February 2020 - 13

MSUE VEGETABLE NOTES
Avoid 'wormaggedon' with Bt sweet corn
By Ben Phillips
MSUE Vegetable Educator
In 2019, the Midwest was
hammered with spring and early
summer rains that delayed or
prevented massive acres of field corn
from being planted. Many sweet
corn growers took their chances and
planted more or less on schedule
into less-than-ideal field conditions.
As a result, sweet corn was present
in the landscape at a much higher
ratio than in a usual year. Field corn
plantings did catch up in some areas,
but this still left a full month window
where caterpillar pests were primarily
finding sweet corn.
This took many growers off guard
with spray programs, and by July
some fields in Michigan were over
90% infested with corn earworm and
European corn borer. The growers
I know who invest in Bacillus
thuringiensis (Bt)-traited sweet corns
reserve them for the latest plantings,
to help when moth flights are at their
peak. But, if they had chosen to plant
Bt sweet corn from the start, would
this have helped? If there was ever a
year to try it, 2019 would have been a
good one.
How does it work?
Genetically engineered field
corn varieties have worked well on
caterpillar pests for many years by
using a range of Bt proteins, which
kill the caterpillars when they eat
the plant. Compared to field corn,
there are fewer sweet corn varieties
with these traits, and they came to
the table well after introduction of
field corn varieties. Currently, there
are five Bt proteins bred into sweet
corn to control caterpillars: Cry1Ab,
Cry1A.105, Cry2Ab2 and Vip3A.
Think of these proteins as the active
ingredient of a sprayable pesticide.
These traits are sometimes inserted
into plant genomes singly, and other
times they are " stacked. " Stacking
traits is a genetic technique similar to
tank-mixing pesticides. They feature
proteins with slightly different ways
of working, but all Bt proteins create
a crystal-shaped
structure in the
presence of high
pH. A caterpillar's
gut has a high pH,
unlike a human's
acidic gut. The
crystals lacerate
the digestive tract
of the caterpillar
and kill it.
Sweet corn growers must pay
BEN PHILLIPS
attention to the bigger picture of Bt
traits in other crops to understand
pest dynamics with these varieties.
Sweet corn genetic traits lag behind
field corn trait packages, and no
assumptions should be made about
their effectiveness against caterpillars
just because they are genetically
engineered like the field corns.
Resistance to resistance
In a biological arms race when
pesticides are overused, the target
can develop counter-resistance to the
control. In the case of Bt corn, some
caterpillars have been overcoming
the Bt traits with their own counterresistance.
Due to the sheer numbers
of acres of field corn planted, most
resistance to Bt proteins is happening
in field corn. However, cotton has
been bred with some of the same Bt
proteins as in corn, and the double
exposure to the same type of control
has essentially sped up the adaptation
to Bt proteins in corn earworm
because they also feed on cotton
(cotton boll worm).
Resistance is also tied to geography.
Pest populations may not be
overcoming Bt proteins everywhere,
but throughout the summer northern
regions of the country " inherit " many
insects from the southern regions
throughout the summer, and sometimes
also their ability to overcome Bt traits.
The effectiveness of Bt-traited sweet
corns in northern regions can change
literally overnight depending on the
direction of a storm front.
Protein breakdown
Cry1Ab is effective on European
corn borer but has never been
effective on western bean cutworm.
It is moderately effective on fall
armyworm and corn earworm.
Field corn with this single trait is
not marketed for control of corn
earworm or fall armyworm, but sweet
corn still is. To slow down counterresistance
in pest populations the
field corn market is moving away
from single-traited varieties.
Cry2Ab2 is always paired with
Cry1A.105 and is effective on European
corn borer and fall armyworm. It is
moderately effective on western bean
cutworm, and in some places is no
longer effective on corn earworm. There
are reports of corn earworm resistance
in North Carolina field corn, and sweet
corn in Maryland.
At this point, Vip3A is the only
effective protein on corn earworm,
Western bean cutworm and fall
armyworm.
Additionally, there is another
protein used in field corn, called
Cry1F. Cry1F by itself was recently
found to be no longer effective on
European corn borers in eastern
Canada field corn, fall armyworm in
southern U.S. field corn, and western
bean cutworm in field corn fields all
over North America. I mention this
to illustrate that there is resistance to
other single-trait proteins than what
is available to sweet corn growers.
What's in your corn?
There are 25 sweet corn varieties
available with Bt traits as a part of
the Seminis Performance Series, and
Syngenta Attribute I, Attribute II,
and Attribute Plus Series. Seminis
Performance Series corn have
Cry1A.105 + Cry2Ab2. Syngenta
Attribute I Series corn have Cry1Ab.
Syngenta Attribute II Series corn
have Cry1Ab + Vip3A. No sweet corn
varieties contain Cry1F. You can view
all the varieties on the Handy Bt Trait
Table for Sweet Corn here: https://
www.texasinsects.org/bt-corn-traittable.html.
Knowing
the situation with
resistance popping up in all proteins
but Vip3A, my bet would be that
if at least one Attribute II series
sweet corn had been planted earlier
in 2019, losses in Michigan would
have been less. However, given the
larger trends in other crop systems,
counter resistance will eventually
appear in Vip3A as well. If you are
wondering whether your sweet corn
varieties could be experiencing
counter-resistance by caterpillars,
be sure to contact your seed
representative and Extension service
for consultation. VGN
Siegers...more than seed
Midseason, 74 days. Resistance: Aal(IR)
Fol1,2(IR), Mi(IR), Ss(HR)
MARIANA
, Vd1(IR)
Extra-large
saladette extremely uniform in shape.
Thick fruit walls, exceptional shelf life.
Jointed with uniform green shoulders.
Very firm.
GRAND VANTAGE
,
80-85 days. Resistance: Foc1(HR)
Attractive green to blue-green color with
very round heads, dense solid interior and
a short core. It has excellent tight wrapper
leaves, a strong frame, good holding
ability and excellent yield potential.
MERCER
72-78 days. Resistance: Pc(IR)
,TMV0(HR)
Xcv0-3,7,8(HR) Well suited for production in
the Northeast where Phytophthora Capsici
is a problem. Plants are strong with good
cover and produce high quality large to
extra-large smooth fruit.
,
XTREME VANTAGE
80 days. Resistance: Foc1(HR)
, Tip Burn(T)
Xcc(IR) Large, round, blue-green heads
with very good uniformity in the field.
Nice disease package.
(800) 962-4999  www.siegers.com
SIEGERS
,
EED CO.
VGN | FEBRUARY 2020 | 11
http://https:// http://www.texasinsects.org/bt-corn-trait http://www.siegers.com
February 2020

Table of Contents for the Digital Edition of February 2020
