Dodder—a Dangerous & Desperate Lover

Dysfunctional relationship visible just left and above mid-photo; closer view below.

Driving through the Mojave Desert south of Baker, California, I saw bushes entangled in what looked like finely shredded orange plastic. It was dodder (Cuscuta sp.), also known as the Love Vine. Like a passionate lover, it becomes firmly and intricately entwined with its partner. But this is an insidious embrace. Unable to photosynthesize, the Love Vine repeatedly pierces the vascular system of its host, sucking out water and nutrients.

Dodder has other names, many of which reflect its parasitic nature, for example Witch’s Hair, Witch’s Shoelaces, Devil’s Hair, Devil’s Guts. Damage to the host varies, from minor to deadly, and some dodder species are serious agricultural pests (1).

Unable to stand on its own, devoid of chlorophyll, and with only tiny scale-like leaves, dodder hardly looks like a plant, or maybe a primitive one at most. Yet it’s an angiosperm, a flowering plant, specifically a member of the morning glory family (Convolvulaceae). Most likely the dodder I saw was Cuscuta denticulata, by far the most common one in the Mojave Desert region.

Dodder stems tightly twine around anything within reach.

When I enlarged this photo, I saw I had captured flowers and fruit! (click on image to view)

Toothed petals are source of specific epithet denticulata (S. Matson, Attribution-NonCommercial license).

If dodder finds itself on an appropriate host (verified by chemical sensing), it tightly twines around stems and leaves, growing haustoria in the process—projections that penetrate the host’s vascular tissue.

Developing haustoria of Cuscuta gronovii (courtesy BlueRidgeKitties, some rights reserved).

Haustorium of Cuscuta reflexa in shoot of Nicotiana benthamiana (Kaiser et al. 2015).

But to live the easy life of a freeloader, dodder must first overcome a major challenge—establishment. Its seeds germinate indiscriminately, no matter where they land (seeds of some parasitic plants germinate only in response to chemicals from suitable hosts). Even worse, a dodder seedling has no roots and little in the way of embryonic food reserves, so it must find a host soon, within just a few days (2).

It would seem that only if a seed falls very close to a host plant could the seedling become established. But the situation is not as desperate as it appears. Dodder seedlings are able to “forage” for host plants! After germination, a seedling immediately sets off in search of a suitable host. If it “smells” one nearby, it adjusts its growth accordingly.

Justin Runyon and colleagues (2006) demonstrated that given the opportunity, seedlings of five-angled dodder grow toward their favorite hosts, tomato plants. Using 30 seedlings, the researchers put the basal end of each in a vial of water in the center of a paper disc 9 cm in diameter, with a tomato plant at the edge of the disc. Over four days, they marked on the paper disc the seedling's path of growth (diagrams below are composites for 30 seedlings). For comparison (i.e., a control), they did the same for 30 more seedlings but using pots with just moist soil—no plants.

X marks positions of “targets”: control pot with only moist soil, and pot with tomato plant.

The results were convincing: 80% of dodder seedlings with a tomato plant nearby grew toward the plant, and fairly directly. Those with only a pot of soil wandered more, and were evenly distributed between semicircles (3). The researchers then repeated the experiment using chemical extracts instead of entire plants, and got similar results—73% of seedlings grew toward the tomato-plant extract, and with less wandering. Thus directed growth was specifically in response to volatile chemicals released by the tomato plants (vs. light or color or something else). In other words, a dodder seedling follows its nose (4).

Five-angled dodder also parasitizes wheat, but it prefers tomatoes if given a choice. When Runyon and colleagues isolated the various chemicals in the volatile extracts, they found out why. In addition to several appealing chemicals, wheat contains one that dodder finds disgusting. Tomatoes have no such defense.

Desperate Love Vine approaches its victim.

Notes

(1) Dodder can cause significant damage to crops, and is on the USDA Top Ten Weeds List. Eradication is difficult to impossible. Rarely can dodder be eliminated without also harming the host plant. Instead, it’s recommended that the host be removed, and a different crop planted. See Dodder (Cuscuta spp.) Biology and Management.

(2) Some reputable sources say a dodder seedling produces an anchoring root, which dies once a host is found. Sources also differ on survival time for unattached seedlings; they might survive for as long as a week. Perhaps these features vary with species. Cuscuta pentandra seedlings can grow just a few centimeters on their own, but even so, in a field of tomatoes or wheat, plenty of seedlings will find a host.

(3) Runyon and colleagues also did these experiments with paper discs divided into quadrants rather than two semi-circles, and obtained similar results.

(4) In their supplementary materials, Runyon et al. provide a very cool movie of a wandering dodder seedling searching for and finding a host! Click on “Movie 1” to download.


Sources

Kaiser, B., et al. 2015. Parasitic plants of the genus Cuscuta and their interaction with susceptible and resistant host plants. Front. Plant. Sci. 6: 45. https://www.ncbi.nlm.nih.gov/pubmed/25699071

Runyon, JB, Mescher, MC, and De Moraes, CM. 2006. Volatile chemical cues guide host location and host selection by parasitic plants. Science 313: 1964-1967. https://www.ncbi.nlm.nih.gov/pubmed/17008532

The Mailbox on the Sill—the short & the long of it

The Palisades in Cimarron Canyon, northern New Mexico.

Browsing through my photos of the Cimarron Canyon Palisades, I spotted an odd form on the crest which I hadn’t noticed when I was there. Is that really rock? Zooming in showed nothing to suggest otherwise. But how improbable!

