Evil Plants? Media Hype about the Buffalo Bur

Solanum rostratum, Buffalo Bur Nightshade Photo By leighannemcc

 A member of our SW U.S. Arthropods Face Book group wanted me to post a warning against the severe dangers posed by a plant that people might encounter while looking for insects.  The concerned and caring reader had found an older post on Fireflyforest.net. I think in the past I have used a very good plant guide to Arizona Wildflowers by the same author. 
But this time I was not impressed: After an anecdotal encounter involving the plant's burs and the author's dog, the native plant was maligned as evil, vicious and dangerous and called highly invasive. The plant: Solanum rostratum, Buffalo Bur Nightshade.
The attribute 'invasive' may stem from the usual reaction of landowners to an opportunistic plant that spreads in disturbed habitats. Worse - this one is said to form tumble weeds that roll about during droughts and spread the plant that way. Of course opportunistic colonizers among plants make no difference between 'disturbed' by natural causes or cleared by ranchers to raise cattle. In Marana, Arizona,  it grows happily on soggy, heavily grazed meadows along the Santa Cruz River. In Sonora Mexico I also found it in grazed meadows close to rivers. The plant is native to the United States and northern and central Mexico. It may be unwanted by the property owners, but ecologically, endemic plants are not considered invasive.

Solanum rostratun with Leptinotarsa haldemani and Coleomegilla maculata (Spotted Lady Beetle)

As for the danger the plants toxins pose and the warning I was supposed to put out on the FB group that I administer with my friend Robyn Waayers, I believe that the situation in the dog anecdote was unfortunate and the conclusions way overdrawn. 

As a Nightshade, Solanum rostratum contains a certain amount of solanine, glycoalkaloid poison, which can cause convulsions and death if taken in large doses. So do many of our wild Solanaceae.  Therefore, do never eat parts of nightshade plants if you are not sure if they are edible.

But the warning went further: the plant would be highly dangerous if simply touched. There are plants that do defend themselves efficiently against touch. I grew to know burning nettles as a little kid in Europe. 

Toxicodendron radicans, Poison Ivy and Mala Mujer, Cnidoscolus angustidens two truely untouchable AZ residents

 Poison ivy and oak are carefully avoided by most Americans. In Arizona, we warn people on excursions not to touch Mala Mujer in the family Euphorbiaceae. All those plants have irritating chemicals contained in glassy hairs that break on contact or glands that exude an oily substance that coats the leaves. 
In contrast, the buffalo bur has strong thorns protecting plant and seed pods, the burs. I have often touched the plant with no adverse effect when I photographed beetles or bumble bees on it (did I mention that this evil weed feeds lots of pollinators?). Of course the thorns prevented me from rubbing my fingers all over it and my usually careful approach kept me from being pierced by those obvious thorns.  So I think that the normal careful behavior of a naturalist will prevent the Buffalo Bur from becoming as evil a menace as described in the internet article. Media hype is all around us, though: A derivative article, mostly copied from the firefly forest post, was actually titled: Buffalo bur! This Arizona plant can kill you and your pets.

Leptinotarsa decemlineata (robably) here on silverleaf nightshade

In fact, there is another much more consequential side to the Buffalo Bur. Solanum rostratum is the ancestral host plant of the Colorado Potato Beetle, Leptinotarsa decemlineata. In eastern Nebraska in the eighteen-fifties, the beetle was accidentally matched up with a great new host, another Solanum originally from South America brought in by humans:  Solanum tuberosum.  (There aren't any convenient potato plantings where I could photograph the beetles here in AZ, so I have only founds them in Arizona and Mexico on wild growing Silver-leaf Nightshade)

The beetle thrived on that new, more succulent host, grown in convenient monocultures, and spread quickly eastward. New host plant and beetle were also carried from the Americas to Europe, where the host plant again went into agricultural mass production. So not only had the beetle combined with a super-optimal host that was further bred, fertilized, and pampered into defenselessness, but the natural enemies of the beetle had not been imported with it. Paradise for the Potato Beetle. The beetles multiplied and became THE most hated pest of potatoes in Europe (not counting fungal blight here).