Cornflower, Bachelor's Button (Centaurea)
Plant Health Problems
This plant has no major disease problems in the Connecticut landscape.
Aphids, Brachycaudus helichrysi.
Brachycaudus helichrysi and a root aphid sometimes attack cornflower. Insecticidal soap or ultra-fine horticultural oil, which are among the compounds registered for control of this pest in Connecticut, can be applied to control aphids on foliage. A malathion soil drench can be applied as needed to treat root aphids. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.
Stalk borer, Papaipema nebris.
This borer infests an occasional stalk of many kinds of herbaceous plants, including cornflower. As a rule its presence escapes notice until the plant begins to wilt. Then it is too late for the plant to recover. The larva tunnels up and down inside the stem, and the top portion usually wilts and later dies. There is one annual generation. The moths emerge in September and October and lay eggs on the stalks of their food plants, in which stage the insect passes the winter. The eggs hatch in May or early June. The young larva begins to feed on the leaves of the nearest food plant, and later tunnels in the stem. The mature larva is nearly 1 1/2 inches in length, grayish brown with one white dorsal stripe and two white lateral stripes on each side. On the front half of the body the lateral stripes are interrupted, and the lower brown stripe extends forward onto the side of the head.
Burning all the old stalks, if allowed, and destroying weeds at the edges of the garden helps control this insect. When needed, methoxychlor, which is among the compounds registered for use against this pest in Connecticut, applied as a dust, in June, should control this pest. Consult the label for dosage rates and safety precautions.