It isn't an epidemic and it won't shut down corn production anytime soon. However, researchers have confirmed that western corn rootworms have developed resistance to Bt corn hybrids that express the Cry3Bba trait in some areas of Nebraska.
University of Nebraska-Lincoln entomologist Lance Meinke said testing in areas of northeast and southwest Nebraska has been conducted over the last few years to determine the cause of "greater than expected rootworm injury," (GPE) observed in some cornfields.
"We started seeing some fields with GPE as early as 2011," Meinke said. "Our testing has included evaluation of factors other than western corn rootworm that could have caused corn plant damage. We've used a bioassay technique to assess the susceptibility of rootworms to various rootworm-active Bt traits. In that process, we collect the western corn rootworm beetles from GPE fields, bring them back to the laboratory and provide an environment for them to lay eggs."
In their lab, Meinke's team placed newly hatched larvae on corn plants at the five-leaf stage under identical conditions.
"We compared survival of GPE collected populations and susceptible control population across the different Bt events on the market," Meinke said. "We also did some on-farm trials, testing across different rootworm Bt traits to evaluate the level of rootworm control each provided in areas where western corn rootworm populations resistant to the Cry3Bb1 trait have shown up."
What Meinke and hit team have found is that farmers who have planted corn hybrids that express the Cry3Bb1 trait numerous years in a row have provided a natural selection environment for western corn rootworms.
"When a few of the rootworms survive the Bt corn trait, they reproduce larger numbers of survivors and then the number of individuals in that population that can survive exposure to the Bt trait continues to expand," Meinke said. "Resistance doesn't mean the entire field will be a failure. Typically, this scenario begins with just a few survivors and it takes several years for the resistant proportion of a population to increase. That means the level of control the Bt corn hybrid provides in that field may decline over time as more rootworms survive exposure to the Bt trait."
What researchers like Meinke hope to convey to corn growers is that western corn rootworm resistance isn't yet a crisis, but it could become much more difficult to manage rootworms if available Bt technologies are overused and resistance to many or all rootworm Bt traits evolves.
"We need to understand that it takes millions of dollars and as long as 10 years to develop a new corn transgenic trait," Meinke said. "That means, if we lose the technology in our current Bt hybrids, we won't necessarily have a new one to replace it."
Alternating tactics on a farm over years is a good way to proactively reduce the chance of resistance occurring or addressing resistance that has evolved.
"Crop rotation is the best tool," Meinke said. "Generally, one year of soybeans in a field with resistant western corn rootworms wipes out that population. The beetles will lay eggs that hatch, but when larvae try to feed on soybean plants, they don't find the nutrients they need and they die."
If some level of crop rotation can be worked into the overall farm plan, over time rootworm populations densities will be suppressed and all tactics will work better.
Corn prices have enticed growers to plant corn-on-corn in recent years, and Meinke said industry specialists know that some operations may face significant risk and/or profit loss if they change their cropping strategy.
"In continuous corn, moving away from single trait Bt hybrids to pyramided trait hybrids (i.e. two or more Bt traits expressed in a hybrid that target the same pest) provides a high level of rootworm control and better resistance management," Meinke said. "Resistance will likely evolve more slowly to a pyramid hybrid than a single trait hybrid because the insect population has to overcome two Bt traits and not just one."
Through research, Meinke and his collaborators have found that strategic use of insecticides can be useful to complement other rootworm management tactics, but should be used on an as-needed basis.
"Use of a soil insecticide with a single trait hybrid that is failing the field can improve rootworm protection, but selection for resistance to the trait will continue," Meinke said. "In most cases, adding soil insecticide over the top of pyramid trait hybrids at planting will not significantly improve rootworm protection but will only add to rootworm management costs."
Integrated pest management (IPM) plans that utilize a variety of Bt or conventional corn hybrids, crop rotation, careful use of insecticide and other pest management practices give growers the best opportunity to maximize profits and manage western corn rootworm.
"IPM isn't a use of every tool in every field during one season," Meinke said. "Working with crop consultants to identify the best integrated plan for each field and farm will bring the best possible results.
"I want to encourage farmers to move away from the 'silver bullet' approach with Bt hybrids," Meinke added. "The Bt traits should be viewed as one of many pest management tools we can incorporate into an effective IPM framework."