Seasonal changes in response ofSenecio ovatus to grazing by the chrysomelid beetleChrysomela speciosissima.

The response ofSenecio ovatus to herbivory byChrysomela speciosissima (Coleoptera, Chrysomelidae) was studied in the Krušné hory Mountains, Czechoslovakia. The following questions were addressed: (1) what is the impact of natural levels of insect herbivory on the performance of individual plants, (2) how do the levels of herbivory change during the growing period, and (3) what is the relative importance of direct and indirect effects of herbivory? Ten experimental plots sprayed with metathion and ten controls were established. Damage to plants was quantified in the period of maximum herbivore activity (June) and again at the end of the growing season (August). In the first half of the season, total leaf area in grazed ramets was reduced by 30.3% compared to ungrazed ramets; half of this reduction was due to beetle consumption and half was due to indirect effects. Individual leaf area was not significantly different between treatments. Insecticide-treated ramets were taller and heavier than the controls. During the second half of the growing period the differences in ramet weight and leaf area disappeared because of compensatory growth. However, the insecticide-treated ramets remained taller and had higher seed numbers per capitulum. Neither number of capitula per ramet nor seed weight were significantly different between treatments. In the grazed population, the leaf area reduction (30.3%) in June resulted in 36.5% reduction in fecundity at the end of August.