Several species of cutworms attack petunias and many other flowers and vegetable plants. The most likely pests of young petunia plants set out early in the season are the surface-feeding cutworms and climbing cutworms. Climbing cutworms shear the leaves off of older plants. Cutworms hide in the soil during the day, and feed only at night. Adult cutworms are dark, night-flying moths with bands or stripes on their forewings.
Apply a bait containing carbaryl or an insecticide containing diazinon around the base of undamaged plants when stem cutting is observed. Since cutworms are difficult to control, it may be necessary to repeat the treatment at weekly intervals. Before transplanting into the area, apply a preventive treatment of an insecticide containing diazinon or chlorpyrifos, or the biological insecticide Bacillus thuringiensis (Bt), and work it into the soil. Cultivate the soil thoroughly in late summer and fall to expose and destroy eggs, larvae, and pupae. Further reduce damage with "cutworm collars" around the stem of each plant. These collars can be made of stiff paper or aluminum foil. They should be at least 2 inches high, and pressed firmly into the soil.
Plant Care for Petunia