Acres of YieldGard Corn Borer corn are up, populations of European corn borers have been down, and scouting for second generation larvae is not on the minds of many crop scouts. However, if you do have concerns about this pest in nonBt corn fields, then now is the time to start scouting. Adult moth flight has begun in Iowa. Some of the first moths were collected July 25 in Woodbury County (western Iowa) and July 28 in Boone County (central Iowa).
|European corn borer egg mass.|
|Corn borers hatching from egg mass.|
Late-planted corn typically has the largest populations of second-generation corn borers. Pay special attention to fields that are actively shedding pollen or have fresh silks.
If insecticide use is warranted, timing of treatments is critical. Scouting is the only effective way to know when to treat. Examine the undersides of the middle seven leaves (three leaves above and the three leaves below the ear leaf) on 20 plants at five locations in the field. Multiply the number of egg masses found by 1.1 to correct for eggs that might be on other leaves of the plants.
From information gathered in a field on egg counts, calculate a cost-benefit analysis by using a table posted on the Internet here . If the population is initially low (below the economic threshold), rescout the field in 5 to 7 days to determine if the egg population has increased.
This article originally appeared on page 143 of the IC-490(19) -- August 4, 2003 issue.