Corn growers may never find a method to control corn rootworms with 100% effectiveness. Even Bt rootworm hybrids sometimes stumble in their attempts to fend off this wily and unwelcome root cruncher, as was documented in two cases in 2006 — one in Iowa and one in Illinois.

“Last fall my colleague Jon Tollefson and I confirmed a Bt rootworm (RW) hybrid failure in Clinton County near Elvira, IA,” says Marlin Rice, Iowa State University (ISU) Extension entomologist. “It was a surprise to us. Until then, we were not aware of Bt RW hybrid failures here in Iowa.”

Bt corn hybrids have been genetically engineered to produce a protein in their plant tissue that is toxic to certain insect pests, such as corn rootworms or corn borers, but not to livestock or humans. This protein is derived from a common soil microbe, Bacillus thuringiensis (Bt).

In the Iowa occurrence, the failure to protect roots with Bt RW technology transpired on a farmer's field, which had been planted with a YieldGard RW Bt hybrid. “It was a Monsanto event,” Rice says. “The reality is that it happened. So even the Bt rootworm hybrids are not fully bulletproof. However, we are not yet seeing this type of failure in our test plots.”

For example, ISU data collected over seven locations and three years (2003 to 2005) showed YieldGard RW to be the most consistent root protection product tested, according to Rice and Tollefson in a December 2005 summary of rootworm control products. “YieldGard RW provided the greatest root protection, although it was not statistically better than Aztec 2.1G applied in furrow,” they wrote. “No product was 100 percent consistent in providing total root protection during 2003-2005; however, YieldGard RW came very close at 99 percent, and it provided the highest average yield — 21 bushels more than any insecticide.”

Illinois trials

University of Illinois corn rootworm control trials show results similar to those in Iowa, says Mike Gray, University of Illinois Extension entomologist. Gray agrees that in most instances Bt rootworm hybrids have provided consistent control of corn rootworms. However, several of the granular soil insecticides also provide consistent root protection. Less consistent performers include liquid insecticides and the high-rate insecticidal seed treatments.

On the other hand, the 2006 corn rootworm trials in Urbana, IL, showed that some Bt RW hybrids sustain significant pruning by early August. “YieldGard rootworm Bt hybrids performed well in July and August at DeKalb and Monmouth, but not at Urbana, particularly in August,” Gray says. “Urbana is in the heart of an area where the variant western corn rootworm lays its eggs in soybeans. So maybe this variant is able to inflict more damage on corn roots than the non-variant type. However, we have so many questions that need answers that we can't really say with certainty why this occurred.”

YieldGard was 35% consistent at Urbana, 75% consistent at DeKalb and 85% consistent at Monmouth, Gray adds. In comparison, Pioneer's Herculex Xtra hybrid was 85% consistent at Urbana, 95% consistent at DeKalb and 95% consistent at Monmouth.

“In our 2006 corn rootworm trials, Herculex was more consistent than YieldGard at all our locations,” Gray says. “Not all hybrids are the same in their expression of a Bt protein, and Herculex has a different protein than YieldGard. However, we still don't know if this is the reason for the differences in performance that we saw at Urbana.”

Varying conditions

Rice agrees that it's still too early to know which Bt proteins perform best in protecting roots across many locations and varying crop-growing conditions. “Our YieldGard and Herculex Bt rootworm hybrid data have looked really comparable in their consistency and root protection so far,” Rice says. “However, in an ISU trial during 2005, AgriSure's Bt rootworm hybrids showed a reduced level of root protection under heavy rootworm pressure, similar to what occurs with granular insecticides.”

All currently available Bt RW proteins ultimately may show instances of product failure under certain circumstances, Rice predicts. For now though, most research shows that product failures are much more likely to occur with high-rate seed treatments and liquid and granular insecticides than with Bt RW corn hybrids, he adds.

Bt RW failures have yet to be confirmed in other states, like Nebraska. “We've heard of higher-than-expected damage on Bt rootworm corn in central Nebraska, but we haven't seen any failures by Bt rootworm hybrids to protect roots at our research plots,” says Bob Wright, University of Nebraska Extension entomologist. “Still, rootworms have shown us that they are very capable of overcoming our efforts to control them, regardless of the products we've used in the past. So, ideally, people should use a diversity of products and methods to control them.”

Crop rotation is the best way to reduce densities of rootworms in the western Corn Belt, Wright stresses. “Farmers really need to do some type of crop rotation,” he adds, “even if it's not in every field, every year.”

What to check

Non-Bt corn refuge areas provide a good check of Bt performance compared to other methods of control, suggest Rice and Tollefson, in an Integrated Crop Management article they co-wrote entitled “Bt Rootworm Corn Failures: Understanding the Issues.”

“The Bt corn and [its] companion non-Bt corn refuge should be checked in July for corn rootworm larval injury,” they advise. “This is the best time of the year to document the success (or failure) of a management tool. If you wait until harvest, when lodged corn is observed while combining, the roots may be diseased and have senesced, making rootworm injury difficult to identify.”

Mike Catangui, South Dakota State University Extension entomologist, points out that roots aren't the only place where Bt corn plants can be attacked. “There are new insects emerging as potential pests on corn ears, where Bt has no effect,” he says. “The insects that we've been seeing on the ears are corn rootworm adults, sap beetles and corn leaf aphids. Western bean cutworms have also been a problem on Bt corn, except on the Herculex hybrids, which do offer some protection against this pest.”

Once the ears are damaged by ear-feeding insects, aflatoxin and other mycotoxins often develop, Catangui cautions. These toxins can cause serious problems to livestock that ingest them, he adds.

Farmers and crop scouts in Nebraska also have reported seeing corn rootworm beetles and sap beetles feeding on ear tips, even on Bt RW corn, Wright agrees. “With a high population of rootworms, you'll see feeding on ear tips, but unless it interferes with pollination, we don't recommend treatment,” he says. “Sap beetles don't attack healthy ears, but once another insect feeds on the ear, the sap beetles follow.”

You'll often see the sap beetle any time another insect opens up the ear tip, agrees Ken Ostlie, University of Minnesota Extension entomologist. “Where there is European corn borer or western bean cutworm activity, their feeding provides an avenue for the sap beetle to have access to the ear,” he explains. “Also, hybrids with loose husks around the ear tip are more prone to sap beetle activity.”

Grain feeding from the western bean cutworm and the sap beetles is the main concern because of the potential yield loss that could result. “If you wait until you see western bean cutworm inside the ear, it's usually too late to treat,” Wright says. “You need to scout just before or at tassel emergence, and treat if needed before the cutworms enter the ear.”

Silk feeding from corn rootworm beetles can be a serious problem in some Bt hybrids. “No Bt rootworm hybrid has high enough levels of Bt toxin in the silks to cause adult corn rootworm beetle mortality or prevent adult corn rootworms from feeding on silks,” Ostlie says. “Both Bt corn and non-Bt corn are equally susceptible to silk feeding from adult corn rootworm beetles.”

Dry conditions also can result in more silk feeding and more yield loss overall. “Drought conditions will retard silk elongation and slow silk growth,” Ostlie says. “This makes drought-stressed corn more susceptible to silk pruning and pollination problems.”

Depending on the growing conditions and insect pressure, Bt RW hybrids may become vulnerable to significant yield loss, either from the corn rootworm larvae or the adult beetles. “There is no completely effective Bt RW or Bt stack that will eliminate corn rootworm feeding entirely,” Catangui cautions. “Even Bt rootworm hybrids will have live rootworms feeding on them. So it's best to remain vigilant and to document and report any major Bt rootworm corn injury to your seed representative or to an extension specialist.”