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Issue No. 2, Article 1/April 9, 2010

Establishment of Refuges Critical in Maintaining Long-Term Durability of Bt Hybrids

It won't be long before planters begin to roll across the landscape of the Corn Belt. With the increasing use of Bt hybrids as the foundation of many corn insect management programs, it's important to remember the requirement to establish refuges to delay or prevent the development of resistant populations of key insect pests, such as the European corn borer and western corn rootworm. To date, we have been fortunate that field-level resistance has not developed for either of these species despite the widespread adoption of this impressive technology (Figures 1 and 2).


Figure 1. USDA--Economic Research Service estimates of stacked gene varieties, Illinois and USA.


Figure 2. Responses to the question "Did you plant a Bt hybrid in 2009?" from attenders at the 2010 Corn and Soybean Classics in five Illinois cities.

Since 2005, producers in Illinois have significantly increased their use of "stacked" gene corn hybrids. These stacks include corn hybrids that express Bt proteins targeted against the lepidopteran complex (e.g., European corn borers, black cutworms, western bean cutworms) and corn rootworms as well as offer herbicide tolerance. In 2009, stacked hybrids comprised nearly 60% of all corn planted in Illinois. Although escalating seed costs remain a viable concern among producers, I suspect we will continue to witness a steady increase in the use of these versatile corn hybrids.

In addition to the stacked hybrids, pyramided corn hybrids (SmartStax) will be introduced to the market in 2010. These pyramided hybrids express several Cry proteins for corn rootworm (Cry3Bb1, Cry34/35Ab1) and lepidopterans (Cry1A.105+Cry2Ab2, Cry1F) and were developed through a cross-licensing agreement between DowAgroSciences and Monsanto. Of particular interest to producers who plant SmartStax hybrids is the refuge reduction from 20% to 5% in the Corn Belt. For 2010, the 5% refuge must still be a structured one (seed mixtures cannot serve as the refuge). For producers who elect to plant other Bt hybrids this spring, the 20% requirement remains in place.

In surveys (using Turning Point Technology) of producers who participated in the 2010 Corn and Soybean Classics, slightly fewer than 80% said they planted a refuge in 2009 according to suggested guidelines (Figure 3). Over time, as more pyramided hybrids become commercialized, I suspect that seed mixtures (transgenic and nontransgenic seed) will form the foundation of resistance management plans. This will ensure grower compliance, and with the pyramided technology in place should help prolong the long-term durability of Bt hybrids.


Figure 3. Responses to the question "If you planted a Bt hybrid in 2009, did you plant a 20% refuge according to the suggested guidelines?" from attenders at the 2010 Corn and Soybean Classics in five Illinois cities.

A large majority (approximately 80%) of producers at the Classics indicated being receptive to using a seed blend as a refuge. Key concerns regarding a seed mixture approach were the potential for significant insect injury to non-Bt seed and the inability to rescue injured plants. However, the convenience offered by a seed mixture refuge strategy seems to trump this concern for most producers, up to a point. Approximately 90% indicated their willingness to use a seed blend that contained non-Bt seed in the 2% to 5% range (Figure 4). However, if the non-Bt seed falls in the 6% to 10% range, interest falls below 60% (Figure 5).


Figure 4. Responses to the question "If you answered 'yes,' would you be willing to use a seed blend that contains non-BT seed in the 2% to 5% range?" from attenders at the 2010 Corn and Soybean Classics in five Illinois cities.


Figure 5. Responses to the question "If you answered 'yes,' would you be willing to use a seed blend that contains non-BT seed in the 6% to 10% range?" from attenders at the 2010 Corn and Soybean Classics in five Illinois cities.

During the 2010 growing season, I would be interested to learn if producers experience corn rootworm control problems despite their use of Bt hybrids. In 2009 there were many reports of severe corn rootworm damage to some Bt hybrids in areas of southeastern Minnesota, northeastern Iowa, and western Wisconsin. Often corn rootworm larval damage goes unnoticed unless severe lodging occurs. In many instances, this lodging is not detected until harvest. It may be worthwhile to check the value of your investment in Bt corn this year. Grab a shovel and remove some corn roots from Bt as well as refuge areas of your fields. Mid-July is the best time to check for corn rootworm pruning, despite the heat and pollen. If you find that nodes of roots have been removed from Bt plants, give me a call or send me an email. I look forward to your observations and comments.--Mike Gray

Author:
Mike Gray

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