Joe also reported receiving an e-mail message describing coneflowers with clipped flower heads. The most likely culprit is the SUNFLOWER HEAD-CLIPPING WEEVIL (Haplorhynchites aeneus), a well-documented pest of cultivated and wild sunflowers (Helianthus spp.) in the Great Plains States. The weevil is also known to infest other members of the Aster Family (Asteraceae = Compositae). However, coneflowers appear to be a less well-known menu item based on the literature; few sources mention this host.
The shiny black to brownish-black weevils are a little over 1/4" long which includes their 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 - 1 1/2" below the flower head. The flower stem is not completely cut; the damaged stem just breaks-over causing the flower head to hang from a thin strand of stem tissue.
Females move into the dangling flower head to feed on pollen, mate with males, and lay eggs. 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 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 July. There is one generation per year.
The best method for controlling this weevil is to remove and destroy the dangling flower heads. 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 and reduce the weevil population. Insecticides are not a good option. First, there are no insecticides labeled for flowering landscape plants that include this weevil on the label. Second, since coneflowers attract a wide array of important pollinators, insecticide applications could potentially cause collateral damage to these "good bugs."