My oxeye sunflowers are covered in aphids. In previous years, they have not usually damaged the plants significantly. We have lots of lady bugs that like to chow down on them. This year, the blooms look stunted and I'm not sure they will flower. Will aphids damage a plant enough to prevent flowering?
We have been seeing a lot of reports of aphids at this time of year. Typically, a few aphids do not affect the overall health of a plant and they will outgrow the damage later in the season. If an infestation is severe though, it can affect plant growth and flowering. In your photo, we see sooty mold on some of the leaves. This is a mold that develops on the honeydew secretion from the aphids; it is a problem because it blocks the amount of sunlight the leaves can receive.
You can use a strong spray of water to dislodge the aphids from the plants, and/or apply an insecticidal soap. Avoid spraying insecticidal soap on a hot sunny day, as it could "burn" the plant.
Take a look at our page about aphids for additional information and how to control them. https://extension.umd.edu/hgic/aphids-trees-and-shrubs