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First In-Field Resistance to Bt Corn Targeting Rootworms Documented in Iowa

This report, written by Christian Krupke, Purdue University Extension
entomologist, appeared this month in Purdue’s Pest & Crop Newsletter.

• Following reports of high damage in Iowa, lab studies revealed
resistance to Bt hybrids expressing Cry3Bb1 toxin (found in Monsanto hybrids
targeting rootworms).
• Field locations where resistance was documented were
characterized by high rootworm pressure, with a history of continuous Bt corn
planting.
• This highlights the importance of refuge planting, and Indiana
producers should remain vigilant.

Regular readers of Pest&Crop may recall that we have
mentioned a few times that corn rootworm Bt toxins are not high dose toxins –
meaning that many larvae survive exposure and reach adulthood on each acre of
these hybrids. This is one of the reasons that the refuge is so critical in
stewardship of this valuable IPM tool. Those points were underscored by the
publication last week of the research findings of Iowa State University entomologist
Aaron Gassman and co-authors. After receiving persistent reports of high damage
to Bt corn in northeastern Iowa, the group collected adults and eggs from the
area. Rearing the larvae in the laboratory on Bt hybrids revealed that the
larvae were able to survive on Bt corn hybrids expressing the Cry3Bb1 toxin at
levels similar to survival on non-Bt corn. Hybrids expressing this toxin
include those formerly labeled as Yieldgard RW and VT3 hybrids. This toxin is
also one of the proteins found in SmartStax hybrids. The good news is that the
study tested the other major toxins deployed in North America against this
pest, Cry34/35 (found in Herculex hybrids targeting rootworms and also in
SmartStax hybrids), and no enhanced survival was found. Although Cry3bb1 and
Cry34/35 toxins are different, they are similar enough that cross-resistance
(where surviving exposure to one toxin confers some level of survival to
another), was a possibility worth investigating. No evidence of
cross-resistance was found in these rootworm populations. The next questions to
tackle involve untangling the mechanisms behind how these insects are able to
survive toxin exposure – what combination of physiological and behavioral
traits are at work here? Understanding these mechanisms will undoubtedly help
find solutions and plan future control technologies.

Given that other researchers have reported that Bt resistance is
fairly easy to select for in the lab, we strongly suspected it was just a
matter of time before we would see it in the field. The majority of corn
planted in the US is Bt corn, and the Cry3bb1 toxin is the major one deployed
against rootworms. So what does this mean for other corn-producing areas,
including Indiana? It certainly is not good news, but it is not a total
disaster either. First off, it demonstrates again what a remarkably adaptable
pest the western corn rootworm is. There is no "putting the genie back in
the bottle," and resistance in these areas is a problem that won't go
away. This is an alert to keep our eyes open for similar occurrences elsewhere
– i.e. Indiana. However, the vast majority of Bt corn continues to perform well
and be a boon for producers. At the same time, remember that rootworms are the
key pest of corn here in Indiana as well, and there is no reason why we could
not see resistance occur at some point, either to this or other toxins. The
fields to watch are those where the selection pressure upon the pest is
highest: namely continuous Bt corn in areas of high rootworm pressure. Some northwestern
and north-central Indiana fields may fit this description. In general, however,
we have a more diverse cropping system than Iowa with a large proportion of
fields rotating corn with soybeans. That helps delay resistance. Planting the
recommended refuge certainly helps, and although compliance with the refuge
requirements has been falling in recent years, this serves as a stark reminder
of how important it is.

Finally, this research also underscores the fact that growers and
consultants are not only the stewards of this technology, but they serve as the
eyes and ears to report problem fields and initiate further study. None of this
research would have been done without reports to ISU Extension staff, so please
keep your eyes open and don't hesitate to report potential problems.

The
full paper can be downloaded and read here:
<doi:10.1371/journal.pone.0022629>.

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