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Issue No. 12, Article 1/June 15, 2007

Hot, Dry Pattern Persists Over East-Central Illinois--Good Survivorship and Early Emergence of Corn Rootworm Adults Expected

So far this summer, many areas of the western Corn Belt continue to receive ample rainfall. This has not been the case for many fields across east-central Illinois, with corn plants over the past week showing signs of drought stress. Hot and dry conditions will speed up the larval and pupal development of many soil insects, including corn rootworms. On June 7, Shawn Jones, area agronomist with Pioneer Hi-Bred International, provided a photo taken by Pat Tomlinson of a third instar corn rootworm larva collected from a field near Decatur (Macon County). Although we typically begin to receive reports of western corn rootworm adults near the 4th of July, we believe adult emergence will be early this season. How early? We anticipate some emergence, particularly in the southern half of Illinois, by the end of the third week of June. By the last week of June, adults may be common in many fields across the state.

Third-instar corn rootworm larva, Macon County, June 7, 2007 (Courtesy of Shawn Jones, Pioneer Hi-Bred, International Inc.)

How do drought conditions affect the performance of Bt hybrids? It is known that drought stress may affect the synthesis of proteins in corn tissue in positive as well as negative ways. The effects of drought on protein production depend on the severity of the drought. Observations over the past several years generally support the belief that Bt plants have fared better than their non-Bt counterparts when droughty conditions developed, especially during pollination. A search of the literature revealed relatively few peer-reviewed journal articles on the topic of drought stress and Bt performance. However, one was published in 2000 by some researchers at Iowa State University (S.B. Traore, R.E. Carlson, C.D. Pilcher, and M.E. Rice. 2000. Bt and non-Bt maize growth and development as affected by temperature and drought stress. Agronomy Journal 92: 1027-1035).

Drought stressed plants in Champaign County, June 12, 2007.

Drought stressed plants in Champaign County, June 12, 2007.

The Bt and non-Bt hybrids used in their experiments, conducted in 1997 and 1998, were MAXIMIZER 454, Cry1Ab, event 176; CIBA 4490 (non-Bt); NK 7333 Bt, Cry1Ab, event Bt11; and NK 7333 (non-Bt). The authors offered the following observations:

"Water deficit delayed leaf appearance up to 6 days, depending on leaf number, and tasseling up to 3 days. It also reduced leaf area as much as 33% and plant height by 15%, depending on leaf number and timing of water deficit. For these characteristics, there was no significant difference between Bt and non-Bt maize plants. There were significant differences among Bt and non-Bt plants for second generation corn borer damage with Bt plants affected the least. This resulted in significant yield differences between Bt and non-Bt plants. Compared with non-Bt plants, Bt plants exhibited significantly greater total plant weight in 1997 (9.7%) and grain yield in 1998 (9.4%)."

Our collective experience with corn rootworm transgenic Bt hybrids is less extensive under a broad range of environmental conditions compared with our experiences with the lepidopteran Bt events. If you encounter rootworm damage greater than anticipated in a field planted with a transgenic corn rootworm hybrid, please share your findings with us. In addition, let us know if you find striking differences in stress levels between plants grown in the refuge versus the Bt portion of your field. Hopefully we will begin to see some much-needed rain in those areas of the state that are very dry.--Mike Gray and Kevin Steffey

Mike Gray
Kevin Steffey

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