NC_OLD205: Ecology and Management of European Corn Borer and Other Lepidopteran Pests of Corn
Statement of Issues and Justification
More than 80 million acres of field corn (Zea mays), worth over $20 billion, is annually grown for grain in the United States. An additional 700,000 acres of sweet corn valued at $770 million also is grown annually. European corn borer (ECB), Ostrinia nubilalis, alone, among several stalk-boring pests, accounts for more than $1.85 billion in control costs and grain losses to field corn growers each year. In 2002, 90% of the fresh market sweet corn acreage was treated with one or more insecticide applications for a total of 479,000 lbs of insecticides applied. European corn borer also attacks many other important crops, such as, sorghum, small grains, cotton, potatoes, snap beans, peppers, and soybeans. The southwestern corn borer, Diatraea grandiosella, causes about $1 million in damage in the Western High Plains (Morrison et al. 1977). Other significant stalk-boring pests include stalk borer, Papaipema nebris, hop vine borer, Hydraecia immanis, potato stem borer, Hydraecia micacea, and sugarcane borer, Diatraea saccharalis. The long-term goal of our research is to develop sustainable ways to manage insect pests of corn. This is a high regional priority, and in the context of demonstrating sustainable practices, it also is an important national priority.
Previous committees extending back to 1950 have focused on the European corn borer and other stalk borers. In addition to stalk borers, we propose to address the other lepidopteran pests of corn, which would include lepidopteran larvae that feed primarily on corn leaves and ears. This is a natural progression for the committee since Bt corn for Lepidoptera affects most of these insects. Importance of the non-stalk-boring Lepidoptera depends on type of corn and area of the country. Corn earworm, Helicoverpa zea, and Fall armyworm, Spodoptera frugiperda, larvae consume leaves, tassels, silk and developing kernels of corn and are particularly important economic pests of sweet corn and popcorn. In southeastern U.S. losses attributed to corn earworm in field corn range from 1.5-16.7%; whereas sweet corn losses can be as high as 50% (Wiseman 1999). Black cutworm, Agrotis ipsilon, is the most devastating of the cutworm complex that attacks young corn in the Corn Belt. Significant loss of plants (>25%) and yields (2,900 kg/ha) are not unusual when infestations occur (Showers et al. 1983; Showers 1999). Western bean cutworm, Richia albicosta, increasingly is a pest of corn ears in the western Corn Belt; and sugarcane borer has emerged as an important pest of corn in Louisiana.
Since the 1996 commercial release of transgenic corn hybrids containing a gene from the bacterium B. thuringiensis subsp. kurstaki (Bt), a revolution in field and sweet corn insect pest management has been underway. This revolution is rapidly moving field corn pest management away from synthetic pesticides to plant-based toxin delivery systems coupled with low dosage commercial seed treatments. The use of Bt sweet corn varieties has been less dramatic but is important because of its higher insecticide inputs due to its higher cosmetic standards. The goal of this movement is to eliminate the need to store and handle toxic chemicals, eliminate the need for special insecticide application equipment, increase the ease of planting, and increase pest control effectiveness. Major seed technology companies continue to develop new transgenic crops for pest protection. These revolutionary changes in corn-hybrid technology are causing major adjustments in the agricultural community, pointing out major knowledge gaps and increasing the need to reevaluate past knowledge about European corn borer and other pests of corn. For instance, an insect resistance management (IRM) program was never a legal requirement for any pest management technology until the introduction of Bt-corn hybrids for protection against European corn borer. Publicity and attention to scientific advisors lead the U.S. Environmental Protection Agency (EPA) to require IRM programs on farms where Bt-corn hybrids are used; consequently, the concept that resistance could be prevented went from a theoretical possibility to an experiment in-progress.
During 2004, 32% of all field corn hybrids planted in the Unites States contained a Bt gene. The area planted to Bt sweet corn has been lower, under 5%. Stacked gene field corn hybrids that have multiple modes of action to prevent injury from both European corn borer and corn rootworm were commercially released in 2004. These stacked hybrids could further increase the percentage of acres planted to Bt corn for European corn borer management since corn rootworm in some areas of the Corn Belt is considered a more important pest than European corn borer. Other new genetically enhanced hybrid types may enter commercial markets within the next five years. As the level of adoption increases, particularly in the western Corn Belt, the potential for resistance evolution increases. 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 suggest that a minimum refuge size of 20% is required to slow resistance development in this species. Although these models were constructed using the best information available on the pest's biology and known population genetic relationships, a number of assumptions had to be made about pest biology for the simulations to be completed. These assumptions need to be tested and research conducted to move them from assumptions to quantified variables with known uncertainty. In addition to addressing information gaps needed to improve these models, information is needed on the economics of this new technology, and the non-target impacts of Bt-corn toxins. Eliminating these information gaps forms the basis for several objectives of the project.
