Hemiparasite – host plant interactions and the impact of herbivory: a field experiment

Schizachyrium scoparium Michx. (Nash) growing naturally with varying numbers of the perennial hemiparasite Pedicularis canadensis L. were randomly assigned to one of four clipping treatments (none, early, late, early and late) to determine how parasitism and herbivory affect the grass and whether herbivory of the host indirectly affects hemiparasite growth. Any clipping eliminated subsequent flowering by S. scoparium in year 1 and reduced the number of plants that flowered in year 2, when no clipping occurred. Only hosts clipped early exhibited depressed growth after one summer. The following year plants that had been clipped twice the previous year produced the least shoot mass, and plants that were never clipped produced the most. Hemiparasite load was negatively associated with host shoot mass, especially in year 1, but did not alter the host’s compensatory response to clipping. The effects of host size and host clipping on the nearest hemiparasite were determined in year 1. Pedicularis canadensis shoot mass declined with host size if the host was clipped late, but increased with host size when clipping occurred once early in the season. Although the impact of hemiparasites and clipping on host growth are independent, clipping can alter the value of the host for parasites.

Fruit and seed set in Mayapple (Podophyllum peltatum): influence of intraspecific factors and local enhancement near Pedicularis canadensis

The breeding system and determinants of fruit and seed production in a population of 114 patches of Mayapple (Podophyllum peltatum L.) were examined. Experimental crosses indicated that Mayapple patches are self-incompatible (clones) and that fruit and seed set were pollinator limited. Mayapple flowers are nectarless yet depend primarily on infrequent visits by nectar-searching bumblebees for pollination. The proportion of flowers setting fruit and ovules setting seed in Mayapple clones was inversely associated with the distance to neighboring clones and the distance to Pedicularis canadensis L. plants. Clones within 45 m of P. canadensis plants (which are heavily visited by bumblebees) showed significant increases in fruit and seed production compared with a similar group of more distant clones. This effect is interpreted as evidence for a facilitative interaction between Mayapple and P. canadensis.

Testing the accuracy of using peroxidase activity to indicate stigma receptivity

Stigma peroxidase activity was tested in flowers of Pedicularis canadensis and Clintonia borealis at discrete age-classes during the course of anthesis. For recipient flowers of each age-class pollen adhesion, rate of pollen germination, and total number of grains germinating on stigmas were scored following hand-pollination. In P. canadensis, the onset of detectable peroxidase activity occurred at the transition from the juvenile to pollen-dehiscing age-class. Concurrently, the stickiness of the stigma surface and total number of grains germinating on the stigma increased significantly. Stigma peroxidase was present to some degree throughout anthesis in C. borealis. However, the percentage of the stigma surface in which peroxidase was detectable increased significantly between straight-sided and medium-curled flower age-classes. Again, corresponding increases occurred in the stickiness of the stigma surface and total number of grains germinating. Results suggest that for both species stigma peroxidase activity is a reliable indicator of receptivity.

OBSERVATIONS ON THE IMMATURE STAGES OF GIMNOMERA CEREA AND G. INCISURATA (DIPTERA: ANTHOMYIIDAE, SCATOPHAGINAE)

G. cerea larvae feed upon the ovules and capsule of Pedicularis canadensis. Distribution of the fly coincides closely with its plant host. Adult flies are active in spring during the plant’s flowering period. Both sexes of flies are predaceous on small insects. Details of mating and oviposition behavior are elaborated. After feeding, larvae form puparia in the soil around roots of the host and undergo protracted diapause. All activities of the single generation per year are closely attuned to host plant’s cycle. Distribution of G. incisurata appears to closely follow that of its hosts. Larvae feed upon the flower parts of three species of Penstemon (hirsutus, digitalis, and calycosus). Duration of adult and larval activity periods along with behavior closely parallels G. cerea. In view of known host plant preferences in the Scatophaginae, it is suggested the way in which adaptation of Gimnomera to members of the Scrophulariaceae has been established is via the parasitic or hemiparasitic members of this plant family.