NC_OLD205: Ecology and Management of European Corn Borer and Other Stalk-Boring Lepidoptera
Statement of Issues and Justification
More than 80 million acres of field corn (Zea mays L.), worth more than $20 billion, is annually grown for grain in the United States. European corn borer [Ostrinia nubilalis (H|bner)] alone, among several stalk-boring pests, accounts for more than $1.85 billion in control costs and grain losses to corn growers each year. This commodity provides feed for livestock and is used as a source of raw products in the production of foods and fuels. Second only to the corn rootworm (Diabrotica spp.) in economic importance on all types of corn, European corn borer also attacks several other important crops, namely, sorghum, small grains, cotton, potatoes, snap beans, peppers, and soybeans.The development and commercialization of Bacillus thuringiensis (Bt) transgenic corn in the past few years, specifically targeted for European corn borer, has greatly changed the paradigm for managing corn pests. One of the most significant concerns associated with this new technology is the possibility of insects developing resistance to Bt. Resistance development could threaten the future use of Bt crops, not only as a highly effective pest management alternative but also as a biological insecticide for conventional insect control. With transgenic Bt corn arising as a totally different integrated pest management (IPM) strategy, there is a critical need for new information relevant to use of this tactic. Although pertinent data applicable to pest management of stalk-boring insects has been acquired through this regional project in the past, this new paradigm has created a demand for basic economic, biological, and ecological information for European corn borer and other associated pests. This information is needed to develop models to predict resistance development and strategies for the management of resistance. Also, questions and concerns have recently surfaced on the impact of transgenic crops on beneficial organisms, such as natural enemies, and nontarget insects, including the monarch [Danaus plexippus (L.)].
We propose to address significant insect management issues associated with the adoption of transgenic crops and approaches to resistance management, especially the requirement for economic assessments, the need for more basic biological and ecological information, and the demand for information on natural enemies and other nontarget organisms. The goal of the project is to develop information leading to enhanced IPM options for corn production that economically benefit producers while minimizing environmental impact.
JUSTIFICATION:
Stalk-boring insects form a complex of pests that significantly reduce yields in field corn and other crops. European corn borer infests more than 200 species of plants, including peppers, snap beans, potatoes, sorghum, cotton, small grains, soybeans, apples, and onions (Mason et al. 1996). Other significant stalk-boring pests include stalk borer [Papaipema nebris (Guenie)], hop vine borer [Hydraecia immanis (Guenie)], potato stem borer [Hydraecia micacea (Esper)], and southwestern corn borer (Diatraea grandiosella Dyar).
Of these stalk-boring species, European corn borer has the greatest impact on corn production in North America, causing $1.85 billion in crop loss annually (Calvin 1995, Ostlie et al. 1997; this figure was modified from the cited publications by using current yields and price structures). In Iowa, a major corn-producing state, losses from European corn borer injury were estimated at 5% when current control methods (insecticides) were applied and 12.4% when no insecticides were applied. Annually, this pest costs Iowa farmers between $15 to $50/acre, even when insecticides are used.
Prior to 1996, management of European corn borer was limited to the use of rescue treatments with insecticides when other tactics (cultural, genetic, and biological) failed to maintain populations below economic levels. The use of insecticides to protect the yield of corn was only cost-effective in a few cornfields, with the highest levels of insecticide use occurring in the western Corn Belt and mid-Atlantic states where corn frequently experiences environmental stress. In many corn production areas, losses from European corn borer were accepted by growers because insecticide applications did not provide long enough residual control to be cost-effective, scouting to properly time applications was difficult and time-consuming, and improper timing of applications resulted in reduced efficacy of the products (Calvin et al. 1988, Bode and Calvin 1990).
With the introduction of transgenic corn hybrids containing the gene that enables corn plants to manufacture the crystalline protein toxin of the bacterium B. thuringiensis subsp. kurstaki, a new highly effective and relatively cheap control alternative for European corn borer was made available to corn producers. This technology lowered the economic threshold for the pest, increasing the percentage of fields that would benefit from European corn borer control from ~12 to 80% (Calvin 1995). For the first time, farmers possessed the technological capability to protect 95 to 100% of the 6 to 7% yield loss (on average) that had been caused by this pest. Because the toxins in the corn plant provide season-long protection against the pest, this technology also eliminated the requirement to scout fields or to use predictive models to optimally time insecticide applications.
