There have been several reports of common stalk borer (Papaipema nebris) in tomatoes and peppers in the past week. It can also attack potatoes and corn. This is an occasional pest that typically affects only a few plants in a field but which can sometimes cause economic damage. An early symptom is a drooping branch. If the affected stem is cut open, a caterpillar can be found inside. The stalk borer has a distinctive appearance, with long stripes on the back half of its body, and solid coloring on the front half of its body (see picture below). This insect takes 2 to 4 months to develop, which is much longer than most other caterpillars; it has up to 14 larval instars or sub-stages. As it grows, it tends to out-grow the stem that it is in, and it moves to a larger stem. By late summer, it prefers large-stemmed plants such as giant ragweed. Like with most borers, any stalk borer that is inside a stem is protected from insecticides. However due to its habit of moving from one stem to a larger stem, insecticide residues can kill a stalk borer when it moves out of a stem in search of a larger stem. Pyrethroid insecticides, which have long residual activity, are suitable for this purpose; these include Asana, Baythroid, Brigade, Hero, Mustang Maxx, permethrin, and Warrior.
Larva of common stalk borer in a maple stem. Photo by James Solomon, USDA Forest Service, Bugwood.org
-Celeste Welty, Extension Entomologist, OSU, Columbus OH