Location: Pest Management ResearchTitle: Cumulative herbivory outpaces compensation for early floral damage on a monocarpic perennial thistle
|LOUDA, SVATA - University Of Nebraska|
Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2017
Publication Date: 12/7/2017
Citation: West, N.M., Louda, S.M. 2017. Cumulative herbivory outpaces compensation for early floral damage on a monocarpic perennial thistle. Oecologia. 186(2):495-506. https://doi.org/10.1007/s00442-017-4027-9.
Interpretive Summary: Flower and seed feeding insects can be a major threat to plant reproduction. Plants that only flower once are particularly vulnerable to seed losses from these herbivores, as such feeding can result in complete loss of lifetime fitness. Plant mechanisms that allow compensation for herbivore damage within a single season should increase the likelihood that at least some reproductive effort is successful. We tested how release of apical dominance (growth and flowering that disproportionately allocates resource investment to the apex region of the plant) affects the number and distribution of flowers and consequent seed production within a growing season. Damage to the plant apex that relaxes apical control allows higher resource investment in low positioned, late flowering, flower heads, which can increase the plant’s seed production potential. Further, we also tested whether controlling insect herbivory (with insecticide) increased the flowering success of these later season flower heads. We found that plants with insecticide-reduced herbivory could compensate, but did not overcompensate, for loss of early damaged apical flower heads. However, when insects were not reduced, plants had significantly lower reproduction success compared to plants with intact apical control. Our results suggest that plants have the resources and capability to compensate for loss of the disproportionate seed production from apical regions, but that the cumulative flower losses due to herbivore damage reduce the potential for the apical damage mechanism to effectively guarantee plant reproduction over a single season.
Technical Abstract: Floral herbivory presents a threat to plant reproductive success. Monocarpic plants should tolerate early apical damage with compensatory reproductive effort by subsequent flower heads during their single flowering season. However, the actual contribution of this tolerance response to net fitness is uncertain under the cumulative insect herbivory plants experience. Using the monocarpic perennial Cirsium canescens (Platte thistle), we quantified total seed reproductive fitness of plants with vs. without experimental damage to the early apical flower head, with and without a reduction of herbivory on subsequent flower heads, for two flowering cohorts. Plants with reduced cumulative herbivory clearly demonstrated the release of apical dominance, and compensation, not overcompensation, for the loss of apical seed. Compensation resulted from greater seed maturation by later flower heads. However, plants that experienced ambient levels of herbivory on subsequent heads undercompensated for the early loss of the apical flower head. Total seed set per plant was lower when the initial apical flower head was damaged. The two main components of the observed compensation response under reduced cumulative herbivory were: a small increase in total flower heads, caused by a higher rate of floral bud survival; and a higher rate of seed maturation by subsequent heads, leading to more viable seeds per matured flower head. We found that compensation for significant early damage to the apical flower head could occur under natural conditions; however, with ambient levels of cumulative floral herbivory, the compensation for apical damage was not sufficient to improve fitness. Variation in the intensity of biological interactions played a role in the success of plant tolerance as a mechanism to maximize individual fitness.