Don't be surprised if you see occasional insect damage on some Bt corn hybrids. According to industry experts, being Bt doesn't mean corn hybrids are totally bug-proof.

“The point is that you should scout all corn, whether it's transgenic or not,” says Bob Wright, University of Nebraska Extension entomologist. “Depending on the insect and the timing, a rescue treatment might still be appropriate.”

All genetically enhanced Bt corn hybrids rely on the genes from a bacterium, Bacillus thuringiensis (Bt), to produce a protein toxic to certain corn pests. However, not every Bt technology targets the same pest or expresses the Bt toxin on the same part of the plant with the same potency.

“The first Bt corn hybrids had very high levels of control for the targeted pest,” says Wright. “Bt corn for corn borers provided 99.9% control. Most other Bt corn hybrids don't provide that level of effectiveness.”

For example, researchers at the University of Illinois (U of I) announced for the first time in September that a rootworm-resistant Bt corn hybrid had “stumbled” during root rating evaluations. In field plots at Urbana this year, researchers gave a rootworm-resistant hybrid a root rating of 3.15.

According to University of Illinois extension entomologists Kevin Steffey and Mike Gray, any root rating above the 3.0 index “could lead to potential yield loss and be deemed unacceptable.” In fact, at the Urbana site, they pointed out that several traditional insecticides provided better root protection than YieldGard Rootworm hybrids.

Some root feeding should be expected, even on rootworm-resistant hybrids, points out Joe Spencer, Illinois Natural History Survey entomologist. “In order for the insects to be killed by the Bt protein, they have to feed,” he says. “If you don't see feeding, maybe you don't have rootworms.”

Rootworms will naturally vary in their ability to ingest a Bt protein without harm, Spencer adds. “Larvae that first encounter YieldGard Rootworm late in their developmental stage may be more likely to survive the experience,” he says. “The younger larvae are the most susceptible, but even some of them, just because of natural variability, aren't going to be as affected as others.”

However, most reports from around the Midwest show farmers and researchers were more than satisfied with rootworm-resistant hybrid performance during 2003 and 2004 (see “Routing Out Rootworms,” page 16). “The goal for YieldGard Rootworm corn was to perform as well or better than soil insecticides do,” points out Spencer. “The data supports that it generally does.”

John Obermeyer, Purdue University Extension entomologist agrees. “In Indiana, we have not heard of one commercial complaint as far as performance goes for YieldGard Rootworm,” he says. “Reports from Ohio State University also indicate good performance there.”

While Purdue researchers did find one instance of a YieldGard Rootworm hybrid “stumbling” in a 2004 research efficacy trial, Obermeyer says the rootworm-resistant hybrids are typically reliable. “Up until this year, the transgenics seemed bulletproof,” he says, “but we've also seen insecticides stumble from one year to another, too.”

Why the difference in Bt control for corn borer compared to corn rootworms? “Corn borers are naturally more susceptible to Bt than rootworms are,” explains Wright. “It's also easier to get a high rate of Bt in the leaves (where corn borers feed) than on the roots, (where rootworms feed).”

Marlin Rice, Iowa State University Extension entomologist agrees. “The level of Bt expression (for YieldGard Rootworm) is a moderate dose, not a high dose,” he says. “Some farmers may expect 100% control, but that won't always happen with YieldGard Rootworm hybrids.”

Despite less-than-total rootworm control, Rice still considers YieldGard Rootworm hybrids a better choice than insecticides. He explains that rootworm-resistant hybrids offer more benefits, such as better root systems, increased yields and decreased insecticide exposure to farm workers and wildlife.

The same advantages apply to Herculex I Bt corn hybrids, Rice says. This new Bt corn technology offers in-plant protection against western bean cutworm, corn borer, black cutworm and fall armyworm, plus intermediate suppression of corn earworm. However, like YieldGard Rootworm, Herculex I fails to provide 100% control of every targeted pest, he adds.

“The data I've seen suggests that Herculex I gives only 70-90% control of western bean cutworm,” says Rice. “It does not provide total control.”

Nebraska's Wright concurs

“The western bean cutworm is not as susceptible to Bt as corn borer,” he says. “In high populations, Herculex I might only provide about 80% control, and that 20% survival may still cause economic injury.”

On the other hand, controlling western bean cutworms with traditional insecticides is difficult, says Wright. “The major concern is timing,” he explains. “Most people aren't scouting for the eggs, but an experienced crop consultant can find them and time rescue treatments before cutworms enter the ear. If cutworms are already in the ear, insecticides won't reach them.”

The bottom line is that even Bt corn hybrids can become vulnerable to insect damage, not just from insects for which they don't control, but sometimes from insects for which they typically do control, says Wright. He adds that farmers need to scout and stay vigilant in order to determine which corn insect control strategies to deploy and when to deploy them on each particular farm.

Biotech Bean Yields Stagnate?

Yearly U.S. soybean yield increases are not keeping pace with corn, says Dale Hicks, University of Minnesota extension agronomist. The cause for the stagnant yields could be a result of increased reliance on glyphosate-resistance technology, he adds.

“Yields are pretty flat on soybeans and have been for some time,” says Hicks. “I think the reason is the introduction of Roundup Ready beans.”

Hicks says he has compared the highest yielding glyphosate-resistant soybean varieties with the highest-yielding non-glyphosate-resistant varieties for several years. In every year, the best-yielding glyphosate-resistant varieties have lagged behind the best-yielding non-biotech varieties.

“On our plots, which are weed-free, that's where we find the 3- to 5-bu. yield difference,” he says. “Nowadays, most varieties are Roundup Ready, so it's hard to make those comparisons anymore. There are just not that many non-Roundup Ready soybeans available.”

“A Roundup Ready yield lag is much more difficult to document now, because ‘sister lines’ are no longer readily available to us,” says Roger Elmore, University of Nebraska Extension crop specialist. However, Elmore adds that since most breeding programs insert the gene into new lines “right from the start, yield lag should not be a problem.”

On the other hand, Elmore questions whether the introduction of Roundup Ready varieties may have slowed advancement in soybean yields.

“First, since the demand for glyphosate-resistant soybeans is high, breeding efforts on non-glyphosate-resistant cultivars by commercial seed firms will likely decrease proportionately,” he says. “Thus, yield potential gains of non-glyphosate-resistant cultivars over time may be less than those of glyphosate-resistant cultivars.

Second, because of this, and due to a “5% yield suppression associated with the glyphosate-resistant gene,” Elmore says future soybean yield potential is likely to be lower than if soybean breeders had maintained their efforts to increase yields in non-glyphosate-resistant soybeans. “If the trend to focus on glyphosate-resistant soybeans continues,” he adds, “we may look back on this time and likely see little or no gain in genetic yield potentials at the beginning of the 21st century.”

At test plots in Illinois, however, there is no apparent yield lag from Roundup Ready soybeans, according to Emerson Nafziger, University of Illinois Extension agronomist.

“In recent years there haven't been enough differences between the two groups, either their averages or the average of the top 10 within each, to definitely say that Roundup Ready yields more or less than non-Roundup Ready,” he reports.

Hicks points out that Minnesota yield trials have failed to show a yield lag from Roundup Ready corn or any other transgenic corn hybrids. The apparent yield lag seems to be isolated to just glyphosate-resistant soybeans.