First generation European corn borer egg hatch has begun with confirmed feeding reports in corn plants in southern Wisconsin. Krista Hamilton, WI DATCP Pest Survey, reported egg hatch on June 8^th. Please visit the Wisconsin Pest Bulletin corn section for this week’s statewide update http://pestbulletin.wi.gov/index.jsp. For the current week, Charlie Hampton (Syngenta Crop Protection) observed first generation European Corn Borer leaf feeding in conventional non-Bt corn fields in the Janesville/Clinton area.

European corn borer first generation development is relevant statewide. Degree day (DD) accumulations for most of WI have exceeded 700 DD, ranging between 600-900 DD throughout the state. Once 800 GDD (base 50F) are reached in your area, be aware of potential for early instar feeding on corn leaves. Field scouting should be conducted during this time and before DD accumulations exceed 1000 DD. Early instar larvae, coinciding with 800-1000 DD, feed on corn plant leaves. Early leaf feeding on whorl leaves results in a line of “pin hole” or “shot hole” circular feeding holes across the leaf as leaves expand. After 1000 DD, larvae begin to bore into leaf midribs – eventually working their way into the stalk.

European corn borers overwinter as 5^th instar larvae in corn stalks, crib corn, and large-stemmed weeds. In spring larvae remain in these protected sites to pupate. After emergence in June, moths spend daylight hours in sheltered areas (weeds or grasses) within or bordering corn fields. Females deposit egg masses on the underside along the midrib of lower leaves of young corn plants.

 

European corn borer degree days are recorded using ambient air temperature above a base temperature, specific to this insect species, of 50F. Accumulation begins after January 1^st. As heat units accumulate, overwintering larval development into pupae and spring moth emergence can be tracked. As the WI DATCP Wisconsin Pest Bulletin stated on May 11^th, “European corn borer life history events are unfolding at a remarkably fast rate”. With more heat units to date in 2007, we are now observing first generation larval feeding earlier than the typical mid-June start time.

 

Moths are attracted to the tallest corn for egg laying. Young corn plants produce DIMBOA, a naturally occurring plant compound that inhibits larval survival on the plant. Once corn plants reach an extended leaf height of 17-24 inches, this plant chemical is sufficiently diluted for larvae to feed and develop on the corn plant. Early planted, conventional field corn is relatively taller during first generation flight and provides an attractive oviposition site for female moths. Therefore, scouting earlier planted fields, nearing or exceeding the 17-24 inch leaf height will help focus scouting efforts.

 

 

 

Refer to the UWEX Publication The European Corn Borer, available at http://learningstore.uwex.edu/pdf%5CA1220.pdf.

 

If economic threshold larval populations are reached, the appropriate time to control first generation in conventional corn with a foliar insecticide application is during the period of 800 to 1000 degree days (above a modified base temperature of 50F). This window brackets the time during which larvae feed on outer plant surfaces prior to tunneling. Scouting during this time will indicate whether treatment is justified.

 

A visit to the WI-MN Cooperative Extension Agricultural Weather Page at http://www.soils.wisc.edu/wimnext/ will help you quickly assess first generation European Corn Borer degree-days for your area. From the home page, click on the “Corn” link. From the Corn Crop Product page click on “European Corn Borer” under the insect phenology section.