Effects of insect herbivory on patterns of seed set in platte thistle (Cirsium canescens Nutt.)

Background/Question/Methods

Insect floral herbivores have demonstrated effects on the density and dynamics of prairie thistle populations. However, little is known about if or how these plants might allocate resources to compensate for or minimize losses in reproductive success due to insect damage under varying resources and natural enemy pressure. For Platte thistle, at the level of an individual thistle plant, flowering is sequential from apex to base, with new flower heads developing at different times throughout the season. Therefore, we hypothesize that one mechanism for plant response to early floral damage might be to alter allocation of resources among later flower heads, in relation to the fate of the earlier heads, to maximize seed output by the end of the single, lifetime flowering episode. To evaluate whether there are recognizable patterns of within plant allocation to flowering that improve subsequent plant seed set, we conducted an experiment to test if and how reducing floral insect herbivory that breaks apical dominance affects the flowering success of subsequent flower heads and total plant seed set. We manipulated moth damage to the initial (terminal) head and compared seed set when subsequent flower heads were or were not protected by insecticide.

Results/Conclusions

Application of insecticide significantly increased plant seed production overall. However, although damage to the apical head did not change whole plant seed set, it did increase seed production per head in lower, later flowers, decreasing the effect of the imposed damage on whole plant seed production. This result provides evidence that plants can partially compensate for damage to their main seed producing head by increasing investment in later heads. However, in this system the effect of this tolerance response was overwhelmed by the high amounts of damage also sustained by the lower, later heads. Insight into how plants allocate flowering resources in response to their herbivores contributes to understanding of plant allocation strategies and potential effects of consumers on the growth rate and persistence of plant populations.