CABERY, Ill. — Since the initial findings of rootworm
resistance to Bt corn, crop rotation is among the first recommendations, but
recent findings have changed that thinking.
Significant western corn rootworm larval injury in
first-year cornfields has been confirmed in Livingston and Kankakee counties in
Illinois, according to University of Illinois Department of Crop Sciences
professor of entomology Michael Gray.
Gray and Joe Spencer, Illinois Natural History Survey
entomologist, recently confirmed injury to rotated cornfields that had been
planted to Bt rootworm hybrids, VT Triple PRO RIB expressing the Cry3Bb1
The fields in question were brought to their attention by
Bryan Johnston of Cabery Fertilizer in Cabery.
“Bryan indicated that many first-year Bt cornfields in the
area had severe root pruning and lodging,” Gray said.
“The fact that rotated corn is now showing susceptibility to
rootworm damage, even when planted to certain Bt hybrids, is evidence that crop
rotation in central and east-central Illinois does not adequately confer a
consistent level of root protection.”
Spencer collected numerous adult western corn rootworms in
the damaged corn and adjacent soybean fields.
“The number of beetles in the soybean fields was reminiscent
of densities in the late 1990s and early 2000s — very impressive,” Gray said.
“The density of western corn rootworm adults in both crops, along with the
severe pruning and lodging, was additional evidence that the Bt hybrids had
failed to offer the necessary root protection.”
Bioassays on the offspring from the adults collected by
Spencer will be required to determine if these rotation-resistant western corn
rootworms also are resistant to the Cry3Bb1 protein.
If the bioassays confirm resistance to the Cry3Bb1 protein,
Gray said producers across a wide swath of Illinois will have a “formidable
insect foe,” capable of overcoming both crop rotation and at least one Bt
Spencer said a couple of growers during Agronomy Day
mentioned to him that they had some poor Bt performance in rotated corn.
“So this was really interesting, in part, because the
expectations based on published reports from Iowa State University and some
populations that Mike was already working on suggested that resistant rootworms
were produced as a consequence of continuous corn and using the same trait year
after year,” he said.
“So rotated corn seemed like an environment where we
probably weren’t going to see Bt resistance, and certainly it wouldn’t be the
first place you would find it within a county. That’s what really surprised
Spencer said the Cry3Bb1 protein has been in the market the
longest and the one for which resistance has been observed in Iowa and a variety
of locations throughout the Corn Belt.
Gray is encouraging producers across east-central and
central Illinois to look for corn rootworm injury in their first-year
“From the roadside, it is very easy to overlook areas of
fields that may be lodged,” he said.
“We were visiting locations in the field that Bryan brought
us to and see the injury. I didn’t personally survey the whole field, but one of
the things we learned in speaking with at least one of the growers was he was
made aware of this problem because one of his neighbors was up using his ultra
light airplane and began to see all this downed corn,” Spencer added.
“I think that’s been one of the ways that people have really
come to appreciate how much of the problem there is. People who have been up in
planes have indicated to me that there’s a lot of downed corn.”
“I suspect that during harvest many producers may be
surprised to see pockets within fields that have been severely damaged by corn
rootworms,” Gray said. “Looking ahead, now is the time to begin thinking about
how to best protect against corn rootworm damage in 2014.”
Producers in the most severely affected areas of central and
east-central Illinois should consider the use of pyramided Bt hybrids — hybrids
expressing more than one rootworm Cry protein — in 2014, the professor
“The use of a planting-time soil insecticide should not be
needed with a pyramided Bt rootworm hybrid,” he said.
“Producers who elect to plant Bt hybrids expressing only a
single Cry rootworm protein may choose to use a planting-time soil insecticide
as an added precaution, particularly in areas where single-traited Bt hybrids
have failed this season.”
Over history, insects have shown an ability to develop
“It’s one of these things that it seems as though we’re
always surprised when some type of resistance happens. But if we go back and
look at history, really the history of insect management is really a history of
resistance,” Spencer said.
“Ever annoying, the insects are going to haunt us forever.
Just when we think we have something that they can’t beat, they teach us another
lesson. I think that may be one of the lessons that we need to learn. If we fail
to learn that lesson, the insects will reeducate us every decade or so or at
least every generation.
“The insects have an amazing capacity to adapt, especially
when you put pressure on them that results in their death — that’s a very
significant selection pressure and any individual that processes some trait that
gives it a slight advantage is going to over time become more abundant and
that’s what has happened.”