I’m hardly the first to spot this weird finger of rock. Years ago someone thought it odd enough to add to the Devil’s collection of landforms and landmarks—Devil’s Postpile, Devil’s Backbone, Devil’s Thumb, Devil’s Racetrack, Devil’s Marbles, Devil’s Gate, etc. In this case, we have the Devil’s Mailbox.

Devil’s Mailbox; Pomona Public Library, Frasher Foto Postcard Collection, 1938.

Unfortunately, the Mailbox story ends here, as my “research” (Google) produced no further details. In contrast, the story of its context is much longer, on the order of 40 million years. The Mailbox is part of the Palisades sill, a massive sheet of igneous rock intruded into sedimentary strata of the Cimarron Range. This may have happened during creation of the range, part of the Laramide Orogeny, the mountain-building episode that built the Rocky Mountains. In a later episode of uplift, maybe 20 or 30  million years ago, the Cimarron River cut down through the range revealing part of the sill, in cross-section. The dramatic exposure is called the Palisades, and is part of Cimarron State Park. There’s a convenient pullout at the base, along US Highway 64.

Actually, the details are still being debated.

As of 2002, geologists were still debating the exact age and composition of the sill. Dating has produced ages ranging from 35 million years to as young as 26 million. Rock composition also has been hard to pin down (see this page for citations):

“Based on mineralogy and chemical composition, the Palisades consist of biotite-diorite porphyry. Other geologists have called the rock type of the Palisades a monzonite porphyry, quartz monzonite porphyry, dacite porphyry, a granodiorite porphyry, or a transition from trachydacite to dacite. Although these terms describe the rock properly according to its composition, some terms are inconsistent with its texture. Therefore, the term porphyritic dacite seems the best description of these sills.”

The rock looked porphyritic to me, i.e., large phenocrysts in a matrix of fine-grained rock. This is easy to see in the blocks lining the pullout.

The columns and towers of the exposed sill are spectacular. They're separated by long vertical joints that formed as the intruded magma cooled, underground. Later, water seeped into the fractures, and froze and expanded in winter, slowly breaking the rock. This went on for millennia, producing talus slopes at the base (and rock blocks for the pullout).

Talus slope at base, apparently stable enough for trees to grow large. 

The Palisades Sill Official Scenic Historic Marker is on US Highway 64 in Cimarron Canyon State Park, about 8.3 miles east of Eagle Nest and 15.5 miles west of the town of Cimarron (36.537449° -105.152374°). The Mailbox stands near the northwest end of the ridge visible from the highway.

Sources

The Palisades sill is included in New Mexico’s Virtual Geologic Tour, a terrific trip-planning resource. The Cimarron Canyon State Park page includes a substantial list of references.

Boxelder News: some sensationalism this month

Photo by M Nelson.

The standard approach shot—boxelder in shady nook, with no obvious changes.

When I rounded the corner of the warehouse yesterday, on my monthly tree-following visit, I could see no obvious changes in the boxelder. But as I got closer, I saw that its neighbors were coming to life, like dandelions along the ramp to the warehouse door.

And what looked like windblown debris from a distance turned out to be sand dock, Rumex venosus, currently in bud. The red winged fruits are spectacular, but we will have to wait at least a month for those.

Sand dock in bud.

As for the boxelder … buds were opening, but only on one branchlet, one of the first to catch sun in the afternoon. These look like flowers buds with emerging anthers, therefore a male tree.

Male boxelder flowers, photo by Kruczy89; source.

Meanwhile, 177 miles south of Laramie and about a thousand feet lower, a boxelder at Lowe’s is fully leafed out, displaying fresh green foliage with a hint of copper. In summer, the leaves will be rich green, and then turn pale red in autumn—hence the name, Sensation Box Elder. It’s said to be “the best known plant discovery of Warren Carnefix, the Idaho plantsman and nurseryman whose family will mark 100 years in the nursery industry this year.” (J. Frank Schmidt & Son Co.)

Sensation Boxelder front center, among arboreal offerings at Lowe’s (M Nelson).

A hint of copper in spring leaves (M Nelson).

Fall foliage (source).

The Colorado Springs Rock Guy was surprised to find a boxelder in a nursery, having lived around boxelders and the notorious boxelder bugs as a kid in Kansas. “I have never, never seen anyone, at least from Kansas, plant a boxelder tree. I have seen many persons cut them down or grub them out.”

But according to J. Frank Schmidt, the “seedless nature of this male clone makes it less attractive to box elder bugs, a pesky but harmless insect [trees show no obvious signs of injury] that feeds on the flowers of female trees and takes refuge in houses in the fall.”

The University of Minnesota Extension says the same: “Starting in mid‑July, they [boxelder bugs] move to female seed-bearing boxelder trees where they lay eggs on trunks, branches, and leaves. They are rarely found on male boxelder trees.” Even female trees get the Extension’s support: “In our opinion, the benefits of having these trees in a landscape outweigh the problem of occasional infestations.”

More than a few people agree. There are even boxelder bug fans, some of whom adopt and name the invaders of their homes. See Thank Goodness for Boxelder Bugs by the Prairie Ecologist, and the numerous comments, almost all positive.

from Orkin Pest Control

This is my May contribution to the virtual gathering of tree-followers, kindly hosted by The Squirrel Basket. More news here.