Nettles

Spurge Nettle (Cnidosculus stimulosus) grows in dry,
sandy soil.  This plant was one of a group in Weymouth Woods State Park,
Moore County, North Carolina.

When I hear the word nettle, my skin starts to itch.  The name accurately describes what these plants do.  Brush up against a nettle and you get pain, itching and inflammation.  Hairs on the stems and leaves of nettles may contain powerful toxins that cause a painful reaction called urticaria.  The plant structures that cause this uncomfortable condition are urticating hairs.  There are several plants in the Southeastern United States called nettles, but they are not closely related to each other.  All these nettles share a common feature, hairs or spines on the stems and leaves.  These structures often contain noxious chemicals and deter grazing animals.  

Solanum carolinense or Horse Nettle is a relative of potato and tomato. 
Horse Nettle is a common plant in fields and along roadsides. 
Rowan County, North Carolina.

A closeup view of Horse Nettle flowers with their five white
petals and five bright yellow anthers.
Rowan County, North Carolina.

Horse Nettle (Solanum carolinense) is a member of the Nightshade family, the Solanaceae.  This family  family includes many familiar plants; potato (Solanum tubersum) and tomato (Solanum lycopersicum), tobacco (Nicotiana tabacum) and Deadly Nightshade (Atropa belladonna).  Horse Nettle is a low-growing herb with lobed leaves.  The leaves and stems bear hairs and large hooked spines.  Horse Nettle produces blueish-white flowers that have five petals and five bright yellow anthers.  The fruits look like yellow cherry tomatoes and each contains dozens of seeds.  Horse Nettle is one of the most toxic plants in this part of the world because they produce solanine glycoalkaloids.  When ingested solanine glycoalkaloids cause nausea, vomiting, headache and even death.  These toxins are in the leaves, stems and fruits, so no part of the plant is safe to consume.

 

Horse Nettle leaves and stems have large spines. 
Rowan County, North Carolina.  

A maturing fruit of Horse Nettle looking very
much like its close relative, tomato.
Rowan County, North Carolina

The Horse Nettle fruit on the left is nearly ripe. The
others will turn yellow in a few days.
Rowan County, North Carolina.

Wood Nettle (Laportea canadensis) is a true nettle in the family Urticaceae.  Wood Nettle grows in Eastern North America near water as a member of the forest understory.  This plant has large bright green leaves that bear stinging hairs.  The hairs break off in the skin and release neurotoxins that cause sharp pain and raise a rash that can last for several days.  Despite their threat of pain, many people harvest and eat Wood Nettles.  These hardy folks boil the plants and this inactivates the toxins.  Wood Nettle is reputed to taste like spinach. 

 

Wood Nettle plant growing on the flood plain
of the Yadkin River.  Rowan County, North Carolina.

The stinging hairs of Wood Nettle.
These fine hairs contain toxins that give a painful sting.
Rowan County, North Carolina. 

The third species on this nettle tour is Spurge Nettle.  It has a marvelous scientific name, Cnidoscolus simulosus and grasping this plant is definitely a stimulating experience.  Spurge Nettle is in the family Euphorbiaceae and is primarily a plant of the Southeastern Coastal Plain where it grows in dry, sandy habitats.  This plant has large, dark green, lobed leaves and produces white flowers with five petals. The fruits of Spurge Nettle are capsules and each contains three seeds.  Of course, since it is a nettle, the leaves, stems and fruits are covered with urticating hairs that contain chemical irritants. Spurge Nettle has a large, starchy taproot that after boiling tastes like pasta.    

 

Spurge Nettle in flower.
Moore County, North Carolina.

Flowers and developing fruits of Spurge Nettle.
Moore County, North Carolina.

Closeup of the utricating hairs of Spurge Nettle.  
Moore County, North Carolina. 

The three-chambered fruit of Spurge Nettle.  Like the rest of the plant,
the fruits are covered with stinging hairs.
Moore County, North Carolina.

