If producers of the television show “Survivor” ever run short of contestants, they can always turn to the lowly corn rootworm.
Each year, this pest costs U.S. farmers $1 billion in lost yields and control measures. Yet, corn rootworm survives and thrives despite widespread use of crop rotation and aerially applied and soil insecticides.
But soon farmers should have the best chance they've ever had to bludgeon this bug. Pending federal approval, Monsanto aims to market Bt hybrids resistant to corn rootworm in 2002. Dow AgroSciences and Pioneer Hi-Bred International also are jointly researching a corn rootworm-resistant product that federal regulators should approve in 2003 or 2004. And Syngenta Seeds also plans to develop similar transgenic hybrids in several years.
These hybrids control corn rootworm with a Bt protein, Cry3Bb, inserted into their chromosomes. In biotech jargon, this is called an “event,” and the protein differs from that used in Bt hybrids resistant to European corn borer (ECB). Rootworm larvae die when they chomp into the hybrids' roots. To control corn rootworm, all farmers will need to do is load their planters with these transgenic seeds and go.
No longer will farmers be limited to corn rootworm insecticides, the leading chemical control option. Because of their toxicity, insecticides require special application, calibration and disposal equipment.
Inconsistent corn rootworm control also results. “There are situations where soil-applied insecticides can and do fail,” says Larry Bledsoe, Purdue University research and extension entomologist. “Excessively dry or wet conditions can impact performance.”
Because transgenic seed contains the fatal corn rootworm protein, control does not hinge upon weather conditions. “They're a more foolproof control tactic than insecticides,” says John McFerson, Monsanto corn technology development manager for market development.
Insecticides also do little to curtail corn rootworm numbers. “Under high populations, rootworm larvae will remove almost all of the root tissue,” Bledsoe says. “When this happens, few rootworms survive and most starve.
“But when you use a soil insecticide, you create a zone of protection around the center part of the root system that prevents excessive feeding. As the protected rootball extends roots into the soil outside the zone, rootworms feed on it. This can support lots of rootworms.”
Because they die when they ingest roots of resistant hybrids, fewer corn rootworm larvae emerge as adults than when soil insecticide treatments are used.
The Monsanto transgenic hybrids, which the company will market and license under the YieldGard Rootworm brand, also give excellent root protection, McFerson says. In Monsanto tests, root protection for the resistant hybrids exceeded that of leading soil insecticides, he adds.
University tests concur. “Rootworm kills with the Monsanto event are slightly higher than what we see with the best insecticides,” says Ken Ostlie, University of Minnesota extension entomologist.
Nor has any yield drag surfaced in three years of Monsanto tests, McFerson says.
For these reasons, corn rootworm-resistant hybrids may sell faster than cold beer on a sultry summer day.
“It sure makes you drool when you compare them to conventional hybrids under high rootworm pressure,” says Steve Sodeman, a Trimont, MN, crop consultant. “Farmers will latch onto them pretty fast.”
Rootworm-resistant corn still faces several hurdles. The United States, European Union (EU) and Japan have not yet approved Monsanto's event, Mon863. Environmental, food and feed approval will likely occur early in 2002 in the U.S. and Japan, says Kelly Fleming, Monsanto's corn rootworm launch team leader. But the EU is not likely to approve the product for next year.
“EU approval is something we are eager to have, but the EU has not approved any new biotech products since early 1998,” Fleming says. Thus, farmers and grain merchandisers will have to segregate any YieldGard Rootworm corn from EU-approved corn.
Some scientists also question the event's impact upon nontarget organisms. Earthworms may be vulnerable to Cry3Bb endotoxins under stressful conditions such as drought, says Charles Benbrook, president of Benbrook Consulting Services, Sand Point, ID. Although U.S. federal regulators may clear the technology, Cry3Bb's impact upon nontarget organisms may cause European regulators to reject it.
“They take so much more seriously the consequences of adverse impacts on nontarget species,” says Benbrook, who submitted comments to the EPA for the Union of Concerned Scientists, a group critical of ag biotechnology.
However, Monsanto studies show that Cry3Bb activity is specific to corn rootworm. Thus, the risk of nontarget organism exposure is low, McFerson says.
Benbrook is also concerned that allergen issues that plagued StarLink corn in 2000 may resurface in transgenic rootworm hybrids. YieldGard Rootworm corn kills rootworm larvae through its roots, which contain high levels of Cry3Bb protein. However, the protein also resides in the plant's grain. With the exception of the protein Cry9c in StarLink corn, Mon863's grain protein levels are at least 50 times higher than in other Bt events, Benbrook says.
