Corn Rootworm Backgrounder

A White Paper by Monsanto



The Corn Rootworm Pest

Managing Corn Rootworm

Corn Rootworm Traits and Performance Questions

Integrated Pest Management Practices

Insect Resistance Management Plans

Monsanto's Ongoing Stewardship Efforts


The Corn Rootworm Pest

Each year, U.S. corn yields are adversely impacted by a number of insect pests. One of the country’s most devastating pests is the corn rootworm family – commonly referred to as “Diabrotica” by entomologists.  There are several types of rootworm including the northern corn rootworm (D. barberi), the western corn rootworm (D. virgifera virgifera) and the Mexican corn rootworm (D. virgifera zeae).

The corn rootworm does its damage as a larva – the immature stage of the insect.  After mating in the late summer, adult corn rootworms lay their eggs in the soil, depositing them in cornfields in many regions.  The eggs survive the winter underground and hatch in the spring, when the larvae can feed on the roots of young corn plants in farmers’ fields. Rootworm larvae feed almost exclusively on corn roots. Rootworm larval feeding inhibits the corn plant's ability to take up water and nutrients, decreases its ability to develop and remain upright, and – ultimately – leads to possible yield loss, depending on the damage inflicted on the roots by the feeding pests and the growing conditions.

The corn rootworm has earned the nickname the “billion-dollar bug.” The U.S. Department of Agriculture (USDA) has previously estimated that the damage caused by the pest and costs associated with controlling it typically total $1 billion annually – including approximately $800 million in yield loss and $200 million in treatment expenses.

Corn rootworms are widely disbursed throughout U.S. corn growing regions east of the Rocky Mountains. It is estimated that approximately 50 million acres of U.S. corn production faces pressure from corn rootworms.

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Managing Corn Rootworm

Growers limit corn rootworm damage primarily through one of the following methods or a combination thereof: 

  • Crop rotation to soybeans or another non-host crop
  • The use of soil- or foliar-applied insecticides to manage larvae and adults, respectively
  • The use of different single and dual mode-of-action (pyramided) Bt-trait products

In addition to limiting damage and optimizing yield potential, farmers can minimize the potential for corn rootworms to develop resistance by combining two of these practices or by planting a Bt hybrid with dual modes of action for corn rootworm control.

Managing Corn Rootworm: Crop Rotation to Soybeans or Another Non-Host Crop

Farmers widely use crop rotation as a key management process for combating corn rootworms throughout the Corn Belt because rotation breaks the pest’s life cycle.

Crop rotation historically ensured that corn rootworms that laid their eggs in corn fields in one year would have their offspring emerge in non-corn fields the subsequent year. While the rootworm pest has adapted to these practices in some geographies, crop rotation is an agronomic practice that is still widely recommended by academics and seed company agronomists to effectively manage rootworm.

In some areas, a portion of western corn rootworm beetles fly out of cornfields and lay their eggs in soybean fields. If the farmer rotates a soybean field to corn the next season, there could be a population of rootworm larvae in the field waiting to feed on the emerging corn crop the next season. This phenomenon is called the “soybean variant.” While this phenomenon is limited geographically, the area is expanding.

In addition to adaptation by laying eggs in soybean fields, northern corn rootworm populations in southwest Minnesota, southeast South Dakota, northeast Nebraska and northwest Iowa have overcome corn and soybean rotations by delaying egg hatch until two winters have passed. By delaying hatch until corn is planted again into the field, hatching larvae have a suitable food source on which to feed. This phenomenon is called “extended diapause.”

In some areas, farmers continue to plant corn year-after-year for economic reasons. Monsanto encourages farmers who choose to plant continuous corn to consider rotating their cropping system and/or ensure that they utilize a combination of the management practices described below to combat rootworms on farm. 

Managing Corn Rootworm: Soil- and Foliar-Applied Insecticide Treatments

In areas where acres are planted to continuous corn, soil- and foliar-applied insecticides have served as important tools in combating corn rootworm. These insecticides are applied to the soil at planting to control larvae or applied to the plant during the growing season to control adults and prevent them from laying eggs that turn into larvae that damage the corn roots the following season. Insecticides used to combat corn rootworm include organophospates or pyrethroids.

While insecticides are reasonably effective most years, their effectiveness varies depending on environmental conditions. These products are weather-dependent: properly-timed rain is needed for the successful functioning of the active ingredient in the soil, and too much rain can reduce their effectiveness. In corn rootworm fields with high pressure, farmers can minimize the potential for corn rootworms to develop resistance to an insecticide treatment by rotating insecticides with different modes or action. In extreme situations, combining two or more insecticide applications with different modes of action for corn rootworm control can be utilized to knock down the high populations.