Economics. Corn growers must weigh yield losses due to corn insect pests against the costs of control. Additionally, with Bt technology they must consider the costs of implementing IRM programs. The economic component of this project will provide growers with the tools to make these decisions plus identify economic incentives that encourage growers to comply with IRM refuge requirements. A grower does not manage the European corn borer in a vacuum, but as part of a larger pest management framework. Growers can choose between Bt-corn hybrids that provide protection from European corn borer, black cutworm, corn rootworm, or a combination of these products. Plus, whether they choose to use the Bt technology or not, they need to decide whether to use seed treatments for soil insect pests, which maturity of corn to plant, and when to plant. To make these decisions the grower must understand how pests influence the potential economic value of each product or practice. During the last project we developed a set of tools that can help address questions about tactical implementation of IRM programs and direct research to assess economic value of new and competing technologies. To make a hybrid selection decision the farmer must understand the potential economic value to Bt corn for European corn borer protection and his/her need to managed other pests. The average net benefit for Bt corn varies across the landscape because of changing synchrony between European corn borer tunneling and plant growth stage. For example, the 33-year average net benefit in northern Illinois (based on simulations) varies from 0 to $4.00 per acre, while in southern Illinois the average net benefit varies from $4.00 to $8.00 per acre. However, in areas infested by the southwestern corn borer, like southwest Kansas and the panhandles of Texas and Oklahoma, the per acre value of Bt corn ranged from $12.49 to $34.60. This is well above the technology cost of ca. $5.25 per acre. The value of a pest management practice also is influenced by year-to-year climatic changes. The net returns of using Bt corn over the past 33 years at one location in Iowa would have ranged from just under $7.00 per acre to -$1.50 per acre. Over the same period, the net returns at one location in Pennsylvania would have ranged from $4.50 per acre to -$2.00 per acre; and in 24 of the past 33 years growers would have received a positive return. New tools that help growers assess the economic value of Bt corn for European corn borer is the first step in helping them determine which technology will serve their needs. Because lepidopteran pests and their behavior and agricultural production practices differ across the Corn Belt, regional economic analyses are required to obtain a complete understanding of producer behavior, factors that drive this behavior, and policies that may modify this behavior in positive ways. These new tools now need to be widely verified and made available to economists and growers for regional analyses of pest management and IRM program economics. The economic benefit to growers using Bt sweet corn has also been substantial. In areas of high insect pressure by the lepidopteran complex, the use of foliar sprays has been reduced from > 15 sprays to only one or two.
Ecology and Genetics. During the past 10 to 15 years, U.S. agriculture has been developing and implementing transgenic corn varieties, 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. Regional 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). Although there have been no cases of insects having developed resistance in the field to Bt plants (Tabashnik et al. 2003), 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 (USEPA 2000). However, there is continuing discussion 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. Several computer-based models have been developed that provide predictions of likely rates of resistance evolution, but all still lack some level of realism that could change how we structure and implement IRM programs. Work with the model system of the diamondback moth, the only insect that has developed resistance in the field to Bt proteins, and Bt crucifers expressing Bt proteins has demonstrated the need for refuges for Bt plants (Shelton et al. 2000, Tang et al. 2001). Additional work with stacked Bt genes, as currently used in some Bt-cotton varieties, has shown this strategy effective for delaying the evolution of resistance (Zhao et al. 2003). Past work by this committee has provided a major part of the research knowledge required to design the IRM program for European corn borer. But, the committee also realizes that two critical key questions continue to need further research to better predict the rate of resistance evolution: the rate and distance of adult movement between fields and the resistant allele frequencies in European corn borer populations. We also continue to lack a landscape perspective on how environmental factors, such as crop growth stage and year-to-year climate variation can influence the pest's movement. During past NC-205 projects, European corn borer developmental models were developed and expanded to provide 1km2 resolution landscape maps of European corn borer development and corn development. These models coupled with population genetic models can further help the committee gain a better understanding of how topography and weather across the nation influences the timing and movement of European corn borer males and females. Simulation models also will help answer questions such as will area-wide IRM programs work and can the size of the refuge be reduced to increase value returned to the farmer through use of the 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.
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.Sugarcane borer has emerged as a common pests on corn in some areas in Louisiana (Castro et al. 2004). Field collections in 2004 showed this insect was the dominant corn borer species, counting 85% of the total collection during 1st and 2nd generations in Louisiana. Laboratory bioassays have demonstrated that sugarcane borer is much less susceptible to Bt toxins in Bt corn compared to European corn borer. There are questions concerning whether the high-dose strategy is a valid approach for this pest.
Natural Enemies. Enhancing natural control is the first line of protection in integrated pest management (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 implement. Key information gaps include understanding patterns of variation in natural enemy communities associated with landscapes dominated by corn and soybeans, 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 in corn may be influenced by natural enemies of pests in soybeans, especially in areas with high populations of soybean aphid, Aphis glycines Matsumura; and 2) 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.
