Corn rootworm resistance present in Bt corn

An Iowa State University entomologist published an article in 2011 confirming the evolution of field resistance to the Cry3Bb1 protein expressed in some Bt hybrids.

University of Illinois entomologist and crop scientist Mike Gray, who has done research on the insect’s severe root damage to Bt corn, shared the results of Illinois corn trials at the Bi-State Crop Management Conference.

He reported that plant bioassay results with offspring from adults from two fields in northwestern Illinois mirrored bioassay results reported by Iowa State in which resistance had been confirmed for some fields in northeastern Iowa.

His results suggest that stacked-gene varieties of corn in some cases are not protecting against the pest.

Guests to the conference were interested to hear how western corn rootworm resistance could become a problem in their fields of continuous corn in northeastern Iowa and northwestern and north-central Illinois.

Gray also mentioned a paper published in Science magazine two years ago that focused on European corn borer populations across the Midwest, which are historically low due to the use of Bt corn and its areawide suppressive effects on the pest.

The article also stated that historically, larval surveys have indicated that O. nubilalis populations have been episodic, characterized by about a six- to an eight-year periodicity indicative of density-dependent population growth and though this periodicity has persisted since Bt commercialization, larval populations have declined relative to the pre-Bt era, particularly since 2002, as evidenced by measures of larva in non-Bt refuge fields and landscape-level areas.

Between 52 percent and 53 percent of corn planted in 2012 is a stacked-gene variety expressing multiple Cry proteins and offering herbicide tolerance, Gray said.

“The ability of the western corn rootworm to develop resistance to transgenic corn was only a matter of time,” he noted. “Researchers are seeing a decrease in the expression of the protein Cry3Bb1 in the root systems of some Bt hybrids from the V4 to the V9 stage of plant growth. There are no high-dose events or silver bullets when it comes to rootworm protection.”

“In addition, you wouldn’t know you have insect resistance by walking into a field,” the scientist said. “Even though Bt plants designed to offer protection against rootworm larval injury may have severe root pruning, this does not prove resistance is present within a field. Plant bioassays must be conducted, and this requires collecting adults from fields with suspected resistance, mating the adults, collecting the eggs, hatching the larvae from eggs and using precise techniques to evaluate whether resistance is present within the population.”

The Illinois researcher and his team conducted a number of rootworm trials in Urbana last summer, using the 2012 growing season to test different Bt hybrids and soil insecticides and evaluated their efficacy in treating rootworm infestations.

To date, western corn rootworm resistance has been confirmed only for the Cry3Bb1 protein, though it is one of three proteins expressed in various Bt hybrids, including Cry 34/35Ab1 and mCry3a, Gray said.

Root pruning has been observed on corn rootworm Bt hybrids that express other Cry proteins. The U of I’s 2011 corn rootworm product efficacy trials unveiled that two Bt hybrids expressing the modified Cry3A protein had about one half of a node of roots pruned in an experiment at the DeKalb Research and Education Center near Shabbona, Gray wrote in the Sept. 23, 2011, issue of The Bulletin .

The checks in the study had root pruning that averaged about 1.5 nodes of roots destroyed.

In 2008, a Bt hybrid expressing the Cry34/35Ab1 corn rootworm proteins had almost one node of roots pruned, again at the DeKalb research site. The level of injury in the checks was intense, with nearly three nodes of roots destroyed, Gray wrote.

Purdue University entomologist Christian Krupke related the results of the observations and tests at the annual Certified Crop Adviser Conference in Indianapolis.

He said while there is no evidence of cross-resistance between YieldGard, VT3 and Herculex, YieldGard and VT3 problems mean SmartStax is not a true hybrid “stack” and the Cry34/35Ab1 and Cry35Bb1 proteins are under heavier pressure in a 5-percent refuge.

In areas where resistance to the Cry3Bb1 protein has been confirmed, in effect, farmers have only one protein providing most of the root protection though pyramided Bt hybrids such as SmartStax hybrids are being used, Gray said.

In the Science paper, it was reported that Bt hybrids targeted at European corn borers generated about $3.2 billion over a 14-year period, but the research also revealed that producers who elected not to plant Bt hybrids benefited the most economically by saving money on seed costs and benefiting from the areawide suppression of corn borers because their neighbors used Bt hybrids, Gray said.

The U of I professor encourages farmers to use integrated pest management, which can include crop rotation, Bt hybrids, soil insecticides and adult control approaches, to deal with resistance issues.

The key to preventing or delaying resistance development is to use integrated management tactics and not rely on any single approach over and over, Gray added.

“The cost of Bt hybrids, insecticides and inputs have gone up quite a bit, so it is important to share with growers the concerns and downsides of repeated use and the importance of going and scouting your fields to be familiar with the life cycles of certain insects so they can decide which densities of pests are really worth treating,” he said.

“Scouting cornfields for corn rootworm larval damage — root pruning — is necessary to assess the effectiveness of a Bt rootworm hybrid or soil insecticide,” he added.

For example, a grower scouting their field for soybean aphids who finds around 250 aphids per plants would have good reason to implement an IPM strategy and spray the plants, while lower populations might not rationalize the cost of the treatment.

Growers have other options for pest management available to them. They may decide to grow a non-Bt hybrid and continue to scout for European corn borer or other pests.

The development of pyramided Bt hybrids such as Herculex corn, which express more than one Cry protein for a single insect, along with the regulatory approval of a reduced refuge from 20 percent to 5 percent, allows farmers to plant a mixture of corn seeds to manage rootworm pests, Gray said.

Krupke noted that resistance in many ways can resemble a lottery, though the higher the crop rotation farmers use, the better off they are in offsetting the factors that may lead to a resistance problem.

“Corn rootworm populations are at very low levels in most of Indiana, but by rotating to different technologies and changing to different Bt traits other than SmartStax or rotating into seed treatments, growers can really strengthen their management against this pest,” he said.

The ability of their corn hybrids to manage pests may be compromised by a reduced refuge, however, Gray said.

Farmers who live in an area where rootworm resistance has been confirmed and are planting SmartStax hybrids with a 5-percent refuge seed blend instead of a 20-percent refuge could compromise the long-term durability of the Cry34/35Ab1 protein expression in the Bt corn, Gray elaborated.

“If we were to plant SmartStax hybrids in areas where resistance is confirmed, we assume a high percentage of the population is resistant to the Cry3Bb1 protein,” he said. “We should get adequate root protection, but we may be compromising the durability of the Cry34/35Ab1 protein because of the reduced refuge.”

“Farmers should go back to more of a long-term, integrated approach to pest management,” he said.

Growers also are looking to the horizon for a new gene silencing process being developed by Monsanto to handle rootworm populations, as well as a new Cry protein for rootworms that has been developed by Syngenta.