No. 23 Article 5/October 5, 2007

Caterpillars That Worked Over Corn Late in the Season (and Some That Didn't)

Throughout September, we received quite a few reports of western bean cutworms being discovered in ears of corn. Some of these reports were accurate; others were not. In northwestern Illinois, where the western bean cutworm is most likely to be encountered (at least based on comparative captures of moths in pheromone traps in 2006 and 2007), there were some legitimate instances of damage caused by western bean cutworm larvae feeding in corn ears. As you know, the western bean cutworm was discovered for the first time in Illinois in 2004, so the pest has not had much of a chance to establish populations that represent consistent annual threats. Nonetheless, several people, including University of Illinois Extension personnel, continue to monitor for adults (moths) captured in pheromone traps during the summer. These reports of captures are reported on the Iowa State University "Western Bean Cutworm Monitoring Network."

Interestingly, a comparison of the number of moths captured in Illinois in 2007 (1,534) and 2006 (6,238) suggests that densities of western bean cutworm larvae in Illinois in 2007 should have been lower than they were in 2006. However, it is important to note that we placed greater emphasis on accurate identification of western bean cutworm moths this year. It's possible that some of the reported captures of western bean cutworm moths in 2006 were actually other species, such as yellowstriped armyworms (Spodoptera ornithogalli). We explained the possibilities in an article titled "Captures of Western Bean Cutworm Moths?" in issue No. 14, June 29, 2007, of the Bulletin.

Western bean cutworm larva in corn ear in Lee County, August 15, 2007 (photograph courtesy of Paul Yoder, Pioneer Hi-Bred International).

We also are aware, however, that some damage to corn ears was mistakenly attributed to western bean cutworms instead of the real culprits, corn earworms. Once again, we emphasize the importance of accurate identification of insects to developing appropriate management plans. Identifying caterpillars is not easy, but there are ways to tell them apart. Check out the article "Corn Earworm, European Corn Borer, Fall Armyworm, or Western Bean Cutworm: Which One Is Causing the Injury I'm Finding on My Corn Ears?" published in issue No. 23, October 7, 2005, of the Bulletin.

The results from our annual survey of second-generation European corn borers have been arriving at our office since late August. The survey is almost concluded, and we will share the results with you in a future issue of the Bulletin. Suffice it for now to say that a lot of the Extension educators have responded to my inquiries with "What borers?" Numbers of second-generation European corn borers seem to be quite low yet again, lending further evidence that Bt corn may be suppressing corn borer populations in a lot of places. Look for further developments to this story.--Kevin Steffey

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