The following articles highlight positive externalities associated with Bt corn and cotton production :
Communal Benefits of Transgenic Corn. Bruce E. Tabashnik Science 8 October 2010:Vol. 330. no. 6001, pp. 189 - 190DOI: 10.1126/science.1196864
"Bt corn planted near non-Bt corn can provide the unmodified plants with indirect protection from pests"
Areawide Suppression of European Corn Borer with Bt Maize Reaps Savings to Non-Bt Maize Growers. Science 8 October 2010:Vol. 330. no. 6001, pp. 222 - 225 DOI: 10.1126/science.1190242W. D. Hutchison,1,* E. C. Burkness,1 P. D. Mitchell,2 R. D. Moon,1 T. W. Leslie,3 S. J. Fleischer,4 M. Abrahamson,5 K. L. Hamilton,6 K. L. Steffey,7, M. E. Gray,7 R. L. Hellmich,8 L. V. Kaster,9 T. E. Hunt,10 R. J. Wright,11 K. Pecinovsky,12 T. L. Rabaey,13 B. R. Flood,14 E. S. Raun15,
"Cumulative benefits over 14 years are an estimated $3.2 billion for maize growers in Illinois, Minnesota, and Wisconsin, with more than $2.4 billion of this total accruing to non-Bt maize growers."
A Meta-Analysis of Effects of Bt Cotton and Maize on Nontarget Invertebrates.Michelle Marvier, Chanel McCreedy, James Regetz, Peter Kareiva Science 8 June 2007: Vol. 316. no. 5830, pp. 1475 – 1477