“StarLink caused a lot of problems because of an allergen issue due to high expression levels in the grain,” Benbrook says. “The pattern of expression in the Cry3Bb is fairly close to that of StarLink.”
Monsanto has evaluated the allergen potential of Cry3Bb through an extensive search of public and private databases that contain information on known protein allergens. This search revealed that the amino acid sequence of Cry3Bb is not similar to that of known protein allergens, McFerson says.
He adds that the EPA considered expression of the gene in all corn tissue — grain included — before establishing a Cry3Bb tolerance exemption last May. This exemption gives EPA's assurance that the corn is safe for both human and animal consumption.
Federal law prevents companies from pricing a product prior to approval. But if the added seed cost hovers around the $15/acre cost of soil insecticides, producers may quickly adopt rootworm-resistant corn.
“Most producers, especially those with continuous corn, incorporate soil insecticides as a standard practice,” says Mike Gray, University of Illinois integrated pest management specialist. “If transgenic hybrids are similar in cost to insecticides, many producers could opt for the convenience of not having to calibrate their planters for soil insecticides.”
If producers forego other control methods and rely solely upon transgenic hybrids, resistance could develop, Gray adds. Historically, farmers have overused effective rootworm control tools, including heptachlor and aldrin in the 1950s and early 1960s. Resistance to these chemicals was common across the Corn Belt by the late 1960s.
Nebraska farmers who aerially applied methyl parathion (PenncapM) and carbaryl (Sevin XLR) in the late 1970s faced resistant western corn rootworm strains in the late 1990s.
For years, farmers controlled corn rootworm by rotating corn with soybeans. Larvae that hatched from eggs laid by beetles in corn the previous year could not survive in soybeans.
Now in east-central Illinois, northern Indiana and southern Michigan, western corn rootworm beetles lay eggs in soybeans that hatch in corn the following year. Meanwhile, northern corn rootworm has thumped the corn/soybean rotation in parts of the Dakotas, Minnesota and Iowa via extended diapause: Eggs laid by adult beetles in corn lie dormant the next year in soybeans before hatching in corn the following year.
Monsanto initially proposed a YieldGard Rootworm refuge similar to that for Bt corn that is resistant to ECB. Within one-half mile of each biotech field, farmers would have to plant 20% of non-biotech first-year corn.
However, university and USDA entomologists disagreed with this proposal, because mating patterns of the ECB and corn rootworm differ.
Currently, no resistance plan exists. However, one will be in place before Monsanto markets these transgenic hybrids, McFerson says.
Gray proposes a voluntary prescription plan to prevent resistance. Growers and crop consultants would monitor fields in summer to estimate adult beetle abundance via beetle counts on plants or the use of sticky traps. If beetle numbers exceeded a certain threshold, producers would then plant transgenic hybrids the following year.
What to do
Should you plant rootworm-resistant hybrids? First, consider your rotation. Although the corn/soybean rotation does not control corn rootworm in some areas, it still works in most regions of the Corn Belt. Corn rootworm incidence drops even further if you plant more than two crops in your rotation.
Next, avoid choosing hybrids based solely upon rootworm resistance. “Getting the trait in the right hybrids is key,” says Rick McConnell, president of Pioneer. “If you put a trait like this in a second-rate corn hybrid, all you have is a hybrid that even an insect doesn't like.”
You also should scrutinize how the cost of rootworm-resistant hybrids compares to that of soil insecticides. “I would find the $15/acre seed charge a bit prohibitive for our farm,” says Dale Koester, who farms near Wadesville, IN. “We do invest $12 to $15/acre in insecticides, but I feel that we get more control of other insects than just corn rootworm.”
But rootworm-resistant hybrids also offer peace of mind. Soil-applied insecticides sometimes fail, says Erik Petry, a grower from Rochelle, IL. “However, you know that the seed technology is there and working,” he says. “Likewise, you cannot put a price on safety from not having to handle insecticide.”
Rootworm-resistant hybrids are a potentially valuable tool to control corn rootworm, Gray says. “But we have to remember not to use the same tool to bludgeon corn rootworm over the head time after time. We need to prolong its usefulness.”
What's on tap for 2002
Even if federal regulators approve Monsanto's YieldGard Rootworm event, hybrid numbers will be limited in 2002.
“Our goal for the launch year is to have growers first try the technology,” says Kelly Fleming, Monsanto's corn rootworm launch team leader. “We will have a number of hybrids for use across the Corn Belt.” Monsanto will emphasize the trial volumes in Nebraska, eastern Colorado and Kansas.
Pending regulatory approval, Monsanto also plans to market a stack of rootworm-resistant and European corn borer-resistant traits in YieldGard Plus hybrids in 2002.