During 1999, ~25% of all corn hybrids planted in the Unites States contained a Bt toxin gene (P. Davis, personal communication). Seed technology companies project that adoption levels may reach 50% by early in the next millennium. In addition, these companies are developing hybrids with stacked genes possessing multiple modes of action and that can control several species of pests. These new hybrids may enter commercial markets within the next 5 years. There is potential to select for evolution of resistance to these crystalline protein toxins in European corn borer populations because farmers can now justify treating up to 80% of their corn acreage.
Research conducted by this committee has been used to develop models predicting the rates of resistance evolution and to investigate the role of refuge structure in preventing or minimizing resistance evolution. These models indicate that a minimum refuge size of 20% is required to slow resistance development in this species. Although these models were constructed based on the best information available on the pest's biology and known population genetic relationships, a number of assumptions about pest biology were required for the simulations to be completed. In addition to addressing information gaps needed to improve these models, information is needed on nontarget impacts of Bt corn toxins. Eliminating information gaps forms the basis for several objectives of the project.
Economics. As noted, economic damage resulting from European corn borer infestations totals more than $1.85 billion in yield loss and control costs each year in the United States (Calvin 1995, Ostlie et al. 1997). Many new economic issues have arisen because of the commercial introduction of Bt corn. These issues include technology adoption, cost differences between alternative refuge configurations, and a desire to identify economic incentives that encourage growers to comply with refuge requirements. Farmers, as business owners, will make these decisions by balancing expected costs and benefits. However, because European corn borer behavior and agricultural practices differ across the Corn Belt, regional analyses are required to obtain a complete understanding of producer behavior, the factors that drive this behavior, and the policies that may modify this behavior in positive ways.
Ecology and Genetics. During the past 10 to 15 years, U.S. agriculture has been developing and implementing transgenic corn varieties that express genes originating from B. thuringiensis, alternative cropping practices, biological control, and landscape-level planning to achieve effective insect pest management. These developments are transforming management strategies of stalk-boring pests of corn. Region wide research and extension efforts are necessary if benefits are to be optimized. The dramatic management of insects via transgenic plants, however, has many scientists (and growers) concerned about high selection pressure associated with broad exposure to these toxins and the subsequent adaptation by pest insects (Gould 1988a, b; Mason et al. 1996). There is a continuing need for research to address resistance problems in stalk-boring Lepidoptera (Gould 1989) and for outreach to transfer research results to the public. There is a need to balance the desire for maintaining long-term durability of this technology with logistical and economic short-term expectations for effective and uniform management.
Resistance management for transgenic corn depends on a refuge strategy complemented by high expression of Bt protein in the plant (EPA 1998). However, there is disagreement concerning the size and placement of non-Bt refuges. Current risk assessment models have been based on incomplete biological information, particularly on dispersal and genetics, perhaps leading to unnecessary constraints as to how corn growers are permitted to use this technology. New biological information will improve the accuracy and precision of resistance management models. Improved models will allow us to investigate the consequences of reducing refuge size and of optimizing refuge placement across variable agricultural landscapes. It may be possible for dense sacrificial plantings of popcorn or nearby plantings of millets to serve as a non-Bt corn source of susceptibility genes because the insects developing within these crops probably escaped B. thuringiensis selection pressure.
European corn borer is not a single, randomly mating population even though it occurs throughout North America east of the Rockies. Clarification of population structure and genetics is necessary to model the likelihood of resistance development and to design resistance management strategies. More information for this species is needed on geographic patterns of genetic variation, voltinism, pheromone blend, sensitivity to B. thuringiensis, and the influence of host plants on genetics and population structure to develop spatial-temporal models of gene flow within an agricultural landscape. Also, little is known of the population structure and genetics of other stalk-boring insects. Acquiring such information will increase our understanding of the spectrum and mechanisms of resistance, genetic basis for resistance, status of cross-resistance, and stability of resistance.
Natural Enemies. Enhancing natural control is the first line of protection in IPM. Even though the basic biology of most of the natural enemies associated with corn has been described, the effects and value of these natural enemies in various corn cropping systems and landscapes is not well enough understood to reliably manage based on knowledge of natural controls. Key information gaps include understanding patterns of variation in natural enemy communities associated with landscapes dominated by corn and more diverse landscapes. Gaps extend to adequately quantifying the role of natural enemies in resistance evolution, improving the use of augmentative biological control agents, and characterizing the economic value of natural enemies in contemporary cropping systems.
