The cinnabar moth, Tyria jacobaeae L. (Lepidoptera: Erebidae) was released in Western Oregon as a biological control agent for a European weed but found a “non-target” host in native perennial Senecio triangularis Hook. (Asteraceae). This non-target interaction could be a significant negative impact of the herbivore introduction if it causes population declines for the plant. Previous work found no evidence that rates of defoliation (0 to 100%) caused reduction in plant size or reproductive output the following year. But an experimental shift in the timing of herbivory produced a 95% reduction in seed production due to floral herbivory.
The sensitivity of S. triangularis to seed loss is unknown. Recruitment rates have not been measured; and reduction of seed-set should only impact recruitment if the plant is seed-limited. This study tested whether recruitment to seedling stage for S. triangularis is limited by seed-rain or availability of suitable microsites. We used a factorial design to test effects of seed-rain (high, low, no seed added) and safe-site manipulation (vegetation removed, clipped, or unaltered); with response as number of seedlings established in 2017. Treatments were fully randomized for 9 treatments x 5 blocks = 45 plots measuring 0.25 m2 on Marys Peak, Oregon.
Results/Conclusions
Our results provide evidence that Senecio triangularis is seed-limited; the median number of seedling established increased 2-fold in plots receiving high compared to low seed-rain. Considering quantities of viable seed sown per plot (low ≃ 300, high ≃ 1080 viable seeds), seedling establishment was small, and widely variable (range: 0 to 73 seedlings). Background recruitment was expected in no-seed plots from mature plants within the study site, but only 1 seedling was counted across 15 no-seed plots. Evidence of an interaction between seed-rain and safe-site shows that the effect of seed-rain on seedling recruitment depends on the type of manipulation for each plot.
This work fills a knowledge gap in the life-cycle of a long-lived perennial necessary for measuring and interpreting interaction strength of a non-target interaction. Sensitivity of S. triangularis to seed reduction shows that floral herbivory by the cinnabar moth can negatively impact recruitment. High rates over consecutive years could lead to population level consequences, and should be considered in an overall assessment of the ecological risk posed to S. triangularis. Ongoing work measures exposure of S. triangularis to seed herbivory by the cinnabar moth at regional scales in the Oregon Cascades and Coast Range.