COVINGTON, Ind. — Western corn rootworm is a disease problem
for many corn farmers, especially those who have been planting continuous corn
in the Corn Belt, and it appears to be evolving resistance to Bt hybrids in
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
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
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
“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,
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
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.