Managing Corn Rootworm: Single and Dual Mode-of-Action Trait Products

Monsanto and other trait providers developed insect-protected plants using Bt (Bacillus thuringiensis) in order to protect the yield potential of plants and significantly decrease chemical pesticide use. Today, U.S. corn farmers use Bt trait technologies with either single or dual modes of action to manage corn rootworm. 

Bt proteins have long been used as topical sprays in conventional and organic agriculture because they are effective and can be used safely. Crops that carry the Bt trait allow farmers to protect their crops while eliminating or significantly decreasing the amount of pesticides sprayed.

Rootworm-protected crops utilize a Bt trait to protect the corn’s root system. Single mode-of-action products utilize one Bt protein to manage corn rootworm, and dual mode-of-action products utilize two Bt proteins to manage corn rootworm.

Monsanto’s Bt corn products will sustain less damage from corn rootworm, but they do not control all insect pests that target the crop. Therefore, it is important to understand that, in some cases, severe infestations of target and/or non-target insects may occur and require additional control measures.

The best way for farmers to preserve the benefits and insect protection of Bt technology is to incorporate Integrated Pest Management (IPM) practices and effectively implement required Insect Resistance Management (IRM) practices. The IRM practices are used to decrease the likelihood of insects developing the ability to survive the ingestion of insect-protected transgenic crops. Every farmer who utilizes Monsanto’s Bt corn products must comply with licensing terms and must follow an Insect Resistance Management (IRM) plan as required by the U.S. Environmental Protection Agency.

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Corn Rootworm Traits and Performance Questions

Recent headlines have questioned the performance of Monsanto’s corn rootworm-protected technologies in some areas of the U.S. Corn Belt.

Importantly, Monsanto’s portfolio of corn-rootworm products continues to demonstrate an outstanding level of performance and grower satisfaction across tens of millions of acres. Field monitoring of these products, which Monsanto has conducted since these products were first introduced last decade, demonstrate that corn rootworm-protected hybrids deliver superior performance on greater than 99 percent of all acres planted to the technology in the United States.

Recent headlines unfortunately overlook the reality that farmers have routinely faced areas with high populations of rootworms for years, even prior to the introduction of insect-protection trait technologies. Today, there are geographical pockets of heavy rootworm infestation in areas where there’s a long history of corn-on-corn plantings. In these areas, farmers see intense rootworm pressure that can overwhelm the plants, leading to damage and some surviving insects. This is not something that is specific to any commercial product category – either soil-insecticides or trait offerings - and farmers in these areas work to effectively manage this problem in their fields every year.

As a business, we take all performance claims of our products seriously, and we work with farmers who report incidents of greater-than-expected damage in a field to better understand what they are seeing and provide them with recommendations for both their impacted field and entire farm. For those fields that were planted with YieldGard® VT Triple® or Genuity® VT Triple PRO® hybrids and experienced greater-than-expected corn rootworm damage during the 2011 season, Monsanto has established  the following best management practices:

  • Rotating to Genuity® Roundup Ready 2 Yield® soybeans or another crop if soybeans are not a suitable recommendation for that specific geography.
  • Switch to dual-mode-of-action Genuity® SmartStax® RIB Complete™ or Genuity® SmartStax® corn to manage and protect against insects on the affected field specifically.  SmartStax® corn is the only product on the market to offer dual modes of action for above- and below-ground insect protection.
  • If a dual-mode-of-action hybrid is not available for a specific geography, use a soil- or foliar-applied insecticide on any fields planted to a single-mode-of-action technology.

These best management practices consider available options, local needs and operational considerations, and were developed in consultation with public sector experts.  Monsanto agronomists and dealers will work with each affected farmer to implement the appropriate management practices for impacted fields and make recommendations for their entire farm.  In addition, third-party representatives will work with Monsanto seed dealers to educate about insect resistance management (IRM), refuge and the importance of an IRM plan.

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Integrated Pest Management Practices (IPM)

Growers of Monsanto’s Bt corn products have found these products to be highly compatible with their goals of IPM and sustainable agriculture, and we work with farmers who choose to plant continuous corn-on-corn to ensure they understand the importance of integrated pest management practices when managing high insect populations on-farm.