People all over the world have a complex relationship with nettles.  North American nettles may be poisonous, sting when touched and some are edible.  Folk medicine traditions describe many uses of nettle from treating asthma and kidney ailments to preventing baldness.  The Tibetan holy man, Milarepa, is said to have lived to a ripe old age in his mountain fastness by eating only nettles.  A side effect of this diet was his skin and hair turned green.  I make no recommendations on nettles as food or medicine, but they are attention grabbing members of our flora.   

The Sphinx, the Weed and the Parasite

 

Carolina Sphinx Moth (Manduca sexta) hovering
over the flower of Sacred Datura (Datura wrightii). 
The moth's proboscis is extended toward the flower.
Rowan County, North Carolina.

This is a story of the integration of three lives.  The sphinx is a giant moth whose caterpillar is a major agricultural pest.  The weed is Jimsonweed is a handsome plant with large showy flowers and produces hallucinogenic compounds.  The parasite is a wasp that lays its eggs in the larva of the Sphinx and new wasps burst forth like aliens from the chest of an unfortunate sci-fi character. 

 

Sacred Datura plant flowering at night.
Rowan County, North Carolina.

First the weed, Jimsonweed.  It goes by many names.  Its scientific name is Datura wrightii, also known as Sacred Datura and it is native to the Southewestern United States and Mexico.  We have several plants in our yard that came to us by way of Austin, Texas.  There are several Jimsonweed species and they are in the same family as potato, the Solanaceae.  This interesting and important plant family includes other well-known plants like Tomato, Tobacco, Eggplant, Pepper and Deadly Nightshade.

Sacred Datura is a desert plant with large gray-green leaves and, as expected from a desert plant, is quite drought tolerant.  It produces a variety of toxic alkaloids that when consumed can induce profuse sweating, drowsiness, visual impairment, hallucinations, psychosis and even death.   The sacred in the name comes from its use in religious ceremonies of native people in the desert Southwest and Northern Mexico.  One of the active compounds in Sacred Datura is scopolamine, a drug now used to combat motion sickness, usually in the form of a transdermal patch.  In the bad old days of spies and enhanced interrogation, scopolamine was a major ingredient in “truth serum” although its effectiveness is questionable.   

 

Early and nearly mature flower buds of Sacred Datura.
Rowan County, North Carolina.

Sacred Datura flower on the afternoon the
flower will open.
Rowan County, North Carolina.

Sacred Datura petals on a flower that is about to open.
Rowan County. North Carolina.

Jimsonweed produces abundant, large, white, trumpet-shaped flowers throughout the summer.  The flower shape and color give this plant another of its many common names, Angel Trumpet.   The flowers develop for several days, with the petals rolled up within the green sepals.  Each day the flower gets longer and finally on flowering day the petals emerge and turn white. In late afternoon, the flowers open to show the five petals fused that make the trumpet.  As evening comes, the flowers release a strong and pleasant fragrance reminiscent of gardenia.  The white color and fragrance gives a clue to what Sacred Datura is doing at night.  The plant is sending an unmistakable signal to its pollinator, the Carolina Sphinx Moth (Manduca sexta).  

 

Carolina Sphinx Moth about to feed.
Rowan County, North Carolina.

Carolina Sphinx Moth sipping nectar from a Sacred Datura flower. 
The moth's long proboscis is visible reaching into the flower.
Rowan County, North Carolina.

Carolina Sphinx Moth approaching a Sacred Datura flower.
Rowan County, North Carolina.

Carolina Sphinx Moth has a four-inch wingspan, is dark gray in color and has five paired, orange spots on the sides of its abdomen.  The moth also has a long proboscis, a feeding structure that can be over eight inches long.  The proboscis is the key to the moth’s relationship with the Jimsonweed.  Datura lures the moth in from a distance with is fragrance, gives a visual target with the white flowers. The payoff for the moth is the sugar-rich nectar.  The nectaries for the flower are at the base of the trumpet-shaped tube, eight inches down.  The Carolina Sphinx Moth hovers over and even flies into the flower to collect the nectar.  But collecting nectar is not straightforward.  In order to drink, the moth must thread its proboscis down one of five nectar tubes on the inside of the petals.  

