A couple of reports this past week prompted me to assemble this article. I received information from both Clark and Adams counties that people suspected they had found southwestern corn borer larvae in a giant ragweed stem (Clark County) and a cornstalk (Adams County). Because we have experienced significant infestations of southwestern corn borers in southern Illinois during the past 3 years, it wasn't unreasonable to think that the pest was present in Clark County. However, Adams County is a bit too far north for southwestern corn borers, so we needed to look into it.
Fortunately, the individual who wondered whether he had found southwestern corn borers in Adams County had photographs of the insect. When I examined them, I saw a dark purple stripe that extended from the prothorax (the "neck" behind the head) onto the side of the yellow-orange head. This characteristic marking convinced me that the insect was a mature stalk borer larva.
We are used to seeing stalk borers when they are much younger and feeding on smaller corn plants earlier in the growing season. After the borers tunnel into the corn stalks, we usually forget about them. However, they continue to feed within the stalks during the summer. As the caterpillars grow, the characteristic dark purple "saddle" that is so obvious on the body of smaller larvae fades; older larvae appear creamy white with some slightly darker pigmentation (tinge of purple) also evident. Small, dark tubercles (small knotlike or rounded protuberances) are evident against the light color of the caterpillar's body, so mature stalk borer larvae appear spotted. Because southwestern corn borer larvae are covered with dark tubercles, confusing mature stalk borers with southwestern corn borers is a distinct possibility. The distinguishing characteristics of the two pests are the dark stripe on the head of the stalk borer and the large, dark tubercles all over the body of the southwestern corn borer.
Even before I wrote this article, the people who had found the caterpillar in a giant ragweed stem in Clark County had confirmed that the insect was a mature stalk borer. Giant ragweed, cocklebur, giant burr-elder, docks, and burdock are among the more than 170 plant species that serve as hosts for stalk borers. Whether the larvae are in corn or weeds, they will pupate soon, and adults will begin emerging during August. Peak emergence usually occurs during the first 2 weeks of September. The females preferentially deposit eggs on nearby suitable grasses such as smooth brome grass, quackgrass, orchard grass, woolly cupgrass, and wirestem muhly, and on some broadleaf weeds such as giant ragweed. These eggs overwinter, and young larvae will hatch and begin feeding next spring.
Because southwestern corn borers can be very damaging pests, proper identification of spotted caterpillars is very important. We certainly don't need rumors of southwestern corn borers in places where the borers don't occur.--Kevin Steffey