Participants in this week’s OGIA/OSU Extension Greater Cincinnati Diagnostic Walkabout observed the trifecta of coneflower (Echinacea spp., family Asteraceae) issues in a mass planting. They included Aster Yellows (phytoplasma disease), Coneflower Rosette Mite (eriophyid), and Sunflower Head-Clipping Weevil.
Aster Yellows and Freaky Flowers
Aster yellows is a serious, chronic disease that occurs throughout North America and may affect over 300 species of plants in 38 families, including several vegetables such as carrots and potatoes. However, as its common name implies, aster yellows occurs most often on members of the aster family.
Coneflowers are particularly susceptible. Disease symptoms include curled foliage, stunted stems, and bizarrely distorted flower parts that are various shades of green, sometimes with splashes of normal colors. The petal-like ray florets may appear as a ring of tiny greenish-yellow spoons arrayed around the base of stunted disk scales and florets. Occasionally, tufted green growth sprouts from the cone.
The disease is produced by unusual bacteria, called phytoplasmas, that are highly specialized plant pathogens. All known forms only infect plant phloem tissue. They cannot live outside of the phloem.
Phytoplasmas are spread by insects that use their piercing-sucking mouthparts to tap into phloem sap. Proven vectors include leafhoppers, planthoppers, and psyllids. However, the species most often implicated in spreading the aster yellows pathogen is the Aster Leafhopper, Macrosteles quadrilineatus. The pathogen can also be spread by vegetative propagation and by seed.
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.
However, the phytoplasma’s obligate relationship with living phloem tissue is its Achilles’ heel. 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.
Mighty Mites and Spiky Hairdos
Spiky disk scales and florets that rise rosette-like from the coneflower cones are symptoms of an eriophyid mite that has yet to be taxonomically categorized, so it has no scientific name. Consequently, it also doesn’t have a common name approved by the Entomological Society of America (ESA). However, the mite is generally called the Coneflower Rosette Mite based on its characteristic damage to coneflowers. The mites infest the straight species as well as cultivars.
The coneflower rosette mites live inside the developing flowers at the base of the disk florets, where they puncture cells with their stylets and suck-up ithe cell contents. As a result, elongated rosette-like tufts of distorted disk scales and florets spoil the symmetrical appearance of the cones. Occasionally, the mites affect all of the disk scales, causing stunting and forming a ball-like structure rather than a cone.
Differentiating rosette mite symptoms from aster yellows, and vice versa, is straightforward. The mites focus their mayhem on flowers; they never affect entire plants. The majority of the damage is confined to the disk scales and florets. However, a heavy infestation may cause the ray flowers to abort.
Eriophyid mites are unique among other mites both in their size and anatomy. Spider mites can be seen clearly with a 10x hand lens, but you need a higher magnification to see the details of the unusual eriophyid mite body plan. However, large collections of coneflower rosette mites attached to the tips of disk scales and florets can cause the florets to look like they’ve been dipped in dust, as shown in the images below.
Unlike spider mites that have oblong bodies and four pairs of legs, eriophyids are cigar-shaped, and they only have two pairs of legs that extend from the front of their body. No other mite has only two pairs of legs at any stage in its development.
Eriophyids are also equipped with a unique structure called a caudal sucker, sometimes called an anal sucker. The sucker adheres to host plants with anal secretions to keep the mites attached as they feed and to hold on while they await a ride to a new plant.
Eriophyid mites can be spread by the wind or animals such as birds. However, coneflower rosette mites enjoy a distinct advantage since they affect flowers favored by insect pollinators, particularly bumble bees (Bombus spp.). The mites crawl to the tips of the disk scales and 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. Imagine how many mites can be carried on the hairs of these large transports to new coneflowers.
Effective management of coneflower rosette mites is straightforward. Cutting and destroying flower heads deformed by mite activity before the mites seek new horizons will reduce mite populations. A bag or other container should be held beneath the tufted flower heads as they are cut to avoid dislodging mites that can drop onto nearby flowers.
The damage caused by the coneflower rosette mite is not only unsightly; it can also significantly reduce seed production and thus natural re-seeding. Consequently, sanitation pruning should be applied as soon as the rosette symptoms are detected.
Coneflower Beheadings
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 ESA-approved common name by clipping the flower heads of cultivated and wild sunflowers (Helianthus spp., family Asteraceae). Yet, it’s seldom a serious agricultural pest.
Weevils are beetles with snouts. Their chewing mouthparts are located at the end of a snout-like structure called a rostrum. Female headclipping weevils are shiny black or brown and measure around 1/4” in length. They have exceptionally long, curved rostrums.
They use their remote mouthparts 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, and the semi-detached flower head hangs from the stem on a thin strand of tissue.
I’ve never observed weevil damage in fields of cultivated sunflowers in Ohio. However, the headclipping weevil has become a serious pest of coneflowers in landscapes and naturalized plantings in central and southern Ohio.
The weevil will also attack several members of the Silphium genus growing in naturalized areas, 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.”
Sometimes things go horribly wrong. Plants in the genus Silphium are notorious for exuding copious amounts of sticky, resinous sap (hence 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.
We also occasionally see the weevil attacking common ragweed (Ambrosia artemisiifolia) and giant ragweed (A. trifida). Based on personal observations, I’ve concluded these “weed” hosts may serve as weevil reservoirs where more common hosts aren’t available.
The weevil’s calling card is the appearance of dangling flower heads. However, the stems that remain on beheaded plants look like collections of soda straws.
The dangling flower heads attract even more females as well as males. Eventually, they all “get acquainted,” and the mated females lay eggs at the base of the disk flower.
Eventually, the flower head breaks from the stem and drops to the ground. The eggs hatch, 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.
As with the damage caused by aster yellows and coneflower rosette mites, the beheading of coneflowers, as well as members of the Silphium genus, can also significantly reduce seed production and thus natural reseeding. I’ve observed mass plantings of straight-species coneflowers gradually fade away owing to annual heavy infestations of the sunflower headclipping weevil.
Insecticides can’t be applied to plants that are in flower; remember, the insecticide label is the law. Consequently, the most effective method for controlling the sunflower headclipping 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.