Infrared video of Carolina Sphinx Moth feeding on Sacred Datura.

Rowan County, North Carolina.

More infrared video of Carolina Sphinx Moth feeding on Sacred Datura. 
Rowan County, North Carolina.

While collecting nectar, the moth brushes against the stamens of the flower picking up pollen.  It then flies to another flower and transfers pollen to the female stigma.  There the pollen makes a tube that grows through the stigma, down the style to the ovary at the bottom of the flower.  There the pollen nucleus fuses with the Datura egg.  The fertilized egg and surrounding tissue develops into the seed that is contained in the fruit.  The fruit of Sacred Datura is covered with spines and these give the plant yet another common name, Thornapple. 

 

Sacred Datura the day after flowering.
Rowan County, North Carolina.

Sacred Datura flower two days after flowering.
The sepals and petals have fallen off but the long
style and stigma are still attached to
the developing fruit. 
Rowan County, North Carolina.

The spiny fruit of Sacred Datura showing why another common
name for the plant is Thornapple.
Rowan County, North Carolina.

Mature fruit of Sacred Datura.  The fruit has broken open
and the seeds are visible.
Rowan County, North Carolina.

The Carolina Sphinx Moth comes from a large caterpillar called the Tobacco Hornworm.  This larval stage eats, as the name suggests, tobacco but also thrives on other plants in the same family, including tomato.  A female moth lays an egg on the host plant where the larva grows and molts several times. The final larval stage of Carolina Sphinx Moth is over three inches long and can quickly defoliate the host plant. 

 

Tobacco Hornworm is the caterpillar of Carolina Sphinx Moth.
This caterpillar is on a Tomato (Solanum lycopersicum) and is eating leaves.
The white structures are cocoons of the wasp Cotesia congregate 
that has parasitized the caterpillar.
Rowan County, North Carolina.

This is where the final part of the story comes in, the parasite.  Actually, it is a parasitoid wasp, Cotesia congregate, that injects its eggs inside the body of the Tobacco Hornworm caterpillar.  The wasp eggs hatch and the emergent larvae consume the caterpillar from within.  Two weeks later wasp larvae dissolve their way through the exoskeleton of the caterpillar and make a white cocoon on the outside. Within the cocoon, the wasps metamorphose into their flying form.  These wasps mate and parasitize more Tobacco Hornworms.  A caterpillar parasitized by Cotesia dies before it can transform into a moth so the wasp is an effective form of biological control for Tobacco Hornworm.  

This intricate relationship, this perfection of interaction, between the plant, the moth and the wasp has been honed for millennia by evolution. Sacred Datura attracts and feeds the Carolina Sphinx Moth.  The moth has the sensory system specialized to find the plant at night and a proboscis long enough to reach the nectar supply deep within the flower.  The moth then pollinates the Datura plant.  The wasp Cotesia parasitizes the Tobacco Hornworm to propagate the species.  All the interactions are pieces of an elaborate living puzzle.

POST SCRIPT - About two weeks after posting this blog I noticed the leaves of one of the Datura's were being eaten.  Sure enough, several larvae of Tobacco Hornworm were consuming the leaves.  

The leaves of this Sacred Datura have been eaten.  
Rowan County, North Carolina.

The Carolina Sphinx moths were not only collecting nectar and pollinating the flowers but were also laying eggs on the leaves.  Although it probably does not, this moth can complete its entire life cycle on one plant.  

The Datura leaves were being eaten by several Tobacco Hornworms,
the larvae of the Carolina Sphinx Moth.  Note this caterpillar has not been parasitized
by the Cotesia wasp and is very healthy, measuring in at 3.5 inches.