NCCC046: Development, Optimization, and Delivery of Management Strategies for Corn Rootworms and Other Below-ground Insect Pests of Maize
Statement of Issues and Justification
The corn rootworm (Diabrotica spp.) complex represents the primary insect pests of maize in the United States Corn Belt. Feeding damage attributable to Diabrotica larvae and associated costs to manage these pests has been estimated at approximately US$1 billion annually for American corn producers. However, controls are not always applied judiciously, and there are significant wasted resources. Gray et al. (1993) were able to document that in 1990 and 1991, 26 of 58 producers had rootworm damage at or above the economic injury index, while in the same area over 88% of the cornfields were treated for rootworm control. Twenty years later the control options have changed, yet risk-averse producers continue to apply treatments where none are needed. Transgenic hybrids (or Bt corn) offering control of both rootworms and lepidopteran (moth/caterpillar pests) dominate the marketplace, composing nearly 50% of all corn in the United States. Because not all of these pests occur each year and in every region, these multi-trait hybrids represent a one-size-fits-all option that results in planting where pressures are extremely low or non-existent.The Diabrotica species makeup varies throughout the United States; however, all are root feeders (which cause yield loss, lodging, and overall plant stress) and management options are similar. The western corn rootworm, Diabrotica virgifera virgifera LeConte, and northern corn rootworm, D. barberi Smith and Lawrence, are often considered the most serious of these insect pests attacking corn from the Rocky Mountains through the Great Plains, the Midwest, Southern Canada, to the US Atlantic coast. In the south, especially Texas, the Mexican corn rootworm, D. virgifera zeae Krysan and Smith, is the dominant species. As a pest complex, Diabrotica spp. threaten a vast North American agricultural zone that transcends corn production systems and crop-use patterns. For instance, rootworms are key pests both of infest irrigated corn in the west and dryland production systems further east. These insects attack corn that will be used for on-farm feed (grain or silage), as well as grain that will be sold for off-farm use (e.g., food, feed, and industrial manufacturing). With expansion of corn production to meet bioethanol/biofuel demand, the rootworm complex not only impacts the economics of food and feed, but in the future may have an increasing impact on fuel production.
Although maize is the only cultivated host for the complex, Diabrotica spp. have developed remarkable adaptations to ensure they maximize their opportunities across many cropping systems. Rotation-resistance, whereby rootworms circumvent crop rotation by either ovipositing in non-corn crops or undergoing extended diapause, plagues much of the Corn Belt. The northern corn rootworm undergoes extended/prolonged egg diapause in parts of the upper Midwest, effectively resulting in a two-year lifecycle. Eggs remain dormant for over a year, bypassing the season when corn roots are unavailable on land planted to an alternate (non-host) crop, then hatch in the following year when fields are rotated back to corn. The western corn rootworm overcomes the practice of crop rotation by altering their behavior: laying eggs in cultivated, but non-host production fields (e.g., primarily soybeans, but also wheat and various forage crops). As the field is planted back to corn the following year, the presence of eggs ensures that larvae will be present, often in numbers capable of causing significant damage. Originating in eastern Illinois and western Indiana, this rotation-resistant behavior of the western corn rootworm has spread into Michigan, Ohio, Wisconsin and west into portions of Iowa, Minnesota and Missouri.
While the traditional management approaches for larval control were mainly crop rotation or the use of soil insecticides, the current primary method of control relies upon the planting of transgenic corn hybrids that express genes for one or more toxic Bt proteins [i.e. mCry3A (from Syngenta), Cry34Ab1/Cry35Ab1 (from Mycogen/Dow + DuPont/Pioneer), and Cry3Bb1 (from Monsanto)] that kill root-feeding Diabrotica neonates. Statistics from NASS (2009) indicated that over 50% of corn planted in the US in 2008 expressed a Bt protein. Over 40% expressed both a Bt toxin and a herbicide-tolerance gene (mostly RoundupReady). Beginning in 2009, a collaboration of DowAgroSciences and Monsanto resulted in the registration of SmartStax" corn with both Cry3Bb1 and Cry34/35Ab1. This allows growers to plant more Bt corn/unit area; EPA regulations allow for a reduction in refuge area to 5% for that product rather than the 20% that had been required for the individual (i.e. non-stacked ) Bt toxins. In spring of 2010, Pioneer Hi-Bred was granted registration for a product that uses a seedmix refuge, called Optimum AcreMax I and Optimum AcreMax RW. These products meet the refuge requirement by having the 10% refuge for rootworm in the bag. In addition to crop rotation, granular insecticides, and transgenic corn, an additional management tactic available to growers is the application of a high rate of neonicotinoid insecticides as seed treatments: either Cruiser (thiamethoxam) or Gaucho (clothianidin), both of which are labeled for rootworm larval control. However, field data from several states suggest that this option does not provide adequate control when rootworm larval pressure is high.
