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Cutworms and caterpillars are the prime insect attackers of petunia plants and your primary concern when caring for petunia plants.

What is it?

Cutworms are 1.5 inches to 2 inches long and may be gray, brown or black. Caterpillars may be smooth, or hairy and grow up to 4 inches in length.

What does it look like?

Cutworms coil when disturbed, and are usually found about 2 inches deep in the soil of damaged plants near the base. The most likely pests of young petunias are the surface-feeding cutworms and climbing cutworms. Caterpillars begin their damage as adult female moths or butterflies, which lay eggs on plants. However, larvae, and mature caterpillars do their part as well.

How does it manifest?

Climbing cutworms shear leaves off older plants as well as young ones. Just one surface-feeding cutworm can sever the stems of many young plants in an evening. Both hide in the soil during daylight hours and feed only after sundown. Once the cutworm matures, its adult form is that of a dark, night-flying moth with stripes or bands on the forewings. The mobility of these cutworms allows them to cause great damage in little time. When female moths or butterflies lay their eggs, they later emerge as larvae which feed on the leaves, flowers and buds of petunias for 2 to 6 weeks depending on species. Mature caterpillars then pupate in cocoons attached to leaves, or buried in the soil structures. These then emerge as moths or butterflies and start the damage process again. There can be several overlapping generations during one growing season.

What can you do about it?

Apply insecticide containing diazinon, chloropyriufos or carbaryl around the undamaged plants if you observe evidence of stem cutting. You will most likely need to reapply weekly, as cutworms are very difficult to control. Applying a preventative treatment of diazinon before transplanting can sometimes stave off cutworm infestation. Further reduce damage by placing cutworm collars, made of aluminum or paper, around the stems of each plant. Make collars at least 2 inches high and press them firmly into the soil. Caterpillar damage can be controlled by an insecticide containing acephate, and occasionally one containing bacillus thuringiensis. Although they require different insecticides, both cutworms and caterpillars can be controlled in late summer and early fall, by cultivating soil thoroughly to expose and destroy worms, including eggs, larvae and pupae.