Corn+Soybean Digest

Scouting Made Simple

Sweltering temperatures and pollinating corn don't make you say, “Let's go scouting!” But, those are generally the conditions under which scouting for corn rootworm takes place.

Scouting rootworms this year will give you a better handle on what could happen in your fields next corn crop, says Ken Ostlie, University of Minnesota Extension entomologist. He adds that it's important to get a handle on rootworm pressure in your fields, and whether the changing pressure means you should shift your management.

“In farming right now, there's a tendency to take a more insurance approach,” says Ostlie. “The move toward recipe farming takes us out of a mode where we can react to changing pest situations.”

And situations are changing, especially with the different rootworms. Northern corn rootworm has developed extended diapause — the eggs laid this year could hatch anywhere from two to four years later to get around crop rotation. Western corn rootworm can also cause potential problems in rotated corn.

“They've lost their fidelity to corn for egg laying in Illinois, Indiana, south Michigan, Wisconsin and east Iowa,” notes Ostlie. “Western corn rootworms now lay their eggs in other crops, and then we move corn to where those eggs will hatch the next year.”

Then there's the possibility of resistance developing to transgenic corn planted to control corn rootworm. “Transgenic corn isn't a high-dose event; the kill isn't complete,” says Ostlie. “Because of extra survival, we can see significant populations remaining in fields. We're concerned about resistance; we have isolated reports of failures in Minnesota and across the Midwest. Are these fields the tip of the iceberg in resistance development?”

That possibility of resistance should be enough to get growers out there scouting, no matter what the conditions. There are two basic techniques for scouting adult corn rootworm (beetles): whole-plant counts and using sticky traps.

For whole-plant counts, avoid the edges of the field (first 100 ft.). Count about 50 plants in around 25 stops, checking plants not next to each other. Remember that “beetles are flighty,” says Ostlie. “If you're smashing through the field, you'll tend to have lower counts than if you approach plants slowly and quietly.”

When you reach a plant, cover the ear tip because beetles like to feed on silks. Count the rest of the plant first, then release the ear tip and tease the silk apart. This will generally give the beetles a flight reaction, but they will fall a bit before they take off. So if you hold your hand under the tip, the beetles will first fall into your hand, Ostlie says.

Scouting with sticky traps is less involved on your part, but not for your pocketbook. They run from $1 to $2-plus each, and each field needs around 12 traps for a reliable count.

The yellow color of sticky traps is attractive to beetles. The traps sould be placed at ear level on corn plants in a representative area of the field, again avoiding the edges (first 100 ft.). Leave the traps out for a minimum of about two weeks, but check beetle numbers weekly.

“An advantage to using sticky traps is that they sample over several days and average out effects of time of day or short-term weather changes,” says Bob Wright, University of Nebraska-Lincoln Extension entomologist. “They also provide a good solution for people who question their ability to adequately count beetles visually.”

NOW YOU'VE VENTURED out into the sweltering canopy to count beetles yourself, or your sticky trap has been out for a week. What do these numbers mean for corn management? What's too high? What's tolerable?

“No studies have developed thresholds specifically for northern corn rootworm,” says Wright. “But northern corn rootworms are no more damaging than western corn rootworms. An acceptable western corn rootworm threshold for whole plant counts is equal to 0.75 beetles/plant with 24,000 plants/acre.” (Depending on area, populations could be higher.)

Also note that there are separate thresholds for corn after corn and for rotated corn.

“Different thresholds have been developed for corn after corn and first-year corn because research has shown that western corn rootworm females have a greater tendency to migrate and make up a greater portion of the population in first-year corn,” Wright notes. “If beetle numbers in a field exceed the threshold, there's potential for economic damage if corn is planted back into the field next year.”

The threshold for sticky traps is 4-5 beetles/trap/day, so 28-35 beetles on a trap when left out for a week, says Ostlie. That same threshold applies to the western variant when scouted in soybean fields.

“Based on research, if a soybean field has a beetle count of 5 or more beetles/trap/day during any trapping week, the field will need to be treated for rootworm if it is planted to corn the next year,” says Bruce Eisley, Extension entomologist at Ohio State University.

However, in areas with extended diapause, rootworm activity in corn also needs to be monitored. “Thresholds for sticky traps haven't been developed yet, but whole-plant counts exceeding 4-5 beetles/plant are associated with higher risk in the following corn crop,” says Ostlie.

Scouting produces an index of corn rootworm pressure that can be used for a number of options. “Does the field need protection with transgenic corn? Does it need rootworm management, period? Will you plant transgenic corn next year, or use insecticide alone?” asks Ostlie. “Pressure may also give a clue as to whether refuge acres need to be treated, and what to use.”

Treatments can include: granular insecticides, liquid insecticides, seed treatments or Bt-rootworm hybrids.

“Farmers have lots of decisions they're making all the time. Growers are operating blind. Getting a handle on risk is important, not only for deciding what to do and what action to take,” Ostlie advises. “Down the road it may help in two regards: determining if pressure is dropping because of continued use of Bt corn and the chance that resistance might be developing. If we're not looking at fields, it's going to catch us by surprise.”


Circumstances may not let you scout every soybean field for the western rootworm variant, so what action do you take when it comes to fields with unknown pressure?

“We know that there are probably more fields that need to be treated, and at the same time, fields that will not need treatment,” says Bruce Eisley, Extension entomologist at Ohio State University. “Since there isn't any way to predict which fields need treatment without trapping information, we suggest that producers take into account several things before a decision is made about treatment needed on the field.”

Eisley recommends asking yourself the following questions to help make a treatment decision for non-scouted fields:

  1. Are the fields located in a county with low or high counts in 2007? We cannot use a few trapped fields in a county to make countywide recommendations, but it does give some idea of the level of rootworm activity in the area.

  2. Has there ever been a problem with the western corn rootworm variant or extended diapause in the field in the past?

  3. Were there any direct observations of root injury and lodging due to rootworm activity in your own adjacent cornfields during the previous growing season? How widespread was the injury and lodging? Was it small and localized, or across the entire field?

  4. Were there any severe weather events that could account for the observed lodging?

  5. Did the lodging have a significant impact on yield or economic returns?

  6. Since there are no rescue treatments for rootworm larvae, how much risk can be tolerated?

Eisley also suggests that if a field is treated for the western variant or extended diapause, that several untreated strips be left in the field.

“This will be the only way we can tell if these problems exist in the field,” he says. “Because of this continued concern with this insect, we urge growers to develop a sampling plan next year in soybean and corn fields, and to sample roots for feeding injury in their first-year corn.”

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