This is the time of the year when “dodders” reach their maximum growth in their annual life cycle and become most obvious. The tangled masses of the string-like stems of these bizarre-looking parasitic plants bear little resemblance to most plants. They more closely resemble a colossal fishing line accident.
Dodders do not start out looking like a jumbled heap of fishing line. They are summer annuals meaning that flowers and seeds are produced in the fall and the seeds germinate in the spring. The tangled mass of dodder stems develop throughout the summer growing season.
Dodders have some nefarious-sounding common names such as strangleweed, wizard's net, devil's guts, witch's hair, hellbine, and hellvine. These names are likely associated with the dodder’s perceived destructive nature. However, dodders also provide some significant benefits, but more on that later.
Much Obliged
Dodders are obligate parasites meaning they can't make a living without their plant hosts. They invade their hosts using specialized, peg-like structures called haustoria (singular haustorium). The dodder's haustoria are considered modified roots and are used to extract water, carbohydrates, and nutrients from their host's vascular bundles.
Dodders can attach themselves to an amazing range of host plants, often at the same time. The capacity to exploit a wide range of hosts across multiple taxonomic categories has been cited as an important survival mechanism for many dodder species.
The image below show swamp dodder twining on the stems of plant hosts representing three plant orders. Tendrils were wrapped around common cocklebur (Xanthium strumarium, order Asterales), American bur-reed (Sparganium americanum, order Poales), and mild waterpepper (Persicaria hydropiper, order Caryophyllales).
Indeed, dodder’s have such a wide host range, it's best to keep moving!
Tangled Taxonomy
Dodders belong to the morning glory family, Convolvulaceae. They were formerly placed in the dodder family, Cuscutaceae, with only one genus in the family, Cuscuta.
Depending on the reference and taxonomic scheme, there are somewhere between 150 – 200 species of dodder worldwide with 10 – 13 species (maybe more) found in Ohio. I think it’s fitting that dodders belong to the Convolvulaceae family because their taxonomy remains convoluted.
Two of the most common dodders found in Ohio are Swamp Dodder (a.k.a. Common Dodder), C. gronovii, and Field Dodder, C. campestris. However, some taxonomic schemes list another species, C. pentagona (Five-Angled Dodder), as also being common in Ohio while others lump the two species together and refer to the amalgam as “Field Dodder.” Some view C. pentagona as a “complex” made up of several subspecies.
Despite its common name, Swamp Dodder appears to be on more solid taxonomic ground. Of course, as its common name implies, it’s usually found in wetlands.
Dodder’s Many Wonders (Wanders?)
Regardless of their tangled taxonomy, the hallmark of all dodders found in Ohio is their thin, tendril-like stems that encircle the stems of their host plants. Dodder stems range in color from yellow to orange to red depending on the species and age. Some species, but not all, have tiny scale-like leaves. Although most dodders have some chlorophyll, it's insufficient to photosynthesize enough sugar to support a singular lifestyle. Mature dodder plants also lack roots.
Dodder seeds are large and loaded with carbohydrates to support the frail seedlings until they can locate a plant victim. The seedlings produce a rudimentary root system; however, if they don't sniff out a host to latch onto within 10 – 15 days, they die.
I used "sniff out" deliberately because that's exactly what they do. Research published in Science in 2006 showed that dodder seedlings are capable of detecting plant volatiles released by prospective host plants. The sensory mechanism used by dodder to detect a host is not known, nor are the exact chemicals or mixture of chemicals that send a "squeeze me" message. However, it was shown that dodder can distinguish between odors emitted by good hosts like tomato and bad hosts like wheat.
As an obligate parasite, a dodder plant would be in trouble if it attached itself to a summer annual host that flowered, produced seed, and died before the dodder plant was able to flower and produce seed. Dodders have evolved a workaround to the problem by producing flowers when they intercept the same chemical message used by its plant host to trigger blooming.
Research published in a 2020 paper in the Proceedings of the Nation Academy of Science showed that dodder plants “eavesdrop” on the mobile protein, known as Flowering Locus T (FT), that signals plants to flower. The FT travels from the host into the dodder via the haustoria to signal the dodder to flower. Will the dodder wonders never cease?
On the other hand, life is not a bed of roses for dodders. Over the years, I’ve observed large dodder colonies one year that disappeared the next. In fact, I’ve rarely seen dodder successfully return year-after-year in the same location. One exception is the swamp dodder that’s featured in many of the images in this Alert. I first found the dodder last season and again this season growing in an ephemeral wetland in southwest Ohio. The end of the season dry out of the wetland allowed me to take images.
Twisted Friends with Benefits
Research published in 2017 in the Proceedings of the National Academy of Sciences showed that dodder's tangled threads can function as important lines of communication between connected plants. Indeed, dodder’s twisted tangled threads bear a remarkable resemblance to our own communications and power lines.
The researchers found that when a plant was attacked by defoliating caterpillars, messages sent to other plants on the “dodder web” caused them to raise their defenses. The caterpillars were less successful with attacking the pre-warned plants.
Dodders also contains a range of compounds that may be beneficial in human medicine. They have long been used in Chinese folk medicine. A review published in 2017 in Biomedicine & Pharmacotherapy reported dodders harbor molecules with possible therapeutic benefits including potential antiviral and anticancer activities.
Of course, some claims published online may be overblown. In preparing this report, I came across a few dodder products touted to reverse hair loss. I'm considering a bulk purchase.
