PerspectivePlant Science

Communal Benefits of Transgenic Corn

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Science  08 Oct 2010:
Vol. 330, Issue 6001, pp. 189-190
DOI: 10.1126/science.1196864

Genetically engineered crops represent one of the most controversial and rapidly adopted technologies in the history of agriculture. First grown commercially in 1996, transgenic crops covered 135 million hectares (ha) in 25 countries during 2009 (1). To reduce reliance on insecticide sprays, corn and cotton have been genetically engineered to make insecticidal proteins derived from the common bacterium Bacillus thuringiensis (Bt). These Bt toxins kill some devastating insect pests, but unlike broad-spectrum insecticides, they do little or no harm to most other organisms, including people (2). Many pests have rapidly evolved resistance to insecticides, however, spurring concerns that adaptation by pests could quickly reduce the efficacy of Bt crops and the associated environmental, health, and economic benefits (37). On page 222 of this issue, Hutchison et al. (8) rein in some of those concerns, documenting a landmark case in which Bt corn has remained effective against a major pest for more than a decade, yielding billions of dollars of estimated benefits to farmers in the midwestern United States.

Hutchison et al. describe Bt corn's suppression of the European corn borer (Ostrinia nubilalis), an invasive insect introduced into the United States in 1917. The caterpillars of this moth chew on leaves and tunnel in corn stalks. Before the advent of Bt corn, it caused losses of $1 billion per year in the United States (8). However, susceptible caterpillars of this pest do not survive on Bt corn that produces toxins active against the larvae of Lepidoptera (moths and butterflies) (8).

Halo effect.

Bt corn planted near non-Bt corn can provide the unmodified plants with indirect protection from pests. European corn borer moths lay eggs indiscriminately on Bt corn and non-Bt corn, but their caterpillars survive and become moths only on non-Bt corn. (A) With only non-Bt corn, moths move between plants and lay eggs, and caterpillars damage plants. (B) Moths do not emerge from Bt corn plants, reducing the number of eggs and subsequent damage on non-Bt corn near Bt corn. Yield is highest for Bt corn (B), lowest for non-Bt corn (A), and intermediate for non-Bt corn near Bt corn (B).

CREDIT: P. HUEY/SCIENCE

Although it is not surprising that planting millions of hectares of Bt corn reduced the damage caused by the European corn borer, Hutchison et al. discovered that most of the economic benefits from 1996 to 2009 were associated with planting corn that does not make Bt toxins (i.e., non-Bt corn). This somewhat counterintuitive result arises from three facts: Farmers paid a premium of about $10 to $20 per ha for Bt corn seed; on average, non-Bt corn was more abundant than Bt corn; and pest populations decreased dramatically on non-Bt corn plants.

The suppression of pests on non-Bt plants near Bt plants—called the “halo effect”—was predicted on theoretical grounds by Alstad and Andow in 1996 (3). The halo effect occurs with European corn borer because females lay eggs indiscriminately on Bt and non-Bt corn, and the caterpillars hatching on Bt corn die (8) (see the figure). If Bt plants account for a substantial percentage of the available host plants, regional pest populations can be greatly reduced, resulting in less damage to non-Bt plants. Although the halo effect was seen before (6, 9, 10), Hutchison et al. are the first to report an economic analysis of this phenomenon based on large-scale, long-term data. They demonstrate that planting non-Bt corn pays off because farmers avoid the extra cost of Bt seed, yet still get some pest control benefits generated from neighboring Bt corn.

The immediate economic gains farmers reap from planting non-Bt corn could boost their compliance with the “refuge” strategy, which is designed to delay the evolution of pest resistance to Bt corn (35). In the United States, the Environmental Protection Agency (EPA) requires farmers to plant refuges of non-Bt corn near Bt corn (2, 11). Refuges promote the survival of susceptible insects to mate with resistant insects that survive on Bt corn. If inheritance of resistance is recessive, the hybrid progeny from such matings will die on Bt crops, substantially slowing the evolution of resistance. This approach works best if the dose of toxin ingested by insects on Bt plants is high enough to kill all or nearly all of the hybrid progeny (12). Moreover, as the toxin dose increases, so too does the magnitude of resistance required for survival on Bt plants (13). Whereas mutations providing small decreases in susceptibility to Bt proteins are relatively common, those conferring sufficient resistance to enable survival on some types of Bt corn are exceedingly rare in the European corn borer (14, 15).

Although refuges have probably helped Bt crops remain effective longer than expected in most cases, some populations of at least four major pests have evolved resistance to Bt crops (12, 13, 16). Analyses of global resistance monitoring data suggest that the evolutionary principles underlying the refuge strategy can explain why some pest populations have evolved resistance faster than others (7, 12). In each case of field-evolved resistance to Bt crops, the high-dose standard was not met, refuges were scarce, or both (7, 12).

New tools to combat pest resistance to Bt crops include a wider array of toxins, including toxins genetically modified to counteract resistance (17). Also, to thwart resistance, plants that produce two or more distinct Bt toxins targeting the same pest are becoming increasingly important. For example, a type of Bt corn registered in the United States in 2009 produces five distinct Bt toxins; three of these target caterpillar pests including European corn borer and two kill corn rootworm beetles (Diabrotica species) (11). Whereas the EPA had previously required non-Bt corn refuges planted in separate fields, rows, or strips, in April 2010 it approved sales of mixtures of corn seeds with and without Bt toxins that kill corn rootworms (11). This seed mixture approach ensures that farmers comply with the refuge strategy, and may be especially useful on small farms in developing countries where planting separate refuges is not practical. With the shift to seed mixtures and multitoxin Bt corn, the EPA has dropped the minimum percentage of corn that farmers must plant in non-Bt corn refuges from 20% to as little as 10% (seed mixtures) or 5% (multitoxin plants) (11). No one knows how fast insects will adapt to Bt corn under these new conditions. As we scramble to stay one step ahead of the pests, let's keep in mind the ground-breaking report by Hutchison et al. affirming the adage that diversity breeds success.

References and Notes

  1. U.S. Environmental Protection Agency, Current & Previously Registered Section 3 PIP Registrations. www.epa.gov/pesticides/biopesticides/pips/pip_list.htm (accessed 26 August 2010).
  2. I thank the U.S. Department of Agriculture–National Institute of Food and Agriculture program for support.
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