Aphids suck plant juices and can be controlled with sprays of malathion, Sevin, or rotenone.
Four-lined plant bug causes circular brown spots on the leaves when it feeds. The spots can be numerous and may be confused with a disease. No chemical control is listed.
Tarnished plant bug punctures shoots and injects a poison causing the shoot to die. Control this insects with sprays of Sevin.
European corn borer may feed on cosmos in late summer. The larvae feed on buds and flowers causing distortion. The stem boring causes wilting and death of plant parts above the point of lowest injury. Remove the dead tip by cutting 2 inches below the wilt point. Sprays of Sevin will control this insect.
Mites cause the leaves to be grayish.