Virus provides novel targeted insecticide option

NPV-infested caterpillar
Larvae infected by nuclear polyhedrosis virus, or NPV, eventually liquify — photo courtesy University of Arkansas

Caterpillars like to eat, and soybeans are often a favorite meal.

In Louisiana and other southern states, worms commonly cause economic damage to soybeans. Corn earworm larvae feed on every major row crop. Though they start in corn, the second generation each season migrates into soybeans and cotton. Soybean loopers migrate from south to north each year, moving into southern soybeans from Central America. Larvae from subsequent generations feed on soybean leaves. Conventional insecticides typically control these pests for less than 14 days, and some infestations require multiple treatments to protect yield.

“Farmers need new alternatives to control these pests,” said Sebe Brown, field crops Extension entomologist for the Louisiana State University AgCenter. “We are studying nuclear polyhedrosis virus, or NPV, as a novel control option.”

Brown’s research, funded by the Louisiana Soybean and Grain Research and Promotion Board, tracks the efficacy and spread of NPV in field-scale trials. He said a company that supplies virus insecticides has reported good success with NPV in Australia, Brazil and other countries. He and other Mid-South entomologists are working to understand how it might fit in the U.S.

Unique insecticide properties

“NPV is a different type of insecticide,” Brown said. “The virus is naturally occurring. It’s not synthetic, like most conventional insecticides, but it’s not a living thing, either. Although it can be considered organic and is approved for use in those systems, it’s technically a virus-based biological insecticide.”

He described viruses as pieces of DNA or RNA. When introduced to an environment, in this case being sprayed in a field, those nucleic acids have no impact on most of what they touch. But when ingested by the target species, they replicate within host cells. Small larvae typically die within four to six days of infection, eventually liquifying and releasing more virus into the environment.

“Each NPV strain is specific to one genera of insect, like earworms/bollworms or loopers,” he said. “That makes it a great integrated pest management, or IPM, tool. The virus is harmless to beneficial insects, people, crops and wildlife.”

Other advantages of NPV include cost and mode of action, Brown said. Its novel mode of action effectively prohibits the development of resistance in target pests. High humidity, rainfall, wind, the movement of beneficial insects, and many other factors spread the virus through a field, controlling target pests as it moves.

It also offers ease of foliar application. NPV products can be tank mixed with any other foliar inputs that need to be applied at a given point in time, and they are much more cost-effective than conventional insecticides.

“The main drawbacks for application is that the product requires cold storage – either refrigeration or freezing – for long-term storage and solution water pH should be 7.0,” Brown said. “Water pH above 8 may damage the virus and reduce efficacy.”

Preventative treatment

The field-scale trials have helped Brown understand the strengths and weaknesses of NPV as an insecticide option. Because each virus strain is specific for the type of lepidoptera species it targets, correct pest identification during scouting is critical.

NPV spread
Wind, humidity, rainfall, beneficial insects and many other factors help nuclear polyhedrosis virus, or NPV, spread throughout a treatment area — photo courtesy LSU AgCenter

“NPV can be very effective, under the correct environmental conditions and insect numbers,” he said. “Louisiana’s environment works well with the virus, as our high humidity helps the virus take off and move through the field. And unlike conventional insecticides, rainfall after application actually improves the effectiveness of NPV because it helps the virus spread.”

For example, he has seen the virus move across a 500-acre field within two weeks.

“We have found NPV to be more valuable as a preventative measure rather than a curative,” Brown said. “It takes time for the virus to infect and control the target species and keep moving through the field. And it may miss large larvae.”

When caterpillars start to appear, Brown and other entomologists working with NPV recommend virus treatment thresholds at about one-third or one-half of the economic threshold for the pest.

For example, the economic threshold for soybean loopers in soybeans is 37 worms 0.5 inch or longer in 25 sweeps. But the treatment threshold with NPV would be about 10 loopers per 25 sweeps. And the threshold for corn earworms is nine per 25 sweeps, but the NPV treatment threshold would be three to five small larvae per 25 sweeps.

“For farmers planning an R3 fungicide application and just starting to see these pests, adding the virus to their tank can prevent the need for future insecticide applications,” he said. “The NPV products are significantly cheaper than conventional products, so they can help save money and protect yield.”

Field trials have shown mixed results. Trials documented up to more than 90 percent control of soybean loopers seven days after treatment. But the corn earworm treatment demonstrated no measurable benefit when applied to corn to prevent future generations from migrating to soybeans. The corn earworm NPV did provide satisfactory control when applied in soybeans.

“I’ve seen good success controlling corn earworms in other fields, but I haven’t been able to replicate those results in my tests,” Brown said. “We continue to work on application methods and timing to understand how to get more consistent results.”

Based on this and ongoing work, he said he believes NPV has solid potential to provide another insect control tool for farmers. It won’t fit in every situation, but it can save input costs and protect soybean yield in some circumstances.

The Soybean Research and Information Network contributed this article.

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