Herbivory affects patterns of plant reproductive effort and seed production

Insect floral herbivory can dramatically reduce plant reproductive success. Thus, plants should have evolved mechanisms that minimize the effect of insect herbivores, particularly in monocarpic species that must maximize fitness in a single flowering year. Tolerance is one such mechanism; however, few experiments to date evaluate underlying mechanisms of plant tolerance under natural conditions. We compared plant seed production by the monocarpic Cirsium canescens (Platte thistle) in control (undamaged) plants versus plants with damage imposed upon the apical, flower head. We hypothesized that C. canescens would tolerate damage to its large, early, apical flower head by increasing reproductive effort in subsequent flower heads, compensating for potential fitness loss under herbivore pressure during its final, fatal, flowering season. In addition, we examined the consequences of each damage treatment in plants with versus without experimentally reduced subsequent floral herbivory. We found that plants under ambient herbivory (unprotected subsequent heads) undercompensated for the loss of apical seed production; total plant seed set was lower when the large apical flower head was damaged. However, when insect herbivory was reduced on the later flower heads, significant compensation for apical seed loss occurred. The compensation reflected greater seed maturation by later flower heads, rather than greater subsequent flower head production, indicating that the total number of flower heads was constrained. These results provide the first analysis of plant mechanisms that reduce the effects of insect floral herbivores on C. canescens, a well-studied species in which the local