How to Handle Bt-Resistant Rootworm

Before you purchase any Bt seed corn varieties this winter be sure you know what you’re getting. Corn rootworm has developed resistance to some Bt corn varieties.

This means that if you have any fields in continuous corn you’re likely at risk for corn rootworm problems even if you plant Bt hybrids. Resistance to Bt has been reported in about a dozen Midwest states. Now entomologists have strong reason to believe resistance has cropped up in some areas of Pennsylvania. The situation is similar in New York as well; rootworms suspected of being resistant to Bt were found on a farm about 30 miles north of Ithaca in 2013.

Corn rootworm is devastating, causing an estimated $1 billion of damage to corn worldwide annually. According to John Tooker, an entomologist at Penn State, the first line of defense against corn rootworm in the eastern states is crop rotation. If continuous corn is a must on your farm—as it is on many dairy operations—then there are further steps you should take.

“In the northeastern U.S., the only reason to manage for corn rootworm is if you are growing continuous corn,” Tooker stated. For many corn producers, the answer has been a Bt corn hybrid. The hybrids promise resistance to corn rootworm.

Pull a few stalks and note the heavily chewed roots, which is typical of rootworm damage.

What about chemicals? Tooker said soil insecticides are “an imperfect solution,” because they are broad-spectrum insecticides and their potency declines over time. Since most rootworm chemicals must be applied at planting and the pest does not emerge for another month or two, the efficacy of the material is reduced. Rootworms usually emerge around the time one first sees fireflies, which is early June.

By the time damage is noticed in late summer or early autumn, there’s little a farmer can do to manage the problem. Treating populations of adult beetles is ineffective, so now is the time to plan solutions that can be implemented next spring.

A look at the problem

For Northeast producers, this has been viewed as a problem that occurs elsewhere. The closest fields with suspected resistance were identified in central Michigan in 2012.

In August 2014, near Belleville in Mif f lin County, Pennsylvania, Tooker met with a farmer and his consultant to explore three fields that were heavily damaged by western corn rootworms.

“There were beetles all over the place,” Tooker said. This shows that there was a huge population of larvae two months earlier. The Bt hybrids appear to have failed to protect against the rootworm.

An area of goose-necked stalks should start a grower thinking about corn rootworm damage.

“The roots didn’t appear to have gained protection,” Tooker said. The fields had many lodged and goose-necking plants. When Tooker made root ratings, he found that rootworms had removed more than two to two and half nodes of roots (on a three-point scale) from many of the plants. “At this point it is ‘suspected resistance,’” Tooker emphasized.

“I’m surprised this is the first field [in Pennsylvania] where I’d heard about suspected resistance,” Tooker admitted. However, he quickly became aware of a second site near Newville in Cumberland County with similar symptoms.

“The field near Newville shared many of the features of the Mifflin County field, and the growers are reporting it to Monsanto as ‘unexpected damage,’ Tooker explained. “There is a good chance both farms are suffering from the same problem.”

With farmers and consultants aware of the potential problem, it’s a safe bet that more Bt-resistant fields will be pinpointed.

While he is sure about what he saw, Tooker cautions that the blame cannot 100 percent be put on Bt resistance until he has had time to do a grow-out in a greenhouse in a controlled situation. Work with offspring of the beetles collected from those fields and raised on Bt and non-Bt plants will confirm whether Tooker’s fears are scientifically accurate. However, all indicators point to Bt resistance similar to that seen across Iowa, Minnesota, Nebraska and much of the rest of the Midwest, save Indiana.

Tooker thinks there’s strong potential for his lab work to confirm his suspicions. “These fields had all the hallmarks of suspected resistance,” he said.

The Mifflin County fields were planted in three corn varieties: two that expressed the YieldGard rootworm trait (Cry3Bb1 toxin) and one expressing one of the Agrisure rootworm traits (mCry3A toxin).

Tooker performed gene checks on the plants to confirm that they were producing the appropriate rootworm-targeting toxins and they were.

“Importantly, all these features are identical to those of fields in the Midwest where resistance to Bt by rootworm populations has been confirmed by university entomologists,” Tooker added.

All about Bt

Bt (Bacillus thuringiensis) is a common soil-inhabiting bacteria. For reasons that are not entirely clear, this bacterial species produces a toxin that’s an effective natural insecticide against many insect pest species.

Some strains of Bt kill insects with toxins called insecticidal crystal proteins or delta endotoxins. Delta endotoxins are triggered in conditions unique to the insect gut and rapidly paralyze the insect’s digestive system, so damage to the plant stops soon after the insect is exposed to the toxin. Mortality may take several days, so the effects of delta endotoxins are different from what we expect from conventional insecticides, noted entomologist F.B. Peairs at Colorado State. Different strains (about 600 are known) of Bacillus thuringiensis produce different forms of delta endotoxins; many are toxic to caterpillars (e.g., European corn borer), while others are toxic to flies (e.g., mosquitoes) or beetles (e.g., corn rootworm).

Bt corn can adversely affect nontarget insects if they are closely related to the target pest, as is the case with monarch butterfly caterpillars. However, Peairs says these adverse effects are considered minor, relative to those associated with the alternative of blanket insecticide applications.

For example, keep in mind that millions of acres of Northeast and mid-Atlantic forests were treated for gypsy moth and other pests with Bt insecticides over the past 30 years. There has been little documented effect on nontarget species.

Producers who have applied materials like Bactimos, Biobit, DiPel, Javelin, Teknar or VectoBac have been using Bt.

Into 2015

“I encourage growers, consultants and other agricultural professionals to be aware of the possibility of encountering this problem in your part of the state,” Tooker said. “Know the warning signs. Recall that this is a problem for continuous corn; many of our dairies are at risk if they plant corn in the same fields year after year.”

The easiest way to fix this problem, or to prevent it from developing, is to rotate fields from corn to soybeans or alfalfa or some other non-corn crop for next growing season, he added.

“Switch to Bt hybrids that have more than one protein active against rootworms,” recommends Penn State’s John Tooker.

“Try to avoid growing corn continuously in any field for more than two or three years,” he continued. Pennsylvania does not have populations of rotational-resistant rootworms—that is, rootworm beetles that lay their eggs in soybean fields rather than cornfields and the larvae hatch from eggs and emerge in first-year corn. “We do not expect them to evolve here any time soon,” he said.

“If rotation out of corn is not possible, switch to Bt hybrids that have more than one protein active against rootworms,” Tooker advised.

There are basically four Bt toxin strains: Cry3Bb1, Cry34/35, mCry3A and eCry3.1Ab.

If you have used hybrids with the Cry3Bb1 or mCry3A toxins, rotate to the Cry34/35. Cross-resistance has been found between Cry3Bb1 and mCry3A toxins, so rootworms resistant to one are resistant to the other. By using a Cry34/35 hybrid, no matter which of the susceptible lines you have been using, you know you are substituting a new Bt toxin, and that should give the field protection it needs. “Use a two-gene hybrid, and that way you know you didn’t use one or the other gene,” Tooker explained.

Finally, ensure that your fields fully comply with refuge requirements. Planting of refuges can help prevent the evolution of resistance.

“The bottom line is that relying on one tactic for too long is a prescription for evolution resistance—just like we have seen with glyphosate-resistant weeds.”

There is no silver bullet. “Rootworm beetles are skilled at adapting to consistent management tactics,” Tooker concluded.