The correlation among B. pilosa flower heads predation and plant size and grouping degree was investigated in ruderal areas at Botucatu, SP, southeastern Brazil. Fifteen plants in reproductive stage were collected monthly, from March to September, 1993. At each collection, ten plants were harvested from grouped and five from isolated plants. The plants were evaluated in relation to biometrics characteristics, biomassa distribution and flower head predation. Plant size showed variation, and the higher frequencies were observed in the smaller size classes. Isolated and grouped plants had no difference in relation to mostly of the biometrics characteristics. A greater number of flower heads was found in bigger plants which exhibited also greater predation level in both isolated and grouped individuals. Therefore grouped and isolated plants showed similar predation levels. Bigger plants, in both types of spatial distribution, had greater number of predated flower heads. This result corroborates the Vigor Hypothesis concerning preference relation between plants and herbivores.
Bidens pilosa; herbivory; preference relations