The peculiar handiwork of the sunflower headclipping weevil (Haplorhynchites aeneus) is just getting underway this season on purple coneflowers (Echinacea purpurea) and other members of the aster family (Asteraceae) in southwest Ohio. The damage includes dangling seed heads and stems that looked like soda straws.
The native weevil is a well-documented pest of cultivated and wild sunflowers (Helianthus spp.) in the Great Plains. In recent years, the weevil has reared its ugly snout in Ohio to become a serious pest of coneflowers in landscapes and naturalized areas.
The headclipping weevil has also shown a fondness for members of the Silphium genus. The alternate common name "Silphium weevil" reflects the weevil's appetite for compass plant (S. laciniatum), prairie dock (S. terebinthinaceum), and cup plant (S. perfoliatum).
I've also found the weevil attacking common ragweed (Ambrosia artemisiifolia) which provides a clear demonstration this weed belongs to the aster family. It also suggests that ragweeds may serve as a reservoir for the headclipping weevil.
The shiny weevils range in color from black to copper-brown. In fact, the specific epithet, aeneus, means "bronze-colored." They measure a little over 1/4" in length which includes an exceptionally long, curved snout. As with all weevils, this beetle's mouthparts are located at the end of their snout (rostrum).
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.
Apparently, some plants can mount a defense against the weevil's stem-puncturing behavior by exuding copious quantities of sticky, resinous sap. I came across a dead weevil yesterday that appeared to be stuck in the gluey sap issuing forth from holes the luckless weevil had chewed into a prairie dock stem.
If females are successful with weakening the flower stems, they 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 after 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 weevil's odd headclipping behavior reduces larval exposure to plant defense chemicals and also prevents other insects from competing for the seed head prize.
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.
Damage is mainly aesthetic, but the reduction in seed production could 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 thus reducing the weevil population for next season.
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!