Bt Sweet Corn:

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Bt Sweet Corn Options, Effectiveness, and
Management
Galen P. Dively Department of Entomology
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Attribute Insect Protected Bt Sweet Corn
Syngenta Seeds
RISPENS SEEDS, INC. RUPP SEEDS,
INC. SEEDWAY SIEGERS SEED CO.

• Expresses the Cry1ab insecticidal protein from
  Bacillus thuringiensis.
• Created by traditional breeding using event BT11
  in transgenic field corn.
• Expresses in all tissues, as in the case of Bt
  field corn.
• Silk and ear expression is generally higher than
  in field corn.

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Secreted VIP proteins
(Cry proteins)

• Naturally occurring soil organism.
• Used safely for more than 50 years.
• Limited use of foliar Bt insecticides.
• Breaks down rapidly, requires complete spray
  coverage, and does not kill all insect stages.
• Protein expression in the plant solves these
  problems.
• Non-toxic to humans

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TripleSweets
SE
GH0851 yellow (new in 2009)
Supersweets

• BC0805 (bicolor), WH0809 (white), GH0851
  (yellow), and BSS0982 (bicolor) are available in
  25M units.

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EUROPEAN CORN BORER
nonBt
Bt

• 100 protection against whorl, stalk and ear
  damage.
• Eliminates all ECB-targeted insecticide
  applications.

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CORN EARWORM

• Eliminates up to 80 of the insecticide
  applications

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FALL ARMYWORM

• Eliminates most whorl and tassel treatments. And
  significantly reduces sprays during silking.

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Attribute GSS0966 (Bt)
Prime Plus (non-Bt)
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• Does it pay to grow Bt sweet corn?
• Seeds cost in the range of 35-40 more per acre.
  
• BtSC results in cost savings, if you typically
  apply a whorl treatment and 3 or more
  applications during silking within a single crop
  season.
• Can significantly reduce the time that growers
  spend in managing insect pests, as well as reduce
  the exposure risks from handling and applying
  insecticides.

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Comparison of insect control programs in Bt and
nonBt plantings
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Comparison of ear protection in Bt and nonBt
plantings without insecticides
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• Are supplemental insecticides required in Bt
  sweet corn?
• Depends on the pest complex, population pressure,
  and marketable ear quality .
• European corn borer - no additional control is
  needed if the only ear-invading pest infesting
  sweet corn.
• Whorl infestations of fall armyworm - eliminates
  pre-silk sprays in most situations.
• Ear infestations of earworm and fall armyworm
  silk sprays are usually required to ensure
  acceptable ear quality when moth activity is
  high.

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• Provides excellent protection against larvae
  hatching and entering ears during fresh silking.
• Bt protein expression is highest in fresh, green
  silk tisuue.

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• Significant numbers of eggs are laid on wilted
  or brown silk tissue when moth activity is high.
• Earworms hatching within 10 to 14 days from
  harvest have a greater chance of surviving and
  invading the ear.
• - Degrading silk tissue expresses less
  active Bt protein.
• - Ear tip becomes more exposed.
• If larvae reach kernels, they are exposed to a
  mixture of Bt expression.
• Become intoxicated (sick), develop very slowly,
  and cause minor injury on the ear tip.

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• Cannibalistic behavior may allow more tolerant
  larvae to exposure to the Bt protein.
• Not uncommon to find 20-30 of the ears infested
  or showing minor tip damage, when moth activity
  is very high.

• Even higher levels of damage can occur in ears
  with poor tip coverage if plants are stressed by
  drought or high temperatures.
• Possible varietal differences in Bt expression
  and early stages of corn earworm resistance
  developing to the Bt protein.

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• Timing of supplemental insecticides in Bt hybrids
  is different from non-Bt sweet corn.
• Most surviving larvae enter the ear after fresh
  silking.
• Apply first supplemental spray at the ear zone
  at full silk (usually 3-4 days later than the
  timing of the first silk spray in non-Bt corn).
• Apply a second spray 3-4 days later if heavy
  moth activity continues sometimes a third
  treatment is necessary.
• Fall armyworms and western bean cutworms are
  more tolerant to the expressed protein however,
  supplemental sprays for corn earworm should
  control these caterpillars as well.