Dodder’s Grand Larceny
It is estimated that dodders cause annual crop losses worldwide that are measured in tens of millions of dollars. These parasitic plants have been found sucking the life out of over 100 species of plants including alfalfa, clover, soybeans, and even solanaceous plants in home gardens such as petunias and tomatoes.
Dodders are stealthy plant invaders using a move that's been described as mimicking a computer virus. Research published in 2018 in Nature showed that dodders send microRNA into their hosts which silences the encoding of genes that would normally support defenses. One of those defenses is a protein that clots the flow of nutrients to the site of the dodder's haustoria. Without the "anticoagulant" protein, the plant's lifeblood keeps flowing into the dodder.
Dodders wreak havoc on their plant hosts in other ways. The “dodder bridge” between several hosts can serve as a passageway for plant pathogens including viruses and phytoplasma. Plant stress caused by the heavy extraction of the host's resources can weaken plants making them even more susceptible to plant pathogens as well as environmental calamities such as drought.
Studies conducted by OSU researchers and published in 2013 in New Phytologist showed that if dodder tangles with soybeans that have been genetically modified to be tolerant of glufosinate herbicides (e.g., Liberty, Rely, Cheetah, etc.), the herbicide tolerance is transferred to the dodder. Applications of herbicides over the top of glufosinate-tolerant soybeans to remove competitive weeds would not kill the dodder.
Dooming Dodder
The annual life cycle of dodder starting with seed germination in the spring means dodder can be effectively managed using a range of preemergent herbicides. This approach is commonly used to suppress dodder in field crops, ornamental nurseries, and landscapes.
Hand-pulling is problematic. While ripping away the threadlike dodder may be satisfying, it is not effective once the dodder attaches to a host. New strands can regenerate from the haustoria left embedded in the host.
One challenge with using preemergent herbicides is the possibility that dodders may occasionally act like a perennial by spending the winter inside their host plant. The authors of research published in 1955 in the Proceedings of the Iowa Academy of Science observed dodder overwintering as haustorial tissues within “galls” induced by the dodder and buried within the stems of its host plant.
In a paper published in 2009 in the journal Flora, the researchers reported a possible perennial characteristic of clover dodder (C. epithymum). They observed the dodder overwintering vegetatively on its most common perennial host Scotch heather (Calluna vulgaris).
I observed the possible harboring of dodder within the woody stems of black willow (Salix nigra) in 2017. As shown in the pictures below, dodder flowers were sprouting from woody gall-like nodules on the main stems of willow. The flowers were not sprouting from dodder stems.
Dodders are notorious for being prolific seed producers and the seed may find its way into crop seed. So, a primary dodder management strategy targets contaminated crop seed to reduce the introduction of dodder into new sites. Consequently, dodder appears on the “Prohibited and restricted noxious weed seeds list” in the Ohio Administrative Code, Rule 901:5-27-06, Testing of Seed.
Of course, dodder seed is also spread through other routes. Seed may be spread by flooding given the propensity for colonies to appear in flood plains or along waterways. Dodder may also appear in landscapes and farm fields owing to contaminated soil or equipment. A paper published in 2016 in the American Journal of Botany revealed that seed ingestion and excretion (endozoochory) by waterfowl play a significant role in the long-distance spread of dodder’s large seeds.
Galling Dodder
Last year, I observed small, subglobose galls embedded within the tangled masses of swamp dodder stems. There are several weevils belonging to the genus Smicronyx (order Coleoptera (beetles), family Curculionidae (snout weevils)) that produce galls on dodder. Weevils are beetles with a snout and gall-making beetles are extremely rare. Just do a web search using “beetle galls” as the keywords.
The gall-making weevils targeting dodder have a worldwide distribution. However, the most likely culprit behind the galls that I found was the Dodder Gall Weevil (Smicronyx sculpticollis). Galls produced by this weevil have been found on both swamp and field dodders.
The biology of the weevil is poorly documented. However, every gall that I photographed had an exit hole and was empty meaning the weevils had left the building when I took the photos in late August.
The weevil galls appeared to have little impact on the overall health of the dodder. They also didn’t appear to be associated with a particular plant host.
The literature notes that the galls commonly develop from the nodes where dodder floral structures arise. Likewise, I observed this, and also found galls that had developed in the middle of the dodder stems far from floral structures.
Interestingly, while the vast majority of the dodder galls were the same tannish-yellowish color as the parasitic dodder stems, some galls were green. As noted above, dodders are generally devoid of chlorophyll or lack enough to create their own food by photosynthesis. However, the green-colored galls may indicate the weevil is able to tap into dormant dodder genes as it directs gall growth to endow the gall tissue with this capacity. Of course, this rampant speculation!
The weevil gall structure that I observed appeared to be more rudimentary and much less organized compared to those typically produced by better-known gall-makers such as gall wasps and midge flies. For example, wasp galls commonly include specialized “nutrient tissue” that surrounds the chamber housing the immature wasp and is constantly being replaced. The wasp consumes the nutrient tissue rather than eating the gall.
I could not discern any specialized nutrient tissue within the weevil galls on the swamp dodder. Instead, it appeared that the immature weevils had created an internal chamber simply by eating the succulent gall tissue.
As far as I can determine based on my cursory literature review, Smicronyx weevils appear to be the only gall-makers found on dodders. I believe it may yet be another example of an insect going the extra mile to take advantage of an underutilized resource.