Cow Parsnip Toxicity

As a teacher working with kids in the field, I have for years told students that cow parsnip (Heracleum maximum) has photo-allergenic properties to some people, because it makes a good story. But nobody I knew ever developed a reaction to it, at least not that I was ever made aware of.

But this year, a few days after looking at a a specimen in the Clatsop Community College Parking lot with my current team of Upward Bound field biology students,  one of them showed me a horrific blister on his hand and said, "What do you think this is from, Mr. Cain?"  We decided he must have bumped into the cut stems of our specimen, was allergic to the plant, and that he was, in my experience of some 30 years of being around the stuff, a rarity.

Then...

Last Monday, my crew of students and I were hacking a trail along a brushy section of Young's Bay. We were setting out minnow traps as part of our annual survey of tidegate effectiveness.  Cow parsnip was abundant along the dike, but I thought of little except to keep Cody away from it, and we trimmed it along with all the other rampant jungle growth.

Alas, I am now a statistic as well. Two days later, this is what my forearm looked like:


Digging a little deeper, this is what I found out.  There are a number of plants (many in the carrot family, such as cow parsnip, parsnips, and celery) which produce furanocoumarins, a class of polycyclic hydrocarbon chemicals which are a defensive strategy against insects.  These are not only toxic, but in many cases are photo-actively toxic -- they are capable of producing fierce rashes in the presence of ultraviolet light.

Googling "cow parsnip rash" or related strings pulls up a long list of unfortunate individuals who have had the same reaction to the furanocoumarins that I had, some far worse than I. Many of these experiences are in southeast Alaska, where the plant is very abundant (they call it "pushki" there) and probably a higher percentage of the population does outdoorsy things.

Despite the present discomfort, I am kinda glad I bumped into this annoying allergy, because it brought to my attention the fascinating recent work of May Berenbaum (entomologist extraordinaire, and author of the excellent book Bugs in the System) on the evolution of these toxins in wild parsnip in response to the grazing of parsnip webworm larvae.  It seems that I stepped into the middle of an arms race.

(click to enlarge)

The webworm moth (genus Depressaria) lays an egg (or several) on the cow parsnip and the caterpillar spins silk to bind together parts of the umbel (flower cluster) into a sort of feeding chamber and house. 

It then chows down on the reproductive parts of the parsnip throughout the summer before metamorphosing into an adult moth. 

The plant, however, fights back through the production of furanocoumarins, which reduce the silk production of the webworm.  And it doesn't stop there, because it has been suggested that not only does the silk shelter hide the webworm from the hungry eyes of birds, but its shade cuts back on the phototoxic activity of the furanocoumarins.


Check out these links to read Berenbaum's words -- she says it much better than I.

• http://www.life.illinois.edu/berenbaum/
• http://www.pnas.org/content/105/12/4547.full.pdf

One thing that still puzzles me is why my student Cody and I developed the reaction during this year of all years, since I have handled and cut cow parsnip hundreds of times.  I am not sure, but I suspect that it might be a downside to all of the gorgeous sunny weather we have been recently having.  It may also be that the plants are in particularly toxic part of the cycle right now.