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) 2008 Beltwide Cotton Conferences (Nashville, TN)

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

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

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

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)

Corn Earworm Larval Counts and Damage to Ears b c a ab Stewart (2007), UT Extension

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

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

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, 2002 99: 16581-16586 Corn Belt may be important refuge for bollworm

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)

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)

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?

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