Antagonistic interactions between plant competition and insect herbivory

Area of Science:

• Ecology
• Plant-insect interactions
• Community ecology

Background:

• Interspecific plant competition and insect herbivory can interact synergistically, negatively impacting plant growth.
• The combined effects of competition and herbivory on plant performance are not always additive and may involve complex interactions.

Purpose of the Study:

• To test if plant competition negatively affects herbivore performance and their top-down impact on host plants.
• To investigate whether competition and herbivory interact antagonistically, reducing the combined negative effects.

Main Methods:

• A greenhouse experiment using Poa annua (annual bluegrass) with competitors Plantago lanceolata or Trifolium repens.
• Poa annua was subjected to the presence or absence of competitors and a Poa-specialist aphid herbivore.
• Measured effects on Poa growth, aphid density, shoot biomass, and nitrogen content.

Main Results:

• Both plant competition and aphid herbivory negatively affected Poa annua growth.
• Plant competition reduced aphid density on Poa, partly due to changes in shoot biomass and nitrogen content.
• Reduced aphid densities in competitive treatments alleviated herbivory's negative impact on Poa biomass, indicating antagonism.

Conclusions:

• An antagonistic interaction between plant competition and herbivory was demonstrated.
• The outcome of this interaction (antagonistic vs. synergistic) was contingent on the competitor's identity, observed only with T. repens.
• Competitor identity and herbivore responses to competition influence the interaction's magnitude and direction.