Insect floral herbivory can be a critical determinant of individual plant reproductive success. However, if plant reproductive effort varies through time, individuals may have opportunities to partially compensate for reproductive losses due to insects. We examined the influence of floral herbivory on the distribution of individual flowering effort, and the consequences of altered flowering patterns for plant seed set. We conducted an experiment examining how reduction of insect herbivores, and the breaking of apical dominance through damage to the apical flower head, influenced the distribution and success of seed production in two native thistles with different life history strategies, Cirsium canescens Nutt. (Platte thistle) and Cirsium undulatum (Nutt.) Spreng. (wavyleaf thistle). Individuals of each species were marked in early spring 2007 and 2008. Half of these individuals received a regular insecticide treatment and the remaining received a water control. Within each of the two treatment groups, half also received researcher and insect imposed damage to the apical flower head. Measurements of plant performance and flowering were obtained in the field, and all seed heads were collected after plant senescence for observation of damage and successful seed production.
Results/Conclusions
Reduction of insects significantly increased plant seed production for both species. In C. canescens (monocarp), damage to the apical head did not change whole plant seed set in the insecticide treatment, and increased seed production per flower head. Plants compensated for damage to their largest seed producing (apical) flower head by increasing investment in later flower heads, lessening effects on whole plant seed production. However, when late flowers were not protected from insects (water controls), potential compensation was insufficient to tolerate high ambient insect damage. For C. undulatum (iterocarp), when insects were reduced (insecticide treatment), damage to the apical flower head resulted in lower overall plant seed production and lower seed production per flower head compared to undamaged controls. In addition, late flower heads did not display a relative increase in seed production. Though reproductive effort in the presence of floral damage varied between species, neither was able to compensate for loss of seed production by the apical flower head under ambient insect loads. Insight into how plants vary reproductive patterns in response to floral herbivory contributes toward identifying the efficacy of strategies plants might use to maximize reproduction in the presence of complex and varying interactions.
