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In wild parsnips, as in other apiaceous plants, essential oils contain not only terpenes but also furanocoumarins.  Because the oil tubes are pressurized, an insect that feeds on a leaf can get a heavy dose of toxic oil very quickly.  How, then, can a caterpillar secure nutrition from a leaf while limiting its exposure to toxins?
One answer is trenching...
trench animation nsf.gif (20996410 bytes)
 

Cabbage loopers aren't the only clever caterpillars stalking parsnips...

 

notching.jpg (60452 bytes)Black swallowtails, Papilio polyxenes, also perform a behavour that disrupts the oil tubes.  They make notches in the petioles.  The top picture shows the notches and the bottom picture shows the UV- fluorescent furanocoumarins released from the oil tubes.   These furanocoumarins and the terpenes in which they were dissolved were prevented from travelling to the leaflets where the larvae subsequently feed.

 

 

last updated 9/16/2005