European Corn Borer On Flowers
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Symptoms - Buds Discolored; Tunnels in Stems; Plants Wilted
European corn borer and stalk borer sometimes attack flowers such as dahlia. The stalk borer is a striped worm is more likely to appear in early summer, whereas the corn borer arrives later in the summer. These nuisances feed on bud tips, flowers and leaves and then, left unchecked, burrow down inside the flower stem. This causes the plants to wilt.
This pest is more common in the vegetable garden.
While these pests are still outside the stem of the plant they can be treated that same way as you control any caterpillar. See the details in Controlling the Caterpillar
Once the borer has invaded the stem, there is little you can do but remove the stem and flower to the trash.