As soon as our article about stalk borers in last week's Bulletin (issue no. 7, May 12, 2000) was available on the Web, we began to get reports from people who were just beginning to observe stalk borer injury in corn. Timing is everything! |
Figure 2 shows the accumulated heat units (base 41°F) from January 1 through May 15 to estimate development and movement of stalk borers. Recall that we indicated last week that stalk borers first begin to move into corn when about 1,100 heat units have accumulated from January 1; 50% movement occurs when about 1,400 to 1,700 heat units have accumulated. We recommend scouting when 1,300 to 1,400 heat units have accumulated, and a decision to treat with an insecticide should be made between 1,400 and 1,700 heat units. Treatment decisions may be appropriate already in some counties in southern Illinois and imminent in some central Illinois counties.
Movement of stalk borer larvae into corn should be taking place throughout the southern half of the state. However, keep in mind that the estimates are guidelines. Movement into corn can begin earlier if the initial weed hosts are killed with herbicides, as was the case in at least one field in Hancock County during the week of May 8.
Watch for stalk borers in fields that had been infested with weeds last year, and watch for them along field margins as the larvae move into fields from weedy borders or waterways. If you find them early enough, you can prevent significant stand loss with a timely insecticide application. Insecticides suggested for control of stalk borers were published in last week's Bulletin.--Kevin Steffey