Home
Up
Reference

A plant escapes its enemies for 150+ years.  What happens to it during that time, and what happens when its arch enemy catches up with it?
herb picture.jpg (121768 bytes) Are the answers to those questions held by herbarium specimens? It seems they are. Wild parsnips were brought to North America by European colonists as a source of food and were in widespread cultivation by 1609.   What the colonists did not import was the parsnip webworm, the parsnip nemesis that consumes the reproductive parts of the plant,...at least until 1869, when the insect was first recorded in Ontario Canada.  Thus, parsnips enjoyed roughly 250 years without this pest. It turns out that the furanocoumarins, which are important defenses against webworms, are phenomenally stable over time, allowing us to reconstruct changes in defense that occurred as the parsnip was reunited with webworms.  Even the physical interaction between plant and insect is preserved in herbarium specimens.  Presence of webworms can easily be confirmed by the presence of characteristic webbing and feeding damage. Indeed, on rare occasions, the perpetrator itself is preserved in the specimen as shown in this picture of parsnip collected in 1958.
None of the specimens collected between 1889 and 1909 showed evidence of   webworm activity.  After that, a significant increase in webworm attacks was observed. 

Furanocoumarin content of seeds increased dramatically with the appearance of webworms.  Two furanocoumarins in particular, sphondin and bergapten, which are implicated in webworm resistance, increased.  In comparison with samples from Europe (EUROPEUROPE), where webworms are native, North American samples were significantly lower (*) in both furanocoumarins prior to establishment of webworms. 

Absent webworms for such a long period of time reduced selection for furanocoumarin defense.  On reassociation with its nemesis, parsnips evolved increased furanocoumarin content. 

graph.gif (4841 bytes)