Plant Plasticity, Phenology, and Herbivore Dispersion: Wild Parsnip and the Parsnip Webworm

The influence of plant plasticity on the dispersion of a specialized insect herbivore was analyzed in an 8— to 9—yr oldfield. The parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae), is a specialist on the flowers and developing seeds of the biennial herb Pastinaca sativa (Umbelliferae). The dispersion and abundance of larvae on isolated plants in mixed vegetation was compared with that on plants in patches in which flowing conspecific density was between 30 and 40 plants per m². On isolated plants the number of larvae per gram umbel was 50% higher at the peak of larval abundance, the cumulative proportion of plants attacked was 80% higher, and the cumulative proportion of umbels on each plant attacked was 40% higher than on high density plants. The dispersion of larvae within the study area was governed by the plastics response of plants to increasing plant density rather than by density per se. Successful oviposition was limited to unopened umbels. Isolated plants had more unopened umbels available at any one time and maintained this availability over a longer period of time than high density plants. Most umbels harbored only 1 larva. Hence, the number of umbels and the synchrony between present of unopened umbels and moth oviposition period limited the number of early instar larvae. Within—plant dispersion of larvae was related to the patterns of umbel size and the phenological staggering of umbel appearance of each plant. Experimental introduction of larvae onto plants either synchronously or asynchronously revealed these patterns. Only large—diameter umbels supported >1 larva, and these umbels were more common on isolated plants. Staggering of the umbel production (or its simulation with asynchronous introductions) led to less aggregation of later instar larvae, because larvae arriving early preempted sections of umbel.