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Issue No. 16, Article 2/July 13, 2007

Western Bean Cutworms . . . The Situation, Thus Far

We're written several articles already about the network of pheromone traps that will keep track of western bean cutworm moths throughout the Midwest this year, and the network is working. Data for 2007 to date can be viewed at Iowa State University's "Western Bean Cutworm Monitoring Network" site. Based upon reports in Illinois thus far, western bean cutworm moths have been found in very few locations. The sites at which western bean cutworm moths have been found are mostly in northwestern Illinois, with a handful of exceptions. If you scroll down through the numbers of western bean cutworm moths being found in Iowa, however, you will get a better sense of the relatively small numbers of moths that have been captured in Illinois.

As I have stated before, I don't think the western bean cutworm has yet established itself in Illinois as a pest of major concern. Fortunately, we began tracking it early enough (2005) to be able to determine when (or whether) we will have to polish our management plans. In the meantime, we can continue to determine timing for scouting and potential management responses by keeping track of accumulated degree-days. Following are estimated percentages of moth emergence based upon accumulated degree-days (base 50°F) from May 1:

  • 25% emergence:1,319 accumulated degree-days
  • 50% emergence: 1,422 accumulated degree-days
  • 75% emergence: 1,536 accumulated degree-days

When I visited the degree-day calculator at the "Water & Atmospheric Resources Monitoring" (WARM) Web site on July 11, I learned that 1,452 degree-days had accumulated at the Freeport (Stephenson County) site through July 10, indicating 50% moth emergence. The projected total for one week after July 10 was 1,611, well past 75% emergence. So, it seems that emergence of western bean cutworm moths in northern Illinois is rapidly approaching its conclusion. (Important note: Information at the aforementioned Web site indicates that accumulation of degree-days for predicting western bean cutworm activities began on January 1. This is a typo. The accumulation of degree-days was initiated on May 1.)

Western bean cutworm females currently are laying eggs in cornfields and lay most of their eggs during peak moth flight. We do not have very much data over time to indicate when peak moth flights occur in Illinois, but we suspect it usually occurs in mid-July. In 2006, peak moth flight generally occurred during the week of July 17-24. Based upon the occurrences of other insects in Illinois this year, you should expect peak moth flight of western bean cutworms to occur earlier in 2007 than it did in 2006.

So, if you are inclined to look for this new pest of corn in Illinois, start looking right now. For information about what to look for and how to look for it, once again I direct you to the videos from a short course located on the North Central IPM Center Web site. As a reminder, you need to scout for this pest before most of the larvae have found their way to the developing corn ears, after which insecticides will not be very effective.--Kevin Steffey

Kevin Steffey

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