There are several caterpillars that bore into herbaceous plants. The larvae of several clearwing moths bore into the stems or roots of the plants. The larvae are creamy white with a brown head. European corn borer, which is a pest of corn, also may attack 200 plant species including perennial sunflowers, dahlia, shasta daisies, chrysanthemum, and veronica. Newly hatched larvae feed on the leaves of plants for a few days and then bore into the stem of the plants. Frass and silk will be visible near the entrance holes. The larvae tunnel through the stems of the plants reducing water and nutrient flow as they feed.
Borer inside chrysanthemum stem. Photo: R. Mulrooney, University of Delaware
To control borers in flowering plants, look for wilting or browning of the plant. Examine the stem or roots for evidence of the borers and prune affected parts or the plant if necessary. There are no insecticides available to control borers.