High-rate insecticidal seed treatments typically perform poorly where corn rootworm populations are plentiful, warn Extension entomologists from several Corn Belt states. These experts remind farmers that scouting fields during previous cropping years is the only way to accurately determine whether corn rootworm population levels will be low enough for seed treatments to succeed.
“The challenge of using the high-rate seed treatments is accurately predicting the corn rootworm pressure in each field,” explains Ken Ostlie, University of Minnesota Extension entomologist. “You need to make sure you don't use seed treatments where corn rootworm pressure will be high, because seed treatments are vulnerable to heavy pressure. So, scouting for adult rootworm beetles in August would go hand in hand with using seed treatments the following spring.”
Most farmers don't scout fields or have any idea of rootworm pressure prior to planting, says Mike Gray, University of Illinois Extension entomologist. “From a convenience angle, a lot of producers would like to use the high-rate seed treatments and be done with control,” he says. “However, if they do so without scouting or hiring a professional crop consultant to determine corn rootworm densities, then they're taking quite a gamble in using high-rate seed treatments as their stand-alone rootworm control product.”
High-rate seed treatments often appeal to farmers who purchase hybrids with in-plant Bt (Bacillus thuringiensis) corn rootworm protection and who need an alternative product to use on refuge acres, says Ostlie. “It's nice to complement the transgenic seed-based system with another seed-based technology, such as a seed treatment,” he says. “That way you don't add extra demands in time, equipment, labor or safety concerns compared to handling soil insecticides at planting. However, the dilemma of using the seed treatments is that they don't provide adequate protection from rootworms when pressure is heavy, which is exactly when we need protection the most.”
University of Illinois research shows high-rate seed treatments fail to protect roots sufficiently where corn rootworm populations abound, says Gray. “Particularly in 2006, Poncho 1250 did not perform very well in our trials, which had very high levels of rootworm pressure,” he says. “In previous trials, Cruiser has been even less consistent than Poncho, so we no longer recommend it as a stand-alone option for corn rootworm control.”
In refuge acres, Gray recommends using a granular soil insecticide rather than a seed treatment in fields where farmers don't know the level of rootworm pressure. “Farmers are more likely to achieve consistent protection with granular insecticides than with high-rate seed treatments,” he says, “particularly if rootworm pressure is heavy.”
Bt rootworm refuge areas could lose significant yield if rootworms are present in high numbers and farmers choose the wrong treatment or no treatment at all, says Jack Bernens, head of Agrisure Corn Traits. “We're looking at ways to help mitigate yield reduction — either by improving soil insecticide applications to make them more convenient or to develop a product that would make having that structured 20% refuge-acre area unnecessary.”
Ultimately, farmers would like to do away with refuge areas and still comply with EPA requirements on insect resistance management, says Bernens. “Take seed treatments, for example — we'd like to maintain their convenience but significantly improve their efficacy,” he says. “We're looking at all types of new product development, in the five-plus year range, that would get us to a place where future products no longer require a structured refuge. So, the goal is either to have no structured refuge-acre requirement or at least to minimize any economic loss on refuge acres that you could plant in a very convenient way.”
In the meantime, research in South Dakota shows that a high-rate seed treatment will work successfully for several consecutive years in fields where corn rootworm levels are initially low. “In general, our studies in South Dakota have shown that a high rate of the Poncho seed treatment is effective in protecting roots until the third year of continuous corn, if fields start out with low rootworm pressure during the first year of continuous corn,” says Mike Catangui, South Dakota State University Extension entomologist. “In South Dakota, it takes about four years of continuous corn for rootworm populations to build to levels where high rates of Poncho seed treatments no longer provide adequate root protection.”
A high-rate seed treatment will likely cost just as much or more as applying a soil insecticide at planting, but a seed treatment offers comparably less root protection, points out Ostlie. “Cost-wise, it's better to go with a soil-applied insecticide if rootworm pressure is high,” he says. “Otherwise, you'll pay a high price for the convenience or the lack of equipment to apply a soil insecticide.”
Cost and convenience are typically the deciding factors in managing refuge acres for Bt rootworm corn, says Marlin Rice, Iowa State University (ISU) Extension entomologist. Having to scout fields for corn rootworms the year prior to using a seed treatment will both decrease the convenience of that treatment and increase its cost, he adds.
“In corn on corn, scouting for rootworms requires time and knowledge,” says Rice. “Most farmers either don't want to do that or may not do it well enough to get the right answers.”
Another important component for deciding on which product is best for managing corn rootworms is consistency, emphasizes Rice. He says ISU research shows Bt corn hybrids with in-plant rootworm protection, such as YieldGard Root-worm, are much more consistent in protecting roots than high-rate seed treatments when corn rootworm pressure is moderate to severe.
“Over seven locations and three years (2003-05), when it comes to root protection, seed treatments in our corn plots did not provide a consistent and high level of protection,” says Rice. “YieldGard Rootworm was 99% consistent; Poncho 1250 was 21% consistent and Cruiser was 8% consistent.”
In addition, ISU studies show liquid or granular insecticides can provide an effective alternative to high-rate seed treatments. “Our trials show a liquid application of Capture insecticide gives better root protection than seed treatments,” says Rice, “and granular insecticides work better than liquid insecticides.”
For farmers without insecticide boxes on their planter, however, liquid insecticides or a SmartBox attachment are the only other options besides a high-rate seed treatment to protect against corn rootworms, says Bob Wright, University of Nebraska Extension entomologist. “Insecticides like Regent or Capture LFR can be mixed and applied with fertilizers,” says Wright. “Regent is used mostly in the western Corn Belt with irrigation systems due to potential performance issues under dry conditions. Regent is also a systemic insecticide that is taken up into the roots, which is not the case with Capture.”
Rootworm management decisions for next year's crop can sometimes be improved by planting an unprotected strip and digging up corn roots to check for root pruning, says Ostlie. “You'd do this between Aug. 1 and Sept. 1 (in Minnesota),” he says. “Digging up roots makes the most sense if you don't know what kind of rootworm pressure you have in a field, but you should also complement digging up roots with scouting for adult beetles.”
While digging up roots and checking plants for root pruning, farmers can also assess compaction, root rot, herbicide injury and planting depth control, says Ostlie. He adds that both scouting and digging up roots in August is very hot, itchy and time-consuming business that some would rather avoid.