At a recent Master Gardener meeting, Susan Kovach asked if anyone had the problem of purple coneflowers having their flower heads almost cut off and left hanging toward the ground. Four summers ago I noticed this problem in my garden.
This is the work of the sunflower head-clipping weevil (Haplorhynchites aeneus) a well-documented pest of cultivated and wild sunflowers in the Great Plains. Sunflowers are the preferred plant but the weevil also attacks purple coneflowers and other plants in the aster family.
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, the mouthparts are located at the end of their snout. The females insert their snouts and chew a ring of holes around the stem one to two inches below the flower head. The flower stem is not completely cut; this weakens the stem to the point where it eventually breaks over, leaving the flower hanging by a thin strand of tissue.
There is only one generation per year. Only females are thought to perform the clipping behavior. “The female enters the flower to feed on the pollen and lay eggs. The flower eventually falls to the ground, eggs hatch and the immature weevil, a worm-like larvae, moves into the ground for winter. Next spring the larvae pupates, then transforms into a weevil and starts feeding on the flower stems in mid-summer.” (Melinda Myers website) It is speculated that the weevil’s odd head-clipping behavior reduces larval exposure to plant defense chemicals and also prevents other insects from competing for the seed head prize.
Removing the flower heads that are still hanging as well as those that have dropped to the ground can help to reduce populations next year. I have found that if the flower head has turned brown before it drops off, it is often difficult to find it on the ground. When removing the flowers from the stem, hold a bucket of soapy water under the flower and clip the rest of the stem, dropping the flower into the bucket. This may help to kill any adults that are feeding in the flowers, reducing the populations and further damage to other plants. It also kills the larvae and prevents them from further developing.
In past summers I have removed the damaged coneflowers and stems well below the weevil cut, put them in a plastic bag, sealed the bag, and put the bag in the garbage. Initially I cut the damaged part of the stalk just below the weevil cut leaving several inches of stem. When checking a few days later for further damage, I was not sure if I had removed the damaged stem and flower or the flower had dropped to the ground leaving a flowerless stalk. To solve this issue, I now cut back to the leaves below the weevil cut and I know that the flower head has not dropped to the ground. I tour my garden about every three days and carry a bag and clippers with me to remove any new damage. This minimizes damage and reduces future infestations.
Cutting off the damaged part of the plant and disposing of it is the best garden control method for the sunflower head-clipping weevil. Pesticides are not a good option. Pesticides should not be used on a plant in full bloom because of the risk of killing pollinators.
This weevil has been at work in Indiana for several years. The first mention I found of the weevil causing damage in Indiana is in an August 2011 Hoosier Gardener article by Jo Ellen Meyers Sharp. Other websites report the weevil in Illinois (2007) and Ohio (2009)
Note. This article is a compilation of information the websites listed below.
https://entomology.k-state.edu/extension/insect-information/crop-pests/sunflowers/sunflower-headclipping-weevil.html (Kansas State University website)
https://bygl.osu.edu/node/1087 (good pictures – Ohio State University)