Gone viral

Targeted, environmentally friendly approach enlists beneficial organism to control corn earworm in soybeans.

• By Vicky Boyd,
Editor •

small corn earworm
Treat corn earworms when they are 1/2 inch or less.

Monty Bohanan, who farms with his brother, Derek, near Stuttgart, Arkansas, likes to joke that the biological worm control they tried on soybeans in 2018 was from a science fiction movie.

The biopesticide — which contains Helicoverpa armigera nuclear polyhedrosis virus — is applied to fields and then consumed by corn earworms. The beneficial virus begins to replicate in the caterpillar’s gut, causing it to stop feeding. Eventually, internal cells burst, and the dying larvae exudes a virus-laden liquid, which then infects other larvae.

HearNPV is a baculovirus that specifically targets Heliothis and Helicoverpa species. It includes corn earworm, also called cotton bollworm, soybean podworm, sorghum headworm and tomato fruitworm. But the virus is harmless to other insects, animals and humans.

The combination of a positive environmental profile with cost-effective worm control has sold Bohanan on using HearNPV
again, should the need arise. The University of Arkansas estimates the product costs about $3-$6 per acre, not counting application, compared to $10-$15 per acre for diamides.

“Going forward, if it’s available and we have issues with worms again, I’m definitely going to check into it,” Bohanan says. “I don’t see any downfall to using it. The whole industry, if it does come to be a popular product, should really use it because that’s what everyone else who are not farmers are looking for — doing things that don’t hurt the environment and going after one particular pest and only it. This should give us a good mark or two.”

Teaching an old dog new tricks

HearNPV is not new, with the natural virus having been commercially available for several years and marketed as Heligen, Helicovex and Gemstar, among other brands.

Gus Lorenz, University of Arkansas entomology professor, began working with Heligen from Australia-based AgBiTech about eight years ago to learn how it might fit the state’s soybean producers.

scouting for small corn earworms
After making 25 sweeps in a field, separate corn earworms from other worm pests. The University of Arkansas and Mississippi State University recommend treating with the HearNPV virus when you have two to five small corn earworm larvae per 25 sweeps — photo by Gus Lorenz, University of Arkansas

The 2018 season marked the first full year of launching Heligen commercially, says Brian Dockery, AgBiTech U.S. commercial director based in Fort Worth, Texas.

Because the product is a virus, it involves a paradigm shift from traditional chemical insecticides, he says.

“You’re getting away from a reactive to a preemptive or proactive approach,” Dockery says. “We want to get ahead of it before the worms are larger and you have a bigger infestation.”

As such, growers and consultants may have to relearn treatment thresholds and pre- and post-treatment scouting. But when used under the proper field conditions, HearNPV can provide excellent corn earworm control, Lorenz said in an email.
Ashley Peters, owner of Peters Crop Consulting in northeast Louisiana, has conducted several field trials with the virus in the past few years with similar results.

“The years we put it out, we got comparable results to other products, such as the diamides, and it’s a lot more economical,” he says. “It acts a little bit differently. When you put it out, you have to give it some time and know how to evaluate the fields afterward.”

Tennessee soybean producers don’t have the same heavy corn earworm pressure that Arkansas and Mississippi growers do.

Nevertheless, those with late-planted beans or along the Mississippi River do see above-threshold populations at times, says Scott Stewart, an entomology professor with the University of Tennessee’s West Tennessee Research and Education Center in Jackson.

“This virus has been very effective, but it’s critical to get the timing right,” says Stewart, who also has conducted HearNPV field trials. “You don’t want to use the product when you have a blow-out or big worms.”

Pull trigger when larvae are small

HearNPV is most effective on small corn earworm larvae that are 0.5 inch or smaller. It is much less effective on larger earworm larvae, when there’s a heavy earworm infestation or on other worm species.

As a result, growers and consultants should begin sweeping fields earlier than they’re used to and use lower treatment thresholds.

After making 25 sweeps in a field, separate corn earworms from other worm pests. The University of Arkansas and Mississippi State University recommend treating with the HearNPV virus when you have two to five small corn earworm larvae per 25 sweeps. Do not use the virus if more than five large larvae are present in the 25 sweeps.

Peters says he typically recommends “pulling the trigger” when corn earworms reach about half the university threshold of nine per 25 sweeps and most of the worms are small.

Be patient

The virus infects caterpillars through their midgut, so larvae must first consume it for it to start replicating internally.

 

dead corn earworm
Infected worms move up the canopy, take on a greasy appearance, then melt— photo by Gus Lorenz, University of Arkansas

Although the larvae remain alive for a few days, they stop feeding shortly after ingesting the product.

Rather than counting the number of live larvae after treatment, Lorenz recommends scouting for active feeding.

“Infected worms move up in the canopy — they get a greasy look and are very lethargic,” he says. “Then they melt. At that point, they’re a virus factory.”

Nevertheless, he says, seeing live larvae after an application can be a bit disconcerting for some growers and consultants.

“You have to be patient … a new concept for some folks,” Lorenz says.

Peters agrees. “It takes some getting used to. You almost have to put it out and walk away for a few days. It may be three or four days and you have the same number of worms. But by five or six days, you’re finding diseased worms. You see them in the top of the canopy and they’re maybe black and hanging there.”

Although the residual control from the spray itself is short, Heligen can provide season-long control due to the replication of the virus in the field from diseased larvae.

“As long as CEW larvae are in the field and becoming infected, you’re making more virus,” Lorenz says. “The virus level in the field goes up dramatically with horizontal transmission of the virus through a population. We saw a field treated last year where virus-dead worms were found 30-45 days after application. Now, I think that represents special circumstances like the rolling population I described.”

He also found in field trials that wind, rain and other insects can spread the replicated virus large distances from Heligen-treated areas.

Keep things cool

Heligen is tankmix compatible with most crop protection materials, Dockery says. The main consideration is the pH of the mixture, which should be 8 or below before adding the virus.

As a living organism, HearNPV needs to be stored under proper conditions to remain viable. The closed, original container should be kept out of direct sunlight and stored at 40 degrees Fahrenheit or below, according to the label. It can be kept for 24 hours or less at 80 F or lower.

What that means, Lorenz says, is to keep it in the shade and not let it remain in a hot pick-up truck. Only mix up enough to apply during the day so you don’t have a partial tank sitting in hot overnight temperatures, he adds.

Heligen remains stable for at least 2.5 years when stored according to label instructions.

To download a fact sheet Lorenz co-authored, titled “What You Need to Know… To Successfully Use Viruses for Control of Corn Earworm in Soybean and Sorghum,” visit https://www.uaex.edu/publications/pdf/AG1306.pdf

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