Depression in Ragwort (Senecio jacobaea) Abundance Following Introduction of Tyria jacobaeae and Longitarsus jacobaeae on the Central Coast of Oregon

This study measured the depression in abundance of dormant and actively growing stages of ragwort (Senecio jacobaea) following introduction of two phytophagous insects, ragwort flea beetle (Longitarsus jacobaeae) and cinnabar moth (Tyria jacobaeae) on the central coast of Oregon, U.S.A. Actively growing stages (seedlings, rosettes, flowering plants) declined to 3% and dormant stages (seeds and vegetative buds) declined to 32% of their former abundance following increase in herbivore populations. Based on evidence that the ragwort population was closed to migration. the depression in host abundance was ascribed to a >95% mortality spread over all actively growing stages followed by a 33% decline in per capita birth rate of the surviving reproductive plants. A decrease in ragwort standing crop was balanced by an increase in standing crop of other plant species (88% perennial grasses), leading to no net change in standing crop of the plant community. Two years following the decline, ragwort standing crop has remained at low levels (flowering plants have disappeared altogether), but densities of seedlings and small rosettes have rebounded to previous levels due to recruitment from the dormant seeds in the soil. Further study is required to determine whether ragwort will remain at low levels in the long term. Reduction de l'Abondance des Senecons Jacobee (Senecio jacobaea) Apres l'Introduction du Lepidoptere Tyria (Tyria jacobaeae) et de l'Altise du Senecon (Longitarsus jacobaeae) Cette etude visait a mesurer Ia reduction de la densite des senecons Senecio jacobaea (parties souterraines et aeriennes) apres l'introduction de deux insectes phvtophages. soft Longitarsus jacobaeae et le lepidoptere Tyria jacobaeae, dans la partie centrale de la cote de l'Oregon aux Etats-Unis. La reduction a ete exprimee selon la formule q = N'/K, ou K represente l'abondance moyenne de la plante hOte avent l'introduction des herbivores et designe le nombre reduit d'hOtes, cet equilibre etant probablement assure par les herbivores. La valeur de q pour les parties aeriennes (jeunes plants, rosettes. plantes a fleurs) etait de 0,03 et de 0,32 pour les parties souterraines (graines et pousses). En se fondant sur des donnees selon lesquelles it n'y avait aucune migration des senecons et que leur distribution aux differents sta ges de croissance etau stable, le reduction du nombre d'hOtes a ete attribuee a la baisse du taux de reproduction, plutOt qu'aux modifications des taux de survie ou de migration. La reduction du stock existant de senecon a ete equilibree par une augmentation du stock existant d'autres especes de plantes (herbes vivaces dans 98% des cas), ce qui n'a provoque aucun changement net du stock existant de Ia communaute de plantes.