant different in yields among the 50%, 25% and check treatments.

armyworm pheromone traps

Dr. Luis Espino, a University of California Cooperative Extension farm adviser, ran a network of 15 armyworm traps during the 2019 season.

“These plots were planted in mid-June, so the yield potential wasn’t even 8,000 pounds per acre,” Espino says. “If this would have been planted at a more normal time, maybe we would see a higher yield potential and possibly some effect from the 50% and maybe the 25% (defoliation) treatments.”

He says growers still struggle with when to treat to see an economic return.

“It really depends on how much of the field is defoliated and the price of rice,” Espino says.

Armyworm infestations typically are localized in corners and along levees and field borders.

If only 5% of the field is defoliated, a grower might lose $20 per acre in reduced yields (dividing the yield loss in the 5% by all acres). Compare that to roughly $40 per acre for an Intrepid SF insecticide treatment together with aerial application.

As the defoliated area climbs to 10% or 15%, then growers may see a return on investment.

“How big an area that is severely defoliated justifies the treatment,” he says.