Transgenerational effects can be important mechanisms by which herbivores reduce maternal plant fitness. Although transgenerational effects often involve changes in off-spring quality, herbivore-induced shifts in seed dispersal phenology may place off-spring into ecological contexts that are more hostile than those encountered by off-spring of undamaged plants. Here we address whether 1) apical meristem mining on tall thistle (Cirsium altissimum) alters seed dispersal phenology? and 2) seed dispersal timing affects off-spring performance? Our data arise from two experiments in south-central Kansas tallgrass prairie. The first experiment involved manipulating insect herbivory on apical meristems of adult C. altissimum through directed application of topical insecticide. We quantified the date on which each flower head on each plant began dispersing seed. The second experiment involved sowing tall thistle seeds (100 seeds / 0.5 m X 0.5 m experimental plot) on three dates in fall 2014 (Sept 14, Sept 28, Oct 5) during tall thistle’s natural seed dispersal interval. We isolated effects of dispersal date from potentially confounded effects of seed quality by drawing seeds for each sowing date from a common pool of seeds that were collected before the experiment began. We are quantifying tall thistle densities and performance in plots.
Apical meristem mining significantly delayed adult tall thistles’ first dates of seed dispersal and shortened their durations of seed dispersal. However, the magnitude of changes in mean date of first dispersal and mean dispersal duration were small, less than a week. In the seed sowing experiment, plots in which seeds were sown on Sept 14 had significantly greater fall seedling germination than did plots in which seeds were sown on Sept 28 or Oct 5. Germination in the following spring did not differ among sowing dates. Over-all (for fall and spring germination combined), plots with seeds sown on Sept 14 had greater seedling germination than plots sown on Oct 5. Seedling survival was unaffected by sowing date. One year after seed sowing, two plots that were sown on Sept 14 contained off-spring that produced seed themselves. No off-spring from plots sown on later dates reproduced in their first year. Shifts in seed dispersal phenology induced by apical meristem mining may not be sufficiently large to strongly affect off-spring performance. However, poor performance of later dispersed seed may limit the efficacy of compensatory flowering, involving developing late-season flower heads, as a tolerance mechanism for herbivory in C. altissimum.