Issue No. 21, Article 1/August 14, 2009
Bt Soybeans: University of Georgia Results Examined
Since 1996 we have witnessed a steady increase, particularly in the United States, in the use of transgenic Bt crops for insect control. Because of the escalation of Bt corn hybrids and Bt cotton varieties that express Cry proteins, conventional insecticide applications have been reduced considerably for several key insect pests. Recently released estimates by the USDA Economic Research Service reveal that 59% of all corn planted in Illinois during 2009 was a "stacked" hybrid. In 2009, stacked cotton varieties were planted on 48% of the cotton acres in the United States. For a complete report, please go to www.ers.usda.gov/Data/BiotechCrops.
To date, Bt soybean varieties have not been commercialized; however, some transgenic Bt lines of soybean have been examined for their effectiveness against lepidopteran insect pests. In this month's issue of the Journal of Economic Entomology (Vol. 102, No. 4, pp. 1640-1648), entomologists Robert M. McPherson and Ted C. MacRae, Department of Entomology, University of Georgia, Tifton, reported on the effectiveness of Bt soybean lines against several species of defoliators. Their work was conducted in replicated field experiments over several growing seasons, 2003 to 2007. The Bt lines they examined were developed with Agrobacterium-mediated DNA techniques. All of the Bt soybean lines evaluated contained a copy of the Bt gene tic107 that resulted in the expression of a synthetic cry1A construct described as similar to Cry1Ac.
Bt soybean lines and their controls were examined for insect defoliation caused by velvetbean caterpillars, soybean loopers, and green cloverworms. The entomologists reported that densities of these defoliators were negligible in the Bt soybean lines each year of the study. However, in the controls, which consisted of isogenic segregants and parental lines, insect densities ranged from 5 to 10 larvae to 20 to 30 larvae per meter row. Defoliation levels in the control treatments (non-Bt lines) ranged from 53% (2003) to 17.5% (2007). The level of defoliation in the Bt soybean plots was near 0% (less than 1.5%). Yield comparisons between Bt and non-Bt treatments were similar throughout this multiyear investigation.
Will the future landscape of Illinois include stacked (herbicide-tolerant and Bt) soybean varieties? I suspect that this might be the case. The most immediate economic benefit regarding the commercialization of Bt soybean varieties will come to producers in the southeastern U.S., where lepidopteran pests reach economic thresholds more frequently. In the north-central U.S., the most common insect threats to soybean production are soybean aphids and Japanese beetles. These insects will not be affected by Bt soybean varieties that have been developed to provide control of lepidopterans. However, recall that the first commercial Bt corn hybrids also targeted lepidopterans, essentially the European corn borer. In 2003, Bt corn hybrids were commercialized for corn rootworms, a coleopteran species. So, in time, will we see Bt soybean varieties sold that express Cry proteins to control Japanese beetle populations? Perhaps.
As I have stated many times in the Bulletin and during some of my extension presentations, we are in the midst of an agricultural revolution dominated by exciting scientific discoveries and the commercialization of new transgenic products that have fundamentally altered the manner in which we manage pests in commercial agriculture.--Mike Gray