I share Pam Bennett's love for coneflowers; she highlighted the delightful range of cultivars in her BYGL Alert! posted on June 30. Of course, as she also noted, mass plantings of this wonderful native may suffer from occasional problems. I'm covering three of the more serious coneflower challenges that may threaten coneflowers in Ohio landscapes in a 3-part series under the banner, "Coneflower Calamities." Fortunately, each of these problems can be effectively managed through accurate early identification and focused management options.
The first is the Sunflower Head-Clipping Weevil (Haplorhynchites aeneus). I came across the developing symptoms of this weevil yesterday in a mass planting in southwest Ohio. The weevil is a well-documented pest of cultivated and wild sunflowers (Helianthus spp.) in the Great Plains and is also known to infest other members of the aster family (Asteraceae = Compositae) particularly members of the Heliantheae tribe which includes both coneflowers as well as members of the Silphium genus. Indeed, the weevil is sometimes called the "Silphium weevil" owing to its strong association with plants in this genus.
For reasons not clearly understood, the Sunflower Head-Clipping Weevil has reared its ugly snout in recent years to attack coneflowers in Ohio. Worse, clipped seed heads are also being found on various members of the Silphium genus including compass plant (S. laciniatum) and cup plant (S. perfoliatum).
The shiny black to brownish-black weevil is a little over 1/4" long with the measurement including an exceptionally long, curved snout. As with all weevils, this beetle's mouthparts are located at the end of their snout. The females insert their snouts into the flower stems to chew a ring of holes around the stem about 1" below the flower head. The flower stem is not completely cut; the damaged stem just breaks-over causing the flower head to hang from the stem on a thin strand of tissue.
Males and females move into the damaged flower head to feed on pollen and mate. The females then lay eggs on the dangling head. Eventually the flower head breaks from the stem and drops to the ground. Heavily de-flowered coneflower plantings look like a collection of soda straws. The eggs hatch once the flower heads drop to the ground and the weevil's grub-like larvae feed on the decaying flower head tissue. It is speculated that the female weevil's odd head-clipping behavior reduces larval exposure to plant defense chemicals and prevents other insects from competing with their off-spring in utilizing the flower head. Mature weevil larvae leave the flower heads and crawl into the soil to spend the winter. Pupation occurs the following spring to early summer and adults appear sometime in late-June to early July. There is one generation per year.
The reduction in seed production caused by damage from this weevil can potentially cause a significant decline in natural re-seeding. The best method for controlling this weevil is to remove and destroy the dangling flower heads as well as heads that have dropped to the ground. This will prevent weevil larvae from completing their development. If the flower heads are removed gently to avoid disturbing the hidden adults, the heads can be dropped into a bucket of soapy water to kill the adults. This will reduce the weevil population and thus reduce damage to flower heads. Insecticides are not a viable suppression option. Insecticide labels will not support making an application to plants in full flower because of the substantial risk of killing plant pollinators. Remember: the label is the law!