King Louis XVI was a little over 6 ft. at the beginning of his reign, but around 5'2" or 5'3" at the end. His abrupt change in stature was impactful.

Although much less impactful, Sunflower Headclipping Weevil (Haplorhynchites aeneus, Family Curculionidae) females are beheading members of the aster family (Asteraceae), with purple coneflowers (Echinacea purpurea) being a primary target. Free of their heads, the coneflower stems look like soda straws.

Sunflower Headclipping - Weevil on Coneflower

However, unlike Louis’ end, the loss of the aster flower heads occurs in stages. At first, the heads dangle from their stems. Without bourbon, this is where the King Louis references end.

Sunflower Headclipping – Weevil Damage in Coneflowers

The weevil will also decollate several members of the Silphium genus growing in native prairie plantings, 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.”

Damage to Compass Plant

Damage to Prairie Dock

Damage to Cup Plant

We also occasionally see the weevil attacking common ragweed (Ambrosia artemisiifolia) and giant ragweed (A. trifida). The connection to the ragweeds is important when considering management tactics.

Damage to Giant Ragweed

Not Just a High Plains Drifter

The sunflower headclipping weevil is native to the U.S. and is a well-documented pest across the Great Plains, where it earned its common name by clipping the flower heads of cultivated and wild sunflowers (Helianthus spp.). However, it’s not considered a serious agricultural pest.

Annual Sunflower Display Field (with Nonconformist)

The headclipping weevil is of greater concern where members of the aster family are used in naturalized areas, pollinator gardens, and mass plantings in landscapes. Aside from the potential loss of flowers for pollinators, there’s also a concern that high weevil populations could reduce natural re-seeding, particularly if the loss occurs year after year.

Coneflowers in Naturalized Area

Bumble Bee on Coneflower

The weevil was virtually unknown in Ohio until 2010, when Dave Shetlar (Professor Emeritus, OSU Entomology, “The Bug Doc”) reported in the July 15 newsletter version of the BYGL that the weevil was causing noticeable damage to purple coneflowers in central and southern Ohio. At first, we believed the weevil had moved from its native range into our state.

Damage to Coneflowers

However, a review of the scientific literature revealed that the native range of the headclipping weevil extends throughout the eastern U.S., including Ohio, and westward to the Continental Divide. Indeed, a paper published in 1973 noted that cut heads on Small Woodland Sunflower (H. microcephalus) had been observed in Ohio in 1967. Consequently, although the weevil had been with us, it had largely flown below our radar until 2010.

Since that time, the peculiar damage produced by the weevil has become a common sight throughout Ohio. Sunflowers are not high on the list of agricultural crops in Ohio. However, many of the weevils’ aster hosts are commonly found in our state’s landscapes and naturalized areas. It's been speculated that the headclipping weevil’s rise from obscurity was fueled by exploiting more bountiful food supplies.

Annual Sunflowers in Mass Planting

Prairie Dock Mass Planting

Damage to Prairie Dock – Note Beheaded Stems

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).

Sunflower Headclipping Weevil on Coneflower

Sunflower Headclipping Weevil on Prairie Dock

Sunflower Headclipping Weevil – Chewing a Prairie Dock Stem

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.

Damage to Coneflower Stem

Damage to Compass Plant Stem

Sunflower Headclipping Weevil – Chewing a Groove in Prairie Dock Stem

Sunflower Headclipping Weevil – Chewing a Prairie Dock Stem

The flower stem is not completely cut. The damaged stem just breaks over, causing the flower head to droop from the stem on a thin strand of tissue.

Damage to Coneflower

Damage to Compass Plant

Damage to Prairie Dock

Damage to Prairie Sunflower

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.

Dead Sunflower Headclipping Weevil on Prairie Dock

If the females successfully weaken the flower stems, the dangling flower heads attract even more females and males, where they all “get acquainted.” Of course, viewer discretion is advised.

Sunflower Headclipping Weevils on Prairie Dock (Viewer Discretion is Advised)

Mated females lay eggs at the base of the disk-flowers. Eventually, the flower head breaks from the stem and drops to the ground.

The eggs hatch after the flower heads drop to the ground, and the weevil's grub-like larvae feed on the pollen and 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.

Management

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 attributed a significant increase in Silphium flowering stems in burn years to fire reducing headclipping weevil populations.

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 the next season.

Cut Flower Heads – A Weevil Management Target

Anecdotally, this management method has been effective in reducing and sometimes eliminating the weevils in mass plantings of coneflowers in Ohio parks and landscapes. Of course, as noted above, it’s also important to inspect non-cultivated members of the aster family, such as ragweeds, for evidence of weevil damage.

Sunflower Headclipping Weevil on Common Ragweed

Damage to Common Ragweed

Obviously, 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!

Selected References

Anderson, R. C., Anderson, M. R., & Corbett, E. A. (2017). White-tailed deer (Odocoileus virginianus) and fire effects on flowering diversity of tallgrass prairie forbs1. The Journal of the Torrey Botanical Society, 144(3), 243-253.

https://doi.org/10.3159/TORREY-D-15-00024.1

Aslam, M., & Wilde, G. E. (1991). Potential insect pests of sunflower in Kansas. Journal of the Kansas Entomological Society, 109-112.

https://www.jstor.org/stable/25085253

Hamilton, R. W. (1973). Observations on the biology of Haplorhynchites aeneus (Boheman)(Coleoptera: Rhynchitidae). The Coleopterists' Bulletin, 83-85.

https://www.jstor.org/stable/3999439

Hamilton, R. W. (1974). The genus Haplorhynchites (Coleoptera: Rhynchitidae) in America north of Mexico. Annals of the Entomological Society of America, 67(5), 787-794.

https://doi.org/10.1093/aesa/67.5.787

Hamilton, R. W. (1981). Description of the larva and pupa of Haplorhynchites aeneus (Coleoptera: Curculionoidea, Rhynchitidae). Journal of the Kansas Entomological Society, 616-624.

https://www.jstor.org/stable/25084199

Pilson, D. (2000). Herbivory and natural selection on flowering phenology in wild sunflower, Helianthus annuus. Oecologia, 122(1), 72-82.

https://doi.org/10.1007/PL00008838

Pilson, D., & Decker, K. L. (2002). Compensation for herbivory in wild sunflower: response to simulated damage by the head‐clipping weevil. Ecology, 83(11), 3097-3107.

https://doi.org/10.1890/0012-9658(2002)083[3097:CFHIWS]2.0.CO;2

Ron Wilson

In The Garden With Ron Wilson