July 17, 1968: 50 years ago

Herbarium specimens can provide insight into plant-insect interactions.  For instance, many specimens show obvious signs of insect damage to the leaves or flowers.  Many other specimens, upon closer inspection under magnification, show damage that is much subtler.  Sometimes, even the larvae or insects were also pressed with the plant!  Some of this damage may have happened from pests in the herbarium that occurred over decades of storage.  Damage from herbarium pests is a serious concern and collection managers are also conservators of these archives, keeping a vigilant eye out for damage from the elements (such as light, temperature, humidity, water, and other chemicals) and introduced pests (such as mold and insects).  However, damage that occurs in the herbarium is minimized for long term preservation and luckily, insect damage that occurs in the herbarium is often easily distinguished from herbivory that happened in the wild, before the plant was collected, dried, pressed, and added to the collection.
 
Few studies to date have used the herbarium record to understand the ecology and evolution of plant-insect interactions. However, there are several pioneering studies at the frontier of this novel use of these collections.
 
This specimen pictured above of wild parsnip (Pastinaca sativa) was collected by Leroy Henry on July 17, 1968 along the Pennsylvania Turnpike not far from Pittsburgh.  Leroy Henry was an influential Curator of Botany at the Carnegie Museum from 1937-1973, having collected thousands of specimens in the herbarium.
 
In addition to the “standard” herbarium label that gives information on what the specimen was first identified as, where it was collected, who collected it, etc., check out another typed label affixed to this sheet.  This annotation label indicates two seeds were removed from for chemical analysis in 2005.  Annotation labels are commonly added to specimens years to centuries later to note its use in a study, a new identification, and/or part of the specimen was removed.
 

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This specimen was one of many specimens included in a study on wild parsnip by Zangerl and Berenbaum published in 2005 in the high-profile journal Proceedings of the National Academy of Sciences (PNAS).  
 
Wild parsnip (Pastinaca sativa) was introduced to the United States from Europe centuries ago and has since spread to become invasive.  The species is a member of the carrot family (Apiaceae).  Like many other species in this plant family, wild parsnip produce phototoxic chemicals called furanocoumarins, which protect the plant from insect herbivory.  These compounds also cause major skin irritations in humans, reacting with sunlight to cause nasty rashes.  This group of chemicals is of the same fame of giant hogweed, poison hemlock, and other well-known toxic plants in the United States.
 
Zangerl and Berenbaum (2005) analyzed herbarium specimens collected over the past 150+ years.  These specimens were collected from before wild parsnip was widespread in the US and compared those to specimens collected more recently, after the species was widely established.  A major herbivore, the parsnip webworm (Depressaria pastinacella) was also accidentally introduced in the mid-late 1800s.  The authors found that as rates of webworm attack increased, so did the toxicity of wild parsnip.  Specimens from 1850-1889 (early stages of introduction) had lower levels of the toxic furanocoumarins than plants collected more recently, as well as than specimens collected in their native range (Europe), where the parsnip webworm is also native.
 
There is so much known and unrealized potential in herbarium specimens.  This particular specimen was not collected with the intention of studying the ecology and evolution of plant-insect interactions.   This study helped inform our current and future use of biological control agents to manage introduced species.
 
These unanticipated uses (often decades to over a century after a specimen was collected!) illustrate the incredible power of natural history collections.
 
See an excellent paper by Meineke et al. just published on this topic (including quantifying the effects of climate change on plant herbivory through herbarium specimens): https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1307.
 
Check out the cool parsnip webworm study here: http://www.pnas.org/content/102/43/15529

Wild parsnip (Pastinaca sativa) in an unmowed roadside median next to Bloomfield Bridge, Pittsburgh PA on June 29, 2018.