Mighty Mites
The handiwork of an eriophyid mite (family Eriophyidae) on coneflowers (Echinacea spp.) is becoming evident in Ohio. Spiky flower parts that rise rosette-like from coneflower cones are symptoms of an eriophyid mite that has yet to be taxonomically categorized, so it has no scientific name or approved common name. However, the mite is generally called the Coneflower Rosette Mite based on the characteristic damage it causes on coneflowers.
The coneflower rosette mites live inside the developing flowers where they feed at the base of the flowers by puncturing plant cells with stylets and sucking in the cell contents. As a result, elongated rosette-like tufts of stunted and distorted flower parts will sprout from the tops or sides of the cones of coneflowers.
Normally, we say that eriophyid mites can’t be clearly seen without some form of magnification; usually at least 40x. However, when large collections of eriophyids are present, they can be seen with the naked eye as illustrated in the image below. The eriophyids make the tips of the coneflower florets look like they’ve been dipped in white dust.
Eriophyid mites can be spread by the wind or sometimes by animals such as birds. Coneflower rosette mites practice an interesting behavior to increase their chances of being moved to new locations. The mites crawl to the tips of the florets and hold on with their caudal sucker so their legs are hanging in the air to snag a passing ride or so they can be blown to neighboring plants.
Cutting and destroying flower heads deformed by mite activity before the mites seek new horizons will reduce mite populations. However, care should be taken not to spread the mites particularly if they are clinging to the tips of the florets. A bag or other container should be held beneath the tufted flower heads as they are cut.
Freaky Flowers
Don’t confuse the symptoms produced by coneflower rosette mites with those produced by the phytoplasma disease, Aster Yellows, and vice versa. The phytoplasma behind aster yellows is, ‘Candidatus Phytoplasma asteris’. Symptoms of the disease include chlorotic, curled foliage; stunted stems; and bizarrely distorted flower parts.
Flower petals may appear as a ring of tiny greenish-yellow spoons arrayed around the base of highly deformed cones. Cones may appear as tightly clustered rosettes. This symptom is sometimes mistaken for damage caused by the aforementioned Coneflower Rosette Mite, and vice versa.
Aster Yellows wreaks havoc on all parts of the plant. There are no sprays that will suppress the disease. Once plants become infected, they remain both infected and infectious which means they serve as a constant reservoir of the phytoplasma to be spread to other plants.
Thus, sanitation is key to managing the disease. All parts of the plant including the root system must be removed and destroyed. As with all phytoplasmas, the Aster Yellows pathogen cannot survive outside of the plant so the soil will not be infectious once the affected plants are removed.
Off With Their Heads!
The Sunflower Headclipping Weevil (Haplorhynchites aeneus, Family Curculionidae) 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.
We reported in the July 15, 2010, BYGL newsletter that the sunflower headclipping weevil was causing noticeable damage to purple coneflowers (E. purpurea) 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.
During this week’s BYGL Zoom Inservice, Dave showed a picture that he had recently taken in central Ohio of the characteristic “dangling flower head” symptom on purple coneflowers. Female weevils are responsible for the slow decapitation of coneflowers and a few other members of the Asteraceae family.
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 its 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.
If the females are successful in weakening the flower stems, the dangling flower heads attract even more females and males. Eventually, they all “get acquainted.” Of course, the spectacle is rated TV-MA (Mature Audiences Only).
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.
In Ohio, the sunflower headclipping weevil primarily focuses its attention on purple coneflowers. However, with the rise in sunflowers being used in mass displays, naturalized areas, or as ornamentals in landscapes, the weevils have found their way back onto their namesake host in our state.
The weevil will also attack several members of the Silphium genus growing in naturalized areas including the 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.”
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.
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. Of course, insecticides are not a viable option. Insecticide labels do 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!
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.
Now, if we could only get the headclipping weevil to selectively remove coneflower seed heads infested with the coneflower rosette mite …
