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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A collaborative research effort by scientists in several states and in Canada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen on its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating exposure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that only a portion of the monarch population utilizes milkweed stands in and near cornfields, and the current adoption rate of Bt corn at 19% of North American corn-growing areas. This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.