Abstract
Simple SummarySince the 1990s, an important innovation in the management of agricultural pest insects has been the commercial cultivation of genetically engineered crops that produce insecticidal toxins, which in turn act to protect plants from feeding injury by insects. To date, these transgenic crops, which include cotton, maize and soybean, have produced insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt). Benefits associated with planting of Bt crops include reduced feeding injury from pest insects, decreased yield losses from pests and less harm to the environment. However, the evolution of Bt resistance by insect pests can diminish these benefits. One serious insect pest currently managed with Bt maize is the western corn rootworm. The larval stage of this insect feeds on maize roots and can substantially reduce yield. In some parts of the US Corn Belt, western corn rootworm rapidly adapted to Bt maize, and currently, some populations show resistance to all commercially available Bt traits. This review summarizes the time course of resistance development in the field, key factors contributing to resistance evolution, and steps that biotechnology companies, farmers and regulatory agencies can take to delay additional cases of pest resistance to current and future transgenic technologies.The western corn rootworm, Diabrotica virgifera virgifera LeConte, is among the most serious pests of maize in the United States. Since 2003, transgenic maize that produces insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) has been used to manage western corn rootworm by killing rootworm larvae, which feed on maize roots. In 2009, the first cases of field-evolved resistance to Bt maize were documented. These cases occurred in Iowa and involved maize that produced Bt toxin Cry3Bb1. Since then, resistance has expanded to include other geographies and additional Bt toxins, with some rootworm populations displaying resistance to all commercially available Bt traits. Factors that contributed to field-evolved resistance likely included non-recessive inheritance of resistance, minimal fitness costs of resistance and limited adult dispersal. Additionally, because maize is the primary agricultural crop on which rootworm larvae can survive, continuous maize cultivation, in particular continuous cultivation of Bt maize, appears to be another key factor facilitating resistance evolution. More diversified management of rootworm larvae, including rotating fields out of maize production and using soil-applied insecticide with non-Bt maize, in addition to planting refuges of non-Bt maize, should help to delay the evolution of resistance to current and future transgenic traits.
Highlights
Planting of transgenic maize that produces insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) has played a prominent role in the management of western corn rootworm, Diabrotica virgifera virgifera LeConte, for nearly two decades
The goals of this paper are to review the time course of field-evolved resistance to Bt maize by western corn rootworm, discuss the factors associated with the evolution of resistance, and consider how the management of western corn rootworm could be improved for current and future transgenic technologies
The presence of minimal fitness costs of resistance favored rapid resistance evolution. It appears that adult western corn rootworm engages in limited dispersal within the agricultural landscape, and this likely contributed to field-evolved resistance in multiple ways
Summary
Planting of transgenic maize that produces insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) has played a prominent role in the management of western corn rootworm, Diabrotica virgifera virgifera LeConte, for nearly two decades. The release of the first transgenic events for management of corn rootworm followed several years of successful management of some other key insect pests of maize and cotton [1,2]. Within six years of the initial release of Bt maize targeting western corn rootworm, the first cases of field-evolved Bt resistance were observed [3]. In the United States, annual economic losses associated with corn rootworm, including both management costs and yield losses, range between $1 to $2 billion [12]. Fields that are planted to maize for several consecutive years provide the ideal habitat for this pest, and can be associated with large populations of western corn rootworm and high levels of larval feeding injury [9]
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