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Other sweet corn pests not affected by Bt sweet
corn
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Ear tip coverage can significantly influence the
infestation rate of sap beetles and caterpillars
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Effects of ear tip coverage on sap beetle damage
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• Recommendations for Managing Insect Pests in Bt
  Sweet Corn
• No control action required if European corn
  borer is the only pest.
• No whorl treatments unless fall armyworm
  pressure is very high.
• One to 3 silk sprays may be required to control
  earworms and fall armyworms in later plantings.
• Optimum timing to apply an insecticide is 100
  fresh silking repeat application 3 or 4 days
  later if moth pressure continues.
• For sap beetles, apply an insecticide when
  50-75 of the ears have wilted silks (coincides
  when adult beetles start to laid eggs).
• Maintain good growing conditions to ensure
  maximum expression of the Bt protein and ear tip
  coverage.
• Use hybrids with good ear tip coverage, such as
  BC0805.
• Control seed and seedling pests with an
  insecticide seed dressing.

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• Requirements for Growing Bt Sweet Corn
• Grower must sign Stewardship Agreement.
• Seed cannot be repackaged or resold.
• Stalks must be destroyed no later than 30 days
  following harvest, but preferably within 14 days,
  either by rotary mowing, discing or plowdown.
• Scout fields and judiciously use conventional
  insecticides to control other pests.
• Report any damage from target pests.
• http//www.rogersadvantage.com/pdf/AttributeGrower
  Guide.pdf

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Future Outlook - Bt Sweet Corn Technology
VIP3A Cry1Ab
More hybrids Other seed companies Herbicide
resistance genes Stacked genes
Cry1Ab
Non-Bt
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Bacillus thuringiensis Growth Characteristics
Stationary Phase No growth, sporulation and
vegetative cell death
Cry endotoxin produced as a crystalline protein
Lag Phase Increasing metabolism, slow growth
Logrithmic Phase Rapid vegetative cell growth
CELL NUMBERS
VIPs produced new discovery by Syngenta
TIME
B.t kurstaki Cry1Aa, Cry1Ab, Cry1Ac, Cry2Ab2,
Cry2B (Lepidoptera) B.t. aizawai - Cry1Aa,
Cry1Ab, Cry1C, Cry1D, Cry1F (Lepidoptera) B.t.
tenebrionis Cry3A, Cry34/35 (beetles), B.t.
israelensis (Diptera) Chimeric protein Cry1A.105
(containing domains of Cry1Ac and Cry1Fa)
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Bacillus thuringiensis Growth Characteristics
Stationary Phase No growth, sporulation and
vegetative cell death
Cry endotoxin produced as a crystalline protein
Lag Phase Increasing metabolism, slow growth
Logrithmic Phase Rapid vegetative cell growth
CELL NUMBERS
VIPs (vegetative insecticidal proteins) secreted
exotoxins no structural or sequence homology
different mode of action activation site not
homologous to that of Cry1 wide spectrum of
activity
TIME
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Stacked Genes vs. Key Pests
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Theoretical distribution of gene expression in
kernels (assuming that MIR162 and Bt11 are
dominant and show expression in the heterozygous
state)
Mir162 (Vip3A) A, Bt11 (Cry1Ab) B
Mir162/Bt11 9/16 A_B_ (56 of the
kernels) Mir162 3/16 A_bb (18 of the
kernels) Bt11 3/16 aaB_ (18 of the
kernels) NonBt 1/16 aabb (6 of the kernels)
instead of 25 nonBt kernels in the single event.
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• Summary
• Attribute Bt sweet corn ideally fits the IPM
  philosophy
• - host plant resistance with prescribed
  insecticides
• - different modes of action to reduce resistance
  risks
• - use of a reduced risk bioinsecticide
• Works best and results in greater cost-savings
  in late plantings.
• Insect control performance may vary under
  adverse growing conditions or very high insect
  pressure.
• Bottom line is that Bt hybrids help growers
  reduce sprays and control costs in an
  environmentally responsible way.