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Transgenic Corn Advances
Ron Meyer, Area
Extension Agent - Golden Plains Area
Date: 11/23/2011
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Advances in transgenic
technology and insect resistance management are occurring at a rapid pace, and
it can be hard to keep current. There are now more than 23 Bt transgenic corn
products in more than five product lines that are resistant to certain insect
pests. The mix of traits, target pests, and resistance management requirements
vary from product to product and can be quite confusing. To help farmers keep
track of the changes to make the best hybrid selections for their farm, Chris
DiFonzo (Michigan State University) and Eileen Cullen (University of
Wisconsin-Madison) developed a table (Click Here to View the Table) that
presents a recent update of transgenic traits for corn. There are several items
to take note of in the trait table.
Insect Resistance. Six species of
caterpillar (European corn borer, western bean cutworm, black cutworm, corn
earworm, stalk borer, and fall armyworm) are controlled or suppressed by Bt
corn products. Suppressed means that while some of the pest population may be
killed by the trait, a significant number will likely survive. If you are
expecting a specific pest problem, choose a product that is labeled to control
that specific pest, not suppress it. These Bt products target caterpillars,
although not all products control the same group of caterpillars.
Beetle-specific transgenic products target corn rootworm larvae; some products
target both caterpillars and corn rootworm larvae.
Herbicide Tolerance.
Herbicide tolerance also varies among products. Some are only glyphosate or
Roundup Ready tolerant, some are only Liberty Link/glufosinate tolerant, and
some are tolerant to both Liberty Link/glufosinate tolerant and glyphosate or
Roundup Ready. A few products only have some hybrids within the product line
that are Roundup Ready/glyphosate tolerant, and one is not herbicide tolerant.
Every year corn fields are mistakenly sprayed with an herbicide that they are
thought to be tolerant of. Be careful with hybrid selection and remember
exactly what traits your hybrid has and where you planted it.
Refuges
for Resistance Management. Insect resistance management requirements are
continuing to evolve. Currently, two basic types of refuge are required –
structured and non-structured. A structed refuge refers to the 5% or 20% non-Bt
corn plantings farmers are required to plant within, adjacent to, or within
½ mile of the Bt corn field. Non-structured refuge refers to the
inclusion of a certain percent of non-Bt corn seed in a bag of Bt corn seed
(currently 5-20%). This is often called refuge-in-a-bag (RIB), and takes the
responsibility of planting a separate refuge out of the hands of the
farmer.
These differences in refuge size and location can be confusing
and beg the question “Why”? Differences in refuge requirements are
based on the biology, behavior, and genetics of the pest, the pest’s
relationship to the crop, and the relative toxicity of the toxin or toxins
within the plant.
Refuge Size. For example, refuge size depends largely
on the probability of resistance developing. The initial transgenic corn
hybrids resistant to European corn borer contain one gene that code for the
production of one toxin that targets corn borers (e.g. Cry1Ab). This toxin
interacts with a specific receptor site in the insect gut. Some of the newer
transgenic hybrids contain two genes that code for two different toxins (e.g.
Cry1F, Cry1Ab) that interact with two different receptor sites in the insect
gut. Either one of these toxins can kill the corn borer. If an insect develops
resistance to one of the toxins, it would still likely be susceptible to the
other. European corn borers are less likely to develop resistance in corn
fields planted with Bt corn that produces two different toxins targeting corn
borers than in corn that produces only one corn borer specific toxin.
Therefore, the refuge can be smaller for fields planted to corn that produces
two toxins targeting corn borers.
Refuge Location. Another example is
the difference in refuge proximity to the Bt corn field. For lepidopteran
resistant hybrids – those resistant to corn borer larvae – the refuge
may be up to ½ mile away; however, for coleopteran resistant hybrids
– those resistant to corn rootworm larvae – the refuge must be in or
adjacent to the Bt corn field. In this case, the mobility of the adult insect
is the primary reason for the difference. Before they mate, female corn borer
moths fly around more than female corn rootworm beetles, so the refuge can be
further away for the corn borer resistant hybrids than for the corn rootworm
resistant hybrids.
Long-term Value. Many biological, behavioral,
genetic, toxicological, and other factors are examined and weighed before
specific insect resistance management requirements are established. As we learn
more and as new products are developed, insect resistance management will
continue to evolve.
Although it does make corn hybrid selection more
complex, resistance management requirements will help reduce the chance of
insect resistance developing and help maintain the efficacy of these products
well into the future.
Source: Tom Hunt, University of Nebraska Extension
Entomology. Haskell Agricultural Laboratory, Concord and Robert Wright
University of Nebraska Extension Entomologist, Lincoln
Transgenic Corn
Table