Cornflower, Bachelor's Button (Centaurea)
Plant Health Problems
This plant has no major disease problems in the Connecticut landscape.
Insect Problems:
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.