During this week’s BYGL Zoom Inservice, Beth Scheckelhoff (OSU Extension Putnam County) showed pictures of Sunflower Headclipping Weevil (Haplorhynchites aeneus, Family Curculionidae) females initiating the slow decapitation of purple coneflowers (Echinacea purpurea, family Asteraceae) in a mass planting on the OSU Ag campus. Reports of this weevil on coneflowers in Ohio have become an annual event.
Weevil damage includes dangling flower heads and stems with decapitated heads that looked like soda straws. “Headclipping” accurately describes the plant-damaging behavior of this weevil.
The sunflower headclipping weevil is native to the U.S. The weevil is a well-documented pest across the Great Plains where it earned its Entomological Society of America approved common name by clipping the flower heads of cultivated and wild sunflowers (Helianthus spp., family Asteraceae). However, it’s seldom a serious agricultural pest.
The weevil will also attack several members of the Silphium genus growing in prairies including compass plant (S. laciniatum), prairie dock (S. terebinthinaceum), cup plant (S. perfoliatum), and rosinweed (S. integrifolium). Indeed, an unapproved common name for the weevil in the Great Plains states is the “Silphium weevil.”
A High Plains Drifter in the Buckeye State
We reported in the July 15, 2010, BYGL newsletter that the sunflower headclipping weevil was causing noticeable damage to purple coneflowers in central and southern Ohio. The identification of the weevil was made by Dave Shetlar (Professor Emeritus, OSU Entomology, “The Bug Doc”). Since that time, the weevil has become a common subject in BYGL Alerts.
However, the behavior of the weevil in Ohio has been perplexing. In 2010, Echinacea did not appear in the literature as a host for the weevil. Indeed, it was difficult to diagnose the damage observed that year because it did not match with known pests of coneflowers.
Once we learned the weevil was a high plains drifter, we speculated that it had moved east into Ohio. However, a literature search revealed that the weevil may be found throughout the eastern U.S. westward to the continental divide. Even more significant, the weevil and its head-clipping handiwork were observed on woodland sunflower (H. microcephalus) in southern Ohio in 1967. It wasn’t new to Ohio, but it had been flying below our radar until 2010.
The weevil’s strong connection to Echinacea in Ohio fueled speculation that the weevil had expanded its host range to exploit a more widely available food source compared to its traditional hosts. Sunflowers are not high on the list of agricultural crops in Ohio. Coneflowers were and are commonly used in mass plantings in Ohio landscapes and usually included in the development of naturalized areas. It's a readily available food source for sunflower headclipping weevil adults and larvae.
Or perhaps the weevil already had a taste for Echinacea; however, the host was never documented. Several species including E. angustifolia are native throughout most of the Great Plains states.
Regardless of the reason(s), coneflowers continue to be the weevil’s favored host plant in Ohio. However, over the past several years, it has begun to appear on members of the Silphium genus as well as on sunflowers. Of course, both are becoming more common in Ohio landscapes and naturalized areas. All of the images in this Alert of the sunflower headclipping weevil damage to sunflowers and Silphium spp. were taken in central and southwest Ohio.
We also occasionally see the weevil attacking common ragweed (Ambrosia artemisiifolia) and giant ragweed (A. trifida). I originally believed this only occurred near mass plantings of more preferred aster hosts, but I’ve also observed damage on ragweeds that are growing far from coneflowers or Silphium spp.
Description and Life Cycle
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).
Male and female weevils are drawn to newly developing flower heads where they feed. The females eventually use their mouthparts at the end of their snouts to either chew holes or carve a groove into the flower stem a short distance 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.
However, sometimes things go horribly wrong. Plants belonging to the Silphium genus are notorious for exuding copious quantities of sticky, resinous sap (thus the name “rosinweed”) in response to wounding. I’ve observed dead weevils stuck in the gluey sap issuing forth from stems damaged by the luckless weevil.
If the females are successful in weakening the flower stems, the dangling flower heads attract even more females and males where they all “get acquainted.” Of course, viewer discretion is advised.
Mated females lay eggs at the base of the disk-flowers, and 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 prevents other insects from competing for the seed head prize and may reduce larval exposure to plant defense chemicals.
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 decapitating damage by this weevil is mainly aesthetic, but the reduction in seed production could potentially cause a significant decline in natural re-seeding. Indeed, in a paper published in 2017 on the effects of fire on tallgrass prairie forbs, the authors note: “… the increase in Silphium flowering stems in burn years on protected and unprotected plots was due to a reduction in the damage to flowering heads caused by the fire-sensitive prairie weevil, Haplorhynchites aeneus Boehman.”
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 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!