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Think ahead for corn rootworm

Snap, crackle, and pop aren’t just limited to a Rice Krispies commercial. It’s also the sound healthy corn roots make when a shovel turns them over this time of year.

Granted, the decisions you made last winter and earlier this year determine the corn stands you have this year. Still, getting out in fields prior to harvest can tell you a lot about what’s been going on during the growing season. It also helps you plan for next year. 

“A shovel can help you tell a lot about your corn,” says Tim Dahl, a Syngenta agronomy service representative. Doing root digs is a time-tested way to detect corn rootworm damage. 

Plunging a shovel into the ground, Dahl samples roots from several field locations. He then takes the root balls back to a farm and carefully washes the roots. “Once the roots are clean, you can check for root feeding and damage,” he says. 

“Corn rootworm causes $1 billion worth of damage each year,” says Dwayne Martin, Syngenta commercial traits manager. The kicker is that by the time a root dig or a field inspection reveals the telltale goosenecking of rootworm-ravaged plants, it’s too late for that year’s corn. 

However, it can reveal fields prone to rootworm damage, particularly corn-on-corn. Another useful predictor is the sticky trap. These traps can indicate the odds of corn rootworm surfacing in corn planted the next year in this year’s adjacent soybean fields. 

Under extended diapause, northern corn rootworm adapts to produce eggs that remain dormant for one or more growing seasons. A twist on this is the western corn rootworm variant. This is when adult western corn rootworm beetles lay eggs in soybeans that hatch into root-ravaging larvae the next year. 

A 2013 Iowa State University study determined a 1.5-rootworm-beetle-per-sticky-trap-per-day level in soybeans (or two beetles per trap per day in corn) indicates rootworm damage may occur the following year, says Joe Spencer, University of Illinois (U of I) entomologist. In these cases, it’s recommended preventive measures like rootworm traits and/or a soil-applied insecticide be used, he says. 

Persistent pest

Scientists first discovered northern corn rootworm in 1820 and western corn  rootworm in 1867. 

“The biology of the insect is tied to corn,” Spencer says. “The most important thing to know is that after the female beetles lay eggs in cornfields, the developing larvae need to feed on corn roots to survive,” he says. 

Corn rootworm has beat back every control measure that’s been tossed its way. 

Rotating corn with a crop like soybeans used to deter corn rootworm. By 1995, though, this practice had selected for the western corn rootworm variant. This led to some Illinois and Indiana farmers experiencing a 50% yield loss in corn after soybeans in 1995, says Spencer. In northern areas, extended diapause also surfaced. 

All this led to an alarming escalation of soil-applied insecticide use, says Spencer. It was a motivating factor for the development of Bt rootworm-resistant traits that first debuted in 2003.

Refuges accompanied the Bt traits that conferred resistance to European corn borer starting in 1996. “With corn borer products already out there, it became problematic to match the corn rootworm biology (and accompanying refuges) with the corn borer refuge,” says Spencer. 

Initially, Bt-susceptible rootworm beetles in refuges were expected to flock to nearby Bt corn, where they would mate with any rare and potentially resistant surviving beetles. 

“We expected this to create lots of susceptible beetles,” says Spencer.  

In addition, trait companies expected farmers to follow refuge requirements and plant the required percentages (20% initially) of refuge acres. 

That didn’t pan out as expected. In 2010, the U of I started a three-year study to see if this was occurring. Western corn rootworm behavior differed greatly from the initial resistance plan. Before corn pollination began, an average of 17% to 25% of adult beetles left refuges for Bt corn each day. Once corn pollinated, though, migration virtually stopped. 

“Structured refuges are poorly suited to western corn rootworm biology,” says Spencer. 

This was aggravated by poor compliance among farmers. Just 75% to 80% of farmers complied with rootworm refuge requirements. Refuge-in-a-bag products that mixed 5% of refuge plants with Bt plants didn’t boost beetles mixing, either. 

Such factors led to the first case of western corn rootworm resistance to Bt corn in Iowa in 2009. Since then, rootworm resistance to multiple Bt toxins has surfaced in the Corn Belt. 

right: Root digs can reveal maladies like chewed-up root masses, notes Clint McGill.

What to do?

We need to pay attention to what insects really do, says Spencer. “We need to stress Integrated Pest Management – and not ever underestimate them,” he adds. 

Planting pyramided hybrids helps. These hybrids aim multiple Bt toxins that differ in modes of action at corn rootworm. One example is Cry34/35Ab1 (known as Herculex RW, which is also contained in other rootworm products). 

Like all control measures, though, sole dependence on it can render resistance. 

“Cry34/35Ab1 is carrying a lot of the water among pyramided hybrids,” says Spencer. 

Rotating traits is another way to deter resistance. However, sorting through all the different offerings is a job in itself. One way to ferret out all the options is to use a cheat sheet, like one offered by Michigan State University.

“The big thing for rootworm management is record keeping,” says Dahl. “Know which trait was put on which field.”

Corn rootworm management took priority for Clint McGill, Albert Lea, Minnesota, after southeastern Minnesota experienced a 2009 corn rootworm infestation. Although several years of moderate to light rootworm infestations have tempered that trend and led to some area farmers planting conventional hybrids, rootworm is still a ticking time bomb, he says.

“It’s just a matter of time when we have another outbreak here,” he says. Corn rootworm-resistant traits aren’t always foolproof, either, as resistance has developed. That’s why an integrated program – using tools like traits, a soil-applied insecticide, and a scouting program including root digs – is a must, he says. 

Dahl concurs. To battle back against corn rootworm, a variety of tools such as root digs, crop rotation, soil-applied insecticides, and traits are needed, he says. 

“Mother Nature has a way of defeating every technology that is not properly stewarded,” he says.

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