Stalk borer larve are developing in grassy areas across Iowa. They feed inside the culms of grasses and also in the stems of giant ragweed, marijuana, and other plants commonly found in fencerows, terrace backslopes, and grassed waterways.
|Early stage stalk borer has a purple midsection and an orange head with a black stripe.|
The larvae feed until they outgrow the grass or plant stem housing them, forcing them to move to new feeding sites, such as corn plants. Thus, damage is often limited to border rows that are adjacent to grassy, weedy areas. "Moving day" for the stalk borers starts when approximately 1,300 base-41 degree days have accumulated. Typically, approximately 10 percent have relocated by the time that 1,400 degree days are reached, and 50 percent have moved by 1,700 degree days.
|Stalk borer damage to corn border rows.|
Insecticide treatments for stalk borer control are only effective when the larvae are in between feeding sites. Once larvae have bored into the young corn, they are shielded from insecticide treatments; thus, scouting based on heat accumulation is critical to properly time treatments. By using degree-day accumulations as of May 27, and average daily accumulations anticipated, the map shows projected the dates for when different parts of Iowa will approach 1,400 degree days. These dates are presented by crop reporting district.
Average daily accumulations of base-41 degree days in the first half of June are approximately 26 in the northern third of Iowa; 27 in the central third, and 28 in the southern third. If weather in an area is significantly colder than average borer development will be delayed, and if it is significantly warmer than average, borer development will be advanced.
This article originally appeared on page 95 of the IC-488(11) -- June 3, 2002 issue.