Editor's Note: A statement from Corteva regarding chlorpyrifos was added to this original story.
Pyrethroids have a place for corn earworm control in soybeans, but rotating chemistries and not relying on them alone is key.
That’s the message Tom Kuhar, professor of vegetable entomology at Virginia Tech, brought to the Virginia Soybean Field Day Sept. 23 at the Eastern Shore Research and Extension Center in Warsaw.
For a long time, pyrethroids were popular insecticides in soybeans because they were inexpensive and offered broad spectrum insect control, killing pests such as stink bugs and corn earworms. “You had a cheap, effective product and you got the job done with it. That went on for a long time,’ Kuhar said at the field day that was moved indoors due to rain.
But in 2008, things began to change. Kuhar noted that at that time corn earworm resistance to pyrethroids started popping up. The issue was particularly pronounced in the Deep South. Soon, farmers stopped using pyrethroids to control corn earworms in soybeans.
In recent years, there has been a turnaround. Today, pyrethroid resistance for corn earworm control is not as big a concern as it once was. In fact, Kuhar said pyrethroids do work to control corn earworms in Virginia.
“We expected that (resistance) to go up and up and up until pyrethroids didn’t work anymore. That’s what everyone predicted. Here we are now in 2021, it has not kept going up and up and up. It’s stabilized and possibly even gone down,” Kuhar said.
Kuhar said the main driver for the change is that farmers have effectively rotated their chemistries and are now using more non-pyrethroid products. In fact, Kuhar noted that farmers in the Deep South don’t use pyrethroids to control corn earworms in soybeans.
“There was a scare, there was a problem, and there was a change in what they were doing,” Kuhar said.
Beginning in 2008, Virginia Tech entomologists almost came to the point where they would tell growers to stop using pyrethroids to control corn earworms in soybeans. But it never came to that.
“Where we are now is it may not be as bad as we thought. You just need to be careful; you need to know that resistance is possible. There are resistance genes out there. But if you rotate chemistries, maybe not just keep hitting them with that pyrethroid, you’re in pretty good shape,” Kuhar said.
In fact, pyrethroids offer excellent control when rotated with other chemistries. In the 2021 Field Crops Pest Management Guide, Virginia Tech entomologists recommend the use of non-pyrethroids such as chlorantraniliprole, which include such products as Prevathon and Vantacor, a new product. The guide also recommends chlorantraniliproleplus bifenthrin which is available in Elevest, a new product.
It also recommend indoxacarb available in Steward EC and recommends lambda-cyhalothrin plus chlorantraniliprole available in Besiege.
In addition, the guide recommends methoxyfenozide plus spinetoram available in Intrepid Edge, spinetoram, available in Radiant SC and spinosyn, available in Blackhawk.
Kuhar emphasized that the use of the insecticide chlorpyrifos — or Lorsban — needs to be taken out of the Field Crops Pest Management Guide and should not be used in soybeans or any other agricultural crop. On Aug. 18, the Environmental Protection Agency issued a new rule banning the use of Lorsban on all agricultural crops.
Corteva issued this statement regarding chlorpyrifos: While Corteva Agriscience no longer produces chlorpyrifos, the Company stands by the safety of the product and its value for the grower community. This action effectively removes an important tool for farmers and, while Corteva continues to review the order, it appears that the rationale used by the Agency is inconsistent with the complete and robust database of more than 4,000 studies and reports that have examined the product in terms of health, safety and the environment.
Given the reduced demand for this product, Corteva made the strategic business decision to phase out our production in 2020.
In addition, Kuhar said the use of Bacillus thuringiensis (Bt) needs to be removed from the Field Crops Pest Management Guide. He said in his vegetable work he has discovered that Bt does not work against corn earworm in soybeans because Bt transgenic crops such as corn and cotton have the original gene, Cry13, which does not kill corn earworm.