The Dirt on Rootworms
This corn pest may be lying low, but it remains a big threat to growers.
For more than two decades, Joe Spencer has spent many a summer evening standing atop 30-foot-tall towers above corn fields, trying to catch adult Western corn rootworm beetles as they buzz by looking for a place to lay eggs.
Lately, he's been a little lonely up there.
"Populations are pretty low right now," says the University of Illinois entomologist, who works for the Illinois Natural History Survey. Widespread rainfall during rootworm egg hatch in June 2015 led to a major collapse in many Corn Belt rootworm populations.
Spencer is worried the lull in rootworm feeding might lure growers into a feeling of false security. After all, this is a pest that cost the industry up to $2 billion a year at its peak. Moreover, when it does surface in fields, the rootworm is increasingly problematic, Spencer says. It has evolved resistance to all four of the Bt traits on the market that target it, and new technology is still at least a couple of years down the road.
RESISTANCE IS GROWING
Cry3Bb1 (YieldGard RW) was the first Bt protein on the market to target rootworm back in 2003. It was followed a few years later by Cry34/35Ab1 (Herculex RW) and mCry3A (Agrisure RW). Industry soon began stacking mCry3A or Cry3Bb1 with Cry34/35Ab1 to create pyramided hybrids with two modes of action. In 2014, a fourth Bt protein, eCry3.1Ab, was added to the landscape and pyramided with mCry3A (Agrisure Duracade).
One by one, Western corn rootworm began developing resistance to each trait. First, in 2009, Iowa State University entomologist Aaron Gassmann collected rootworm populations in Iowa that were surviving the Cry3Bb1 toxin. By 2011, Gassmann had also collected populations that could survive mCry3A, and he documented that they had cross-resistance -- meaning a Cry3Bb1-resistant rootworm would also be resistant to mCry3A and vice versa.
Because of its structural similarities to the other Cry3 traits, cross-resistance to eCry3.1Ab (Duracade) was found almost immediately after it was commercialized. By 2016, scientists from Iowa and Minnesota confirmed field populations of rootworm that were resistant to eCry3.1Ab, as well as the other Cry3 traits.
Now, resistance to these three Bt traits is fairly commonplace, Spencer says. "From what I've seen from all the testing over the years, there is very little susceptibility to the Cry3 traits -- Cry3Bb1, mCry3A and eCry3.1Ab," he says. "And, due to cross-resistance, insects that are resistant to one are resistant to all three."
Cry34/35Ab1 fared better until recently, when Gassmann confirmed partial resistance to the trait in Iowa rootworm populations in 2016 and full resistance in 2018.
That is a deeply concerning development for corn growers since most pyramided Bt corn hybrids on the market rely on Cry34/35Ab1 for efficacy against the Western corn rootworm, Spencer warns.
"Pyramids with Cry34/35Ab1 are planted over a large area, and a lot of rootworms are exposed to it," he says. "That's putting a lot of pressure on this trait."
"There is more resistance continuing to develop," he adds. "When I do bioassays, I get 20 to 30% survival on this trait, and that's to be expected. Insects have been exposed to this product since 2005."
Growers have been waiting a long time for the next generation of rootworm traits from the industry. So far, the only new mode of action on the horizon is Bayer's SmartStax PRO, which uses RNA-interference technology to target the rootworm, in addition to
Once a SmartStax PRO hybrid is ingested by a rootworm, the new RNAi trait switches off a gene in the rootworm's DNA, which halts the production of a key protein and kills the insect.
The RNAi trait has made its way through the U.S. regulatory system but remains in limbo as it waits for import approvals from two key export markets, the EU and China, says John Fietsam, Bayer's corn systems lead for North America.
"We're targeting a commercial launch in the U.S. market early in the next decade," Fietsam says. SmartStax PRO hybrids would contain three rootworm traits: Cry3Bb1, Cry34/35Ab1 and the new RNAi trait. They would be sold as 5% Refuge-in-a-Bag products.
Spencer hopes Cry34/35Ab1-based hybrids can keep up the fight until then.
"In many places, it's the only trait out there providing protection," he notes. "But, I think a lot of growers are getting good advice now thanks to new rootworm-monitoring programs, and they're thinking twice about using Bt when they don't need it -- and hopefully helping to preserve the efficacy of this Bt trait."
The Many Faces of Corn Rootworm:
Here's a quick review of the two major types of rootworms Midwest farmers face today:
Western corn rootworm (WCR): This is the dominant rootworm species in the Midwest and Great Plains. Adult beetles have a yellow body with black or black-striped wing covers. In addition to Bt-resistant populations, the WCR has one other variant population:
Rotation-resistant: This type of WCR has evolved to outsmart the traditional corn-soybean rotation by laying its eggs both in corn and in neighboring fields, usually soybeans. When first-year corn is then planted to those fields, the rootworm eggs are waiting in the soil. Like other Western corn rootworms, this variant's populations have remained low recently, but it has been problematic in parts of Illinois, Indiana, Iowa and other Midwestern states in the past, Spencer notes.
Northern corn rootworm (NCR): This was the original native rootworm found in the Midwest, but WCR moved eastward during the mid-20th century and became the dominant Midwest species. As the more cold-hardy rootworm species, NCR has remained abundant in the northern Corn Belt, Spencer says. It may be seeing a resurgence recently, as some reports of NCR surviving in Bt corn fields have been surfacing, he warns. Adult beetles of NCR are smaller than WCR, with wing covers solidly colored and ranging from light tan to pale green. Like WCR, Northern corn rootworms have one variant that outsmarts crop rotation:
Extended Diapause: When this type of NCR lays eggs, a certain percentage do not hatch the first year and instead lie dormant through the subsequent soybean crop before hatching in the second year, when a corn crop is planted once again.
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