1. Stacked Bt Corn: Efficacy and Potential Implications for Bollworm Control in Cotton S. D. Stewart (UT Extension), B. R. Leonard (LSU AgCenter) & F. Musser (Mississippi State University) Beltwide Cotton Conferences (Nashville, TN)

2. Relevant Bt Traits Corn Cotton YieldGard® = Cry1Ab Herculex® = Cry1F
YieldGard VT Pro® = Cry2Ab + Cry1A.105
Anticipated registration in late 2008 or 2009
Cry1A.105 is a hybrid protein of Cry1F and Cry1A
Activity spectrum of hybrid is unclear but provides better control of corn earworm than either parent protein alone (in part because of higher expression in plant)
Cotton
Bollgard® = Cry1Ac
WideStrike® = Cry1Ac + Cry1F
Bollgard II® = Cry1Ac + Cry2Ab

3. YieldGard VT Pro Refugia Requirements
Proposed refuge plan for the Corn Belt = 95:5
Currently 80:20
Proposed refuge plan for the Cotton Belt = 80:20
Currently 50:50
Corn growers have been "penalized" by refuge requirements in areas with high corn borer populations
Attributes and Potential Benefits
Extremely effective control of corn borers
Increased efficacy on corn earworm (i.e. bollworm), fall armyworm and other less common caterpillars
Dual toxin may prevent or delay resistance to Bt corn in corn borer populations

4. Southwestern Corn Borer Tunnel Length per 20 Plants and Yield
a a a b 50
100
150
200
250
300
350
400
450
500
Tunnels (cm)
1.2
1.6
0.8
479
Bushels/Acre
151
165
182
130
VT Pro
YieldGard
Herculex
Non Bt
Musser (2006), Mississippi State University

5. Background - Corn Earworm
YieldGard and Herculex traits have low impact
Delay in maturation; Some mortality (≈ 20-50%)
Relatively low selection pressure
Bollgard has moderate impact (≈ 65% mortality)
BG II/WideStrike have high impact (≈ 90% mortality)
But not generally considered a "high dose"
No refugia requirement
Laboratory identified resistance genes in corn earworm have been dominant traits
Or at least not single gene, recessive traits
Recent, hotly debated claims of field resistance to Cry1Ac (B. Tabashnick)
Associated fitness costs with resistance (W. Moar)

6. Corn Earworm Larval Counts and Damage to Earsb c a ab
Stewart (2007), UT Extension

7. Corn Earworm Relative Kernel Damage at Harvest (%)b a
* Natural infestations of SWCB inflated damage in non-Bt hybrid

8. Summary for Corn
YieldGard VT Pro significantly reduced corn earworm populations compared with YieldGard or Herculex
≈ 85-95% Mortality (however, there was no direct measure of mortality or moth production)
Does this represent a "high dose"?
No evidence that better control of corn earworms increased corn yields despite relatively high pressure
Because of corn borers, corn growers in some areas could financially benefit from adoption of YieldGard VT Pro technology
Assuming approval of reduced refuge size

9. Observations About Bollworm
A high percentage of the bollworm population is "funneled" through corn
In the Midsouth, an even higher percentage during May and June
C3/C4 analyses of moths indicate that C4 plants (corn and other grasses) are the primary source of bollworms
Cotton and soybean are key and competing late summer hosts in the South
There is some potential for reverse, fall migration from the North to the South
F. Gould et al., PNAS, :
Corn Belt may be important refuge for bollworm

10. Bollworm Population Dynamics (Southeast Scenario)
Estimated moth production in North Carolina
based on hypothetical acreage
Host
Acres
Moths/Acre*
Moths in State
% of Total
Alfalfa
20,000
2,000
40,000,000
0.42
Corn
850,000
6,000
5,100,000,000
52.97
Cotton (non Bt)
50,000
1,800
90,000,000
0.93
Cotton (Bt-2)
800,000 10
8,000,000
0.08
Peanuts
100,000
3,400
340,000,000
3.53
Sorghum
12,000
1,000
12,000,000
0.12
Soybean
1,300,000
3,100
4,030,000,000
41.86
Other hosts
200,000 40
Total
3,332,000
------
9,628,000,000
100
* Gustafson, D.I. & G.P. Head, 2005 (application to amend BG II registration)

11. Bollworm Population Dynamics (Midsouth Scenario)
Estimated Moth production in Mississippi
based on hypothetical acreage
Host
Acres
Moths/Acre*
Moths in State
% of Total
Alfalfa
1,000
2,000
2,000,000
0.02
Corn
850,000
12,000
10,200,000,000
80.09
Cotton (non Bt)
50,000
1,800
90,000,000
0.71
Cotton (Bt-2)
600,000 10
6,000,000
0.05
Peanuts
20,000
3,400
68,000,000
0.53
Sorghum
180,000
360,000,000
2.83
Soybean
2,000,000,000
15.70
Other hosts
250,000 40
10,000,000
0.08
Total
3,951,000
-----
12,736,000,000
100
* Gustafson, D.I. & G.P. Head, 2005 (application to amend BG II registration)

12. Implications - Bollworms in Cotton
Widespread adoption of stacked Bt corn traits will reduce the magnitude of bollworm flights observed in cotton
YieldGard VT Pro, Bollgard II and WideStrike cause more mortality than the original Bt technologies
More selection pressure
Do these new technologies represent a high dose?
If not, can we effectively manage resistance with a realistically sized refuge?
Will bollworm resistance be a single gene, recessive event?
Is there really an IRM value of two Bt traits for corn borers
What other Bt traits are under development?
Do we know enough to predict how new Bt corn traits will influence IRM …… if not, then what?

13. THE END
Not proposing alterative IRM plans, but some discussion is warranted