Paradoxical host shift by Depressaria pastinacella in North America: Is enemy-free space involved?

Herbivorous insects often preferentially oviposit and develop on hostplants that support suboptimal growth relative to alternative hostplants available in the same habitat. In some cases, this preference results from reduced predation risk on the nutritionally suboptimal hostplant. Thus, the availability of enemy-free space has been postulated to affect hostplant choice for herbivorous insects. In the midwestern United States, populations of the parsnip webworm, Depressaria pastinacella Duponchel, introduced from Europe over 150 years ago, feed not only on wild parsnip, Pastinaca sativa L., one of its hosts in Europe, but also on cow parsnip, Heracleum lanatum Michx., a native North American plant. Survival of larvae in the laboratory on fruits of H. lanatum, however, is lower than on fruits of P. sativa, suggesting nutritional or chemical unsuitability. In view of the persistent use of this novel hostplant, we compared susceptibility to predation on these two hostplants in the field. Over a four-year period, contrary to expectations, webworms pupating in cow parsnip stems consistently experienced intense predation, whereas pupae in parsnip stems escaped predation. The pattern persisted when stems of both hostplants containing pupae were placed in pots at a single location. The predator appears to be avian, as only exclosures that prevented access by both birds and mice eliminated predation, whereas exclosures preventing access by mice alone did not reduce predation. The apparent existence of enemy-free space in wild parsnip may be due to differences in stem characteristics; parsnip stems contain significantly greater amounts of the highly photoactive furanocoumarin xanthotoxin and are also significantly tougher compared to stems of cow parsnip. The persistent use of cow parsnips by parsnip webworms in the face of high levels of predation and without clear benefits in terms of growth and development suggests that webworms may have lost genetic variation for host discrimination as a consequence of introduction.