We hypothesize that 1) natural enemy abundance and diversity may be greater in landscapes where European corn borer has historically not been a consistent significant pest compared with landscapes where it has been a pest, and 2) with wide-scale adoption of Bt corn, natural enemies that are specialized on either European corn borer or southwestern corn borer will become less abundant. Some predators and host-specific parasitoids may have such a limited food source that only those with superior host-finding behavior will persist in landscapes dominated by Bt corn.
Nontarget Effects. Management strategies with Bt corn to control European corn borer and other stalk-boring insects may have direct and indirect effects on nontarget pests, and other organisms that could result in positive or negative impacts (Ostlie et al. 1997, Schuler et al. 1999). Secondary corn pests, such as corn earworm [Helicoverpa zea (Boddie)], fall armyworm [Spodoptera frugiperda (J.E. Smith)], dusky sap beetle (Carpophilus lugubris Murray), western bean cutworm [Richia albicosta (Smith)], Banks grass mite (Oligonychus pratensis Banks, and twospotted spider mite (Tetranychus urticae Kock) are either tolerant to or not affected by the endotoxins expressed in Bt corn. With the exception of spider mites, these organisms may have reached greater economic status where Bt corn culture has resulted in reduced use of broad-spectrum insecticides (Horner et al. 1998, Dively et al. 1999). These nontarget pests require a new set of pest management practices that will need to be compatible with strategies used to delay European corn borer resistance to transgenic corn. Bt corn also may have nontarget impacts on European corn borer and other lepidopterans in noncorn host plants grown near cornfields (Losey et al. 1999). The exceptional efficacy of Bt corn may suppress local populations on diversified farms where other crops are grown that share European corn borer and corn earworm as major pests. Because most commercial Bt corn hybrids express the endotoxin in the pollen, it is possible that wind-dispersed pollen may accumulate on noncorn host plants of nontarget Lepidoptera. A regional effort among participating states is essential to provide policymakers, interest groups, and the public with unbiased, scientifically based information on the actual exposure risk of nontarget lepidopterans to Bt pollen.
IPM Education, Policy, and Regulation. Growers rely on a variety of sources for making decisions regarding implementation of traditional and transgenic pest management technologies. During the next 5 years, we expect that single management traits, such as Bt resistance to European corn borer, will no longer be available as stand-alone options. Instead, genes for European corn borer management will be stacked with other pest management and enhanced quality traits (e.g., herbicide tolerance, corn rootworm resistance, high oil or other nutritional factors). To help growers integrate these inputs and options into practical and implementable pest management programs, the results of this project must be made available in a timely fashion for use by policymakers and be packaged as unbiased recommendations for the agricultural and public sectors. It is also important to obtain feedback from growers about their constraints and willingness to adopt resistance management practices as part of their European corn borer suppression program. This input is needed to balance short-term logistical and economic expectations for effective management with the desire to maintain long-term durability of the transgenic technology.
Overall. Collectively, a multistate approach to researching these knowledge gaps and implementing effective technology transfer strategies is appropriate and necessary. Lack of knowledge has led to fears by the general public about the potential environmental and health risks associated with broad adoption of new technologies. The recent controversy about nontarget effect of the technology, particularly the potential effect of Bt corn pollen on monarch populations, has further fueled public concerns. These fears have the potential of forcing legislation to ban or slow the introduction of genetically modified organisms. Answers to questions regarding Bt corn impacts (if any) on these nontarget organisms should help focus the public's perception of this technology and, where benefits are clearly demonstrated, allow farmers to gain the dramatic pest control advantages provided by this and future technologies.
These multistate plans will be a paradigm for the development of science-based resistance management programs for other pests, other crops, and future crop protection technologies. Our efforts will provide fundamental advances in the knowledge of pest ecology, genetics, and evolution. Our work will continue to provide scientifically based assessments essential to the policy decision-making process and should help to increase the public's acceptance of these safe technologies and to identify potential negative impacts that need further investigation. We also view it as part of our responsibility to provide unbiased, scientifically based information that fosters subsequent investment in promising novel approaches to pest management. There is ample evidence that the NC-205 research group has the skills, collaborative working relationships, and commitment to provide the missing biological information and to incorporate this new information into current resistance management models.
Back to Top