Economic Benefits of Bt Maize Maize containing a transgenically expressed toxin originating from Bacillus thuringiensis (Bt maize) is planted across the United States to combat insect herbivory. Non-Bt Maize is also planted alongside Bt maize fields to provide refuges for the insects, which helps to prevent resistance to Bt maize from evolving. Hutchison et al. (p. 222 ; see the Perspective by Tabashnik ) analyzed how Bt maize affected the economic impact of the European corn borer moth in the midwestern United States, as well as its population dynamics. Larval density, a predictor of corn borer population size, has dropped in correlation with the percentage of Bt maize planted. In the highest Bt maize producing state, the positive effects of Bt maize in controlling insect herbivore populations extended to non-Bt maize. Furthermore, the decrease in insect populations demonstrated an overall economic benefit outweighing the overall extra costs associated with planting Bt maize.

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The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean-growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field-testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north-central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid-resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid-resistant soybean, and thus a strategy for optimal deployment of aphidresistant soybean is needed to ensure sustainability of this technology.

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During August of the 2000 growing season, Aphis glycines Matsumura, an aphid native to Asia, was found colonizing soybean plants in Illinois. Accepted for publication 1 February 2001. Published 5 February 2001.

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the north central United States. The effectiveness of crop rotation for management of corn rootworm has begun to fail in many areas of the midwestern United States, thus new management strategies need to be developed to control rotation-resistant populations. Transgenic corn, Zea mays L., effective against western corn rootworm, may be the most effective new technology for control of this pest in areas with or without populations adapted to crop rotation. We expanded a simulation model of the population dynamics and genetics of the western corn rootworm for a landscape of corn; soybean, Glycine max (L.); and other crops to study the simultaneous development of resistance to both crop rotation and transgenic corn. Results indicate that planting transgenic corn to first-year cornfields is a robust strategy to prevent resistance to both crop rotation and transgenic corn in areas where rotation-resistant populations are currently a problem or may be a problem in the future. In these areas, planting transgenic corn only in continuous cornfields is not an effective strategy to prevent resistance to either trait. In areas without rotation-resistant populations, gene expression of the allele for resistance to transgenic corn, R, is the most important factor affecting the evolution of resistance. If R is recessive, resistance can be delayed longer than 15 yr. If R is dominant, resistance may be difficult to prevent. In a sensitivity analysis, results indicate that density dependence, rotational level in the landscape, and initial allele frequency are the three most important factors affecting the results.

Observations of fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), larvae infesting plots of Miscanthus x giganteus Greef and Deuter ex Hodkinson and Renvoize prompted laboratory-based tests of survival, development, and feeding preferences on leaf tissue from M. x giganteus and switchgrass, Panicum virgatum L. Survival from hatch to pupation was >70 and 50% for fall armyworms reared on switchgrass and M. x giganteus, respectively, although survival of the S. frugiperda rice strain was significantly greater than the corn strain on both crops. Developmental times from hatch to pupation or adult emergence showed effects of crop and S. frugiperda host strain, but analysis of an interaction revealed developmental times for the rice strain were similar on both crops, whereas corn strain larvae showed delayed development on M. x giganteus relative to switchgrass. Analysis of larval (10 d) and pupal masses showed a similar pattern, with effects of crop and an interaction (at 10 d), but only the mass of corn strain larvae feeding on M. x giganteus was reduced relative to the other crop and strain combinations. In choice tests, neonates of both corn and rice strains showed a strong preference for feeding on young tissues rather than mature leaves of M. x giganteus or switchgrass, but they also clearly favored corn, Zea mays L., leaves over either of the perennial grasses. Results indicate both plants are potential hosts for S. frugiperda, but additional information is needed to understand under which scenarios and to what degree fall armyworms may damage perennial grasses grown for biofuel production.

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