Non-target Effects and Pest Replacement. Another major adjustment for the agricultural community (growers, seed technology industry, ag-input dealers, extension and researchers) has been the need to more fully understand the impacts of gene-based technologies on non-target organisms and the possibility of human or livestock health effects. Never before in the history of pest management has there been as much pressure placed on the scientific community by the general public to understand the biology of pests and the ecological impacts of new pest management tactics. This is a major problem that must be addressed because no one truly understands how to determine whether a new technology will cause important changes to biodiversity and the functionality of an ecosystem. There is a general lack of knowledge about how field-corn pest management influences biodiversity, in both agricultural and nearby non-agricultural ecosystems. Members of this committee have contributed knowledge about the non-target impacts of transgenic crops, but no one has developed a sound way of judging how much and to what extent change in biodiversity is important. Therefore, research to help understand how ecosystems function, and how new practices are changing the biodiversity in and near cornfields are badly needed.
Management strategies with Bt corn to control European corn borer and southwestern corn borer may have direct and indirect effects on non-target pests, and other organisms that could result in positive, neutral or negative impacts (Ostlie et al. 1997, Schuler et al. 1999). The seed-treatment technologies associated with Bt corn to extend the spectrum of insect control have been shown to have non-target impacts as well. Secondary corn pests, such as corn earworm, fall armyworm, western bean cutworm, dusky sap beetle, Carpophilus lugubris, Banks grass mite, Oligonychus pratensis, and two-spotted spider mite, Tetranychus urticae, are either tolerant to or not affected by the endotoxins expressed in Bt corn. With the exception of spider mites, these organisms may prove to be more problematic with the use of Bt corn and the reduction of broad-spectrum insecticides (Dively et al. 1999). Spider mites are an exception because problems with them are often exacerbated by insecticides. The increased use of Bt corn actually could reduce the need for miticides. Non-target 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. Possible effects of transgenic corn on non-target organisms will be investigated with laboratory and field-based studies. This has been high priority research, since questions arose about possible effects of Bt-corn pollen on monarch butterflies, Danaus plexippus (Losey et al. 1999, Hellmich et al. 2001, Stanly-Horn et al. 2001, Sears et al. 2001).
IPM Education, Sustainable Pest Management, and Outreach. A major adjustment facing the agricultural community is the need to move toward more sustainable production systems. Sustainability means a lot of different things to different groups and individuals, but for the purposes of this project its definition will be to develop insect management programs that are compatible with the economic, environment, health and social needs of both the agricultural and non-agricultural community. The major focus of this project on new genetically enhanced hybrids as a pest management tool, must be balanced with research that increases our understanding of how field crop and nearby non-agricultural ecosystems are affected by insect management efforts. We must also focus on research that is more systems oriented to determine how biodiversity can be maintained through management of fencerows and other non-agricultural areas. These efforts will include studies to investigate and improve conservation biological control of lepidopteran pests of corn and cultural practices. Growers rely on a variety of sources for making decisions regarding implementation of traditional and transgenic pest management technologies. Corn hybrids with Bt resistance to European corn borer now can include additional quality traits such as herbicide tolerance, and corn rootworm resistance. Soon transgenic corn could include enhanced traits related to nutrition and pharmaceutical, fuels, etc. To help growers integrate these inputs and options into practical 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 multi-state approach to researching these knowledge gaps and implementing effective technology transfer strategies is appropriate and necessary. As shown above, the geographic location plays an important role in the economics of Bt-corn technology or other technology value and how the pests interact with other organisms in their environment. It is this significant spatial effect of population and community dynamics that make a regional project necessary. Lack of site-specific knowledge has led to fears by the general public about the potential environmental and health risks associated with broad-scale adoption of new technologies, particularly when those technologies are genetically modified. Controversy about non-target effect of transgenic technology, particularly the potential effect of Bt corn on non-target organisms and human health, has fueled public concerns. These fears, whether unfounded or real, have the potential of forcing legislation to ban or slow the introduction of genetically modified crops. Answers to questions regarding Bt-corn impacts (positive, negative or none) on these non-target organisms should help focus the public's perception of this technology and where benefits are clearly demonstrated, allow growers to gain the dramatic pest control advantages provided by this and future technologies. Identifying acceptable methods of determining how changes in biodiversity caused by these technology influence functionality of ecosystems will blaze the trail for how to accomplish effective future risk assessments of other new technologies or interventions by humans.
These multi-state plans will be a model for the development of science-based resistance management programs and risk assessment 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 technologies and to identify potential negative impacts that need further investigation. Our work also will help lead to more sustainable pest management systems for lepidopteran pests of field corn. 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.
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