The focus upon corn rootworms with transgenic approaches has the potential to result in opportunities for other pests. Corn rootworms are not the only insects that can attack the below-ground portions of maize plants. A variety of other insect pests (e.g., grubs, wireworms, seedcorn beetles, seedcorn maggots, and grape colaspis, etc.) often referred to collectively as "secondary pests", can cause serious economic damage to corn. Damage caused by these pests can be highly variable depending on environmental conditions and infestation level. In some corn production areas, the severity of their injury can rival or even surpass that caused by corn rootworms. They were often considered with corn rootworms in terms of management because the broad spectrum soil insecticides that dominated rootworm management during the 1970s - 1990s were usually sufficient to control many of these secondary pest species in the soil. However, transgenic corn hybrids for rootworm management are, by nature and by design, highly specific. While this approach has many benefits, growers may desire additional control measures, viz. insecticide seed treatments, when using transgenic hybrids to ensure control of secondary pests. As a result, seed companies currently make an automatic application of a low (250-600 mg/kernel) rate of neonicotinoid insecticide (primarily clothianidin (Poncho) or thiamethoxam (Cruiser)) to all transgenic seed containing a Bt protein for secondary pest control.
The narrative above demonstrates that the management of corn pests has changed dramatically since 2004, when the first Bt corn hybrids targeting rootworms were offered. Keeping abreast of these changes with timely and relevant research over an area as large and diverse as the United States Corn Belt lends itself to a coordinated, committee approach. This avoids duplication of effort and facilitates sharing of ideas and resources where appropriate.
The original corn rootworm Multistate Research Coordinating Committee was approved in 1964 as a NCR project, and the committee has operated continuously since then. The intent of the project originally was to "study, on a regional basis, the biology of the corn rootworm complex in relation to current and projected cultural practices and to identify vulnerabilities of the pests that could be used as control measures benefiting farmers in the North Central Region." At that time, the northern corn rootworm was an economic pest in the central Corn Belt and the western corn rootworm had just begun its spread out of Kansas and Nebraska. By 1964, the western corn rootworm had become established in South Dakota, and southern Minnesota, and the western half of Iowa. The "official" membership of the earlier research coordinating committees came from 12 North-Central state agricultural experiment stations and the USDA-ARS Northern Grain Insects Research Laboratory in Brookings, SD. As the western corn rootworm continued to spread, scientists from the newly infested corn-growing states and Canadian provinces began attending the committee meetings as "associate" members; several have since become official members. The 2001 renewal proposal continued as a regional research committee with internal and external linkages to resources from nearly all corn production areas infested by this pest complex in the United States and Canada. More recently, the 2006 renewal proposal changed from a Research Committee (R) to a Coordinating Committee (CC), becoming NCCC46, which provided a mechanism for addressing critical regional issues related to corn rootworms where multistate and province coordination or information exchange was appropriate.