Growers can enhance the sustainability of their corn agricultural systems by following recommended IPM practices, including cultural and biological control tactics, and appropriate use of pest thresholds and sampling.

Recommended IPM practices commonly adopted by growers of Bt crops include:

  • Using recommended cultural control methods to reduce pest overwintering, such as crop rotation and other soil management practices
  • Employing regular, appropriate scouting techniques and treatment decisions, especially during periods of heavy or sustained pest presence
  • Consulting a local crop advisor or extension specialist for the most up-to-date pest control information
  • Selecting insecticide treatments that have minimal negative impact on beneficial insects, which are conserved by Bt-protected crops and contribute to insect pest control
  • Selecting cultivars well-adapted to their local ecology and giving appropriate attention to the impact of crop maturity and timing of harvest on pest severity

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Insect Resistance Management Plans

The goal of an IRM plan is to decrease the likelihood of an insect developing the ability to survive a pest-control technology and, in the case of Bt crops, a key component of IRM plans is planting a refuge. A refuge is simply the planting of a non-Bt corn hybrid in a block, strip, or interspersed within the Bt field (e.g., refuge in bag, RIB). The lack of exposure to the Bt protein in refuges means there will be susceptible insects nearby to mate with any rare resistant insects that may emerge from the Bt crop. To help reduce the risk of insects developing resistance, the refuge should be planted with a similar hybrid/variety, close to, and at the same time as, the crop containing Bt technologies.

The Environmental Protection Agency (EPA) requires the planting of a refuge when planting Bt field corn. Key points to consider about planting a refuge include:

  • Refuges for rootworms must be planted within or adjacent to the Bt corn field, but can be planted in a variety of patterns, including interspersed refuges (Refuge in the Bag or RIB) in the same field, strips within the field or in an adjacent field.
  • Refuge requirements vary depending on the Bt crop being planted and region of the country, and are determined by the EPA based on scientific data. Historically, the minimum required refuge for Bt corn has been 20 percent in Canada and the U.S. Corn Belt and 50 percent in the U.S. Cotton Belt for single mode-of-action Bt trait products. Growers are legally required to comply with these refuge requirements.
  • Products that contain multiple modes of action or more than one Bt trait, as is the case in Monsanto’s Genuity® SmartStax® RIB Complete™ and Genuity® VT Double PRO® technologies, present a lower risk of resistance and have lower refuge requirements.
  • Genuity SmartStax and Genuity VT Double PRO have the lowest refuge requirement at 5 percent in the U.S. Corn Belt.
  • The latest Monsanto Insect Resistance Management (IRM) Grower Guide contains more detailed information on planting a refuge.

In order to help ensure compliance, growers can utilize the new IRM Refuge Calculator. It's a tool designed for growers (customized to their zip code) to help illustrate the appropriate refuge calculation, the quantity of standard seed bags to purchase for planting proper percentages of both trait and refuge, and possible planting configurations for planting certain corn products in the United States.

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Monsanto’s Ongoing Stewardship Efforts

Monsanto takes IRM and the stewardship of our products very seriously. In addition to the actions that can be taken by growers, we are focused on:

  • Continually developing and updating agronomic recommendations for growers, especially with information tailored to local growing conditions, and typically including advice on refuge compliance, scouting techniques, the addition of soil-applied insecticides, maturity and harvest schedules, soil management practices, crop rotation, and adoption of products with dual mode-of-action.
  • Expanding our offering of multi-gene corn hybrids that provide dual mode-of-action and increase protection for growers. We encourage farmers to begin trying these seeds with greater protection as the product line expands in their area.
  • Researching and developing other genes in our pipeline so we can continue to deliver products with new and increased modes of action.
  • Continuing annual, wide-scale monitoring of insect populations through the Agricultural Biotechnology Stewardship Technical Committee (ABSTC), a consortium of agricultural biotechnology companies and associations, as required by EPA.
  • Monitoring and studying the occasional performance inquiries in fields with very high insect population densities that exceed control thresholds.
  • Meeting our required commitments to the EPA to report on these performance inquiries.
  • Collaborating with industry through ABSTC and the National Corn Growers Association (NCGA) in general farmer and dealer education and training on the importance of proper IRM (i.e. billboards, postcards) as well as the developing tools for growers (IRM Calculator).
  • Biotechnology company members of ABSTC conduct annual grower surveys and on-farm visits to find ways to continuously improve our education efforts, as well as assess grower compliance to refuge requirements.
  • Partnering with our farmer customers as well as academics in actively investigating and studying claims of potential insect resistance.

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