Population differences in Pastinaca sativa and its suitability as a host plant for Depressaria pastinacella

The parsnip webworm (D. pastinacella) feeds almost exclusively on wild parsnip (P. sativa). These plants produce furanocoumarins, compounds, which aid in deterring insect herbivory. Despite the capacity to produce defense compounds, some parsnip populations are more susceptible to herbivores than others. In this study, we were interested in assessing herbivory and furanocoumarin levels in four populations of P. sativa. Two populations, one from Alma, IL and one from Farina, IL, showed little to no evidence of damage or active feeding from D. pastinacella while populations from Urbana, IL and Perrysville, Indiana were heavily infested with D. pastinacella. Male-stage primary umbels from each population were collected and brought back to the laboratory where they were bioassayed with ultimate-stage larvae reared on artificial diet. Larvae fed P. sativa tissue from Alma had a mean growth rate that was significantly less than that of larvae fed tissue from the Farina, Urbana, and Perrysville populations. No correlation was found between final and initial larval mass, so there is no evidence that growth was limited by proximity to pupation. Chemical analyses of the umbels were undertaken to determine whether furanocoumarin content and composition account for differential performance across populations.