The original intent of the previous committees was to study the biology of the corn rootworm complex to identify vulnerabilities of the pests that could be exploited for control purposes. In many ways, this goal has not changed. However, as outlined above, the prevalence of transgenic hybrids has significantly altered the backdrop for studies of pest biology. Pooling data across states/provinces has generated several important management decision guidelines involving chemical control tools. Examples of tangible results that were achieved and delivered to growers as a result of this long-term regional coordination include: 1) an understanding of how repeated applications of carbamate insecticides to some soils can result in an accelerated rate of degradation by soil microbes that diminishes the insecticides' effectiveness; 2) a large multistate database with differing environments and production systems demonstrated that the application rate of many insecticides could be safely reduced by 25%, thus, lowering grower input costs and maintaining effective pest control; 3) the concept of controlling adult corn rootworms with reduced rates of insecticides before females lay eggs was refined and demonstrated across a variety of Midwestern corn production systems; 4) writing of a regional Corn Rootworm Management Guide by members of NCCC46 that will go to press in late 2010 along with a commitment to an ongoing web presence of updated versions; 5) the NCCC46 group worked with industry to remove the bag-tag restrictions on research involving Bt corn hybrids. This agreement, developed in 2010, will allow public sector scientists more freedom to evaluate the short and long-term implications of using these ubiquitous and powerful tools for production agriculture. This research can now include agronomic and yield comparisons, comparative efficacy studies with other transgenic hybrids, studies on interactions of the trait with pest biology and pest management practices, including interactions related to resistance management, and studies on interactions of introduced traits with the environment. This agreement came about through discussions held at previous NCCC46 meetings both within the group and with the seed industry.
The introduction of two new and transformative management approaches, i.e., Bt transgenic corn hybrids and neonicotinoid seed treatments, have increased the need for addressing critical regional issues largely because the landscape has changed. To wit, changes in corn rootworm biology, pest management technologies, and regulatory issues over the last several years have illustrated the fact that much of our past information and research about these systems have to be re-evaluated. Promotion of an IPM-based approach to the deployment of these products is central to the NCCC46 agenda. The committee also focuses upon the use of these tactics in an environmentally benign fashion that will prevent or delay the onset of insect resistance to widely-implemented Bt-proteins and seed-applied insecticides. Ensuring that these tools remain durable for the long-term necessitates developing resistance management strategies that are compatible with production practices. Most current resistance management plans are based largely on models; there is critical need for more biological data to inform our models. These data are time-consuming, labor-intensive, and thus, difficult to generate. A committee approach ensures that the group can match critical areas of need with expertise of researchers. The consequences of resistance are a real, quantifiable threat to the sustainability of transgenic hybrids: two recently published studies document the rapid development of resistance in laboratory and greenhouse experiments to the two leading commercial Bt corn toxin offerings. In one study, after western corn rootworm were exposed to hybrids containing the MON863 event (expressing the Cry3Bb1 toxin) for three generations, there was evidence for complete resistance to the toxin in the laboratory,i.e., there was no difference in survivorship of larvae reared on an isoline (non-Bt corn) and a Monsanto Yieldgard hybrid expressing the toxin (Meihls et al. 2008). Similarly, Lefko et al. (2008) documented a 15.1 and 58.5 fold increase in survivorship after nine generations (using two different field-collected strains) of exposure to event DAS-59122 containing the Cry34/Cry35Ab toxins (sold under the Pioneer Herculex label). Although the mechanism of resistance is presently unknown, these selection experiments demonstrate that field populations of western corn rootworm contain the genetic architecture to overcome currently deployed Bt toxins relatively quickly.
Corn is most widely grown crop in the US, and members of the corn rootworm complex of beetles are found in most places wherever corn is grown. The insect is highly mobile and has demonstrated regionally distinct adaptations to different geographies. For example, the rotation-resistant soybean variant occurs commonly in the Upper Midwest, but not the Great Plains states. This variant population is not static in its distribution; multistate efforts have been and are required to document movement, spread of the trait, and impacts. NCCC46 coordination promotes a regional approach to studies of corn rootworm biology and management. Breadth in the expertise and the geography of investigations assure that the products and messages available to stakeholders are broadly relevant and yet informed by regional constraints. In an era when advanced biotechnological tools have become the new gold standard for rootworm management, our diverse stakeholders need the independent voice of NCCC46 for advice on technologies impacting rootworm management, including insect resistance and mitigation. The work of NCCC46 will continue to instill confidence in our stakeholders through our collective critical evaluation of novel pest management offerings, especially for multi-trait transgenic hybrids that are increasing in prevalence and yet have little or no independent data on their efficacy or stewardship.
The overall impact of curtailing NCCC46 would be a lost public capability to promote science-based pest management. It would also mean the destruction of a collaborative forum for exchange of information, observations, and recommendations that provides value to constituents and the inspiration for extramural research, subsequent publications and outcomes in the spirit of the Land Grant mission.
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