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Issue No. 5, Article 4/April 28, 2006

Scouting and Use of Thresholds for the Variant Western Corn Rootworm: An Overlooked and Ignored Management Approach?

Soon after the variant western corn rootworm began to cause significant problems in first-year corn across east-central Illinois in 1995, we began to develop scouting methodologies and an economic threshold for this pest. In 1996 and 1997, we deployed Pherocon AM traps in producers' soybean fields, and in subsequent years root injury was assessed in untreated strips (no soil insecticide used). A sampling protocol and an economic threshold (five beetles per trap per day) were established following this research. For a full discussion of the project, please consult the following paper: O'Neal et al. 2001. "Predicting western corn rootworm larval injury to rotated corn with Pherocon AM traps in soybeans." Journal of Economic Entomology 94: 98-105.

Pherocon AM trap in soybeans--a neglected sampling tactic for variant western corn rootworms?

Although some producers utilize this management approach for the variant western corn rootworm, the great majority do not. Use of the Pherocon AM traps is generally greatest in areas where variant western corn rootworm adults are just beginning to colonize soybean fields. Once a producer is convinced that the variant has established in a given county, the traps are seldom used. Yet we know that corn rootworms are not present at economic levels in every cornfield. In fact, during the early 1990s, economic infestations in Illinois (continuous cornfields) occurred in about 50% of fields. Because of increased soil insecticide use and the planting of transgenic corn rootworm hybrids, it's difficult to assess how many first-year cornfields actually have economic levels of corn rootworms. We hope to conduct surveys at some point in the future with producers who are willing to leave untreated check strips for us.

At recent meetings in St. Louis, Missouri (National IPM Symposium, April 4-6, 2006), and Bloomington, Illinois (North Central Branch Meeting of the Entomological Society of America, March 26-29, 2006), several discussions were conducted concerning the state of IPM in the corn and soybean agroecosystem. During the St. Louis discussions, many participants argued that some slippage in IPM implementation had occurred in the production of corn and soybeans. In fact, some argued that different metrics may be in order, to more accurately quantify or define IPM in the field crop arena. Significant concerns were raised regarding the overreliance on single management tactics for many pests. In essence, have we forgotten the importance of the "I" in IPM?

At the North Central Branch ESA meeting in Bloomington, a very interesting discussion occurred regarding the use of thresholds for the variant western corn rootworm. Tom Green, IPM Institute of North America, Madison, Wisconsin, recently shared some notes with me from this session. Several challenges and obstacles to the implementation of scouting and use of economic thresholds for corn rootworms were identified.

  • Current thresholds may not adequately address cumulative stress, such as drought.
  • Current thresholds do not account for price of corn, cost of specific control measures, or efficacy of specific control measures.
  • Expectations about the accuracy of the thresholds may be unrealistically high (i.e., the threshold will not predict damage accurately 100% of the time).
  • After one drought season and a higher-than-usual failure rate for the threshold, producer participation declined.
  • Producers are working to simplify operations, especially where off-farm activities are an important revenue source and where consolidation results in increasing acreage under management.

These are legitimate concerns, and clearly more work is warranted if we hope to increase the adoption of scouting and use of economic thresholds for the variant western corn rootworm. As discussed at the ESA meeting, a new scouting system and threshold that utilize monitoring efforts in field edges may increase adoption rates of this IPM tactic. I thank Tom Green for sharing these notes.

I would be interested in your thoughts on the current state of IPM in the production of corn and soybeans across the Corn Belt.--Mike Gray

Mike Gray

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