The indirect effects of ant-hemipteran mutualisms on host plant fitness: comparing the cascading effects of two ant species on coffee production

Ant-hemipteran mutualisms are common in natural and managed ecosystems, and have varying effects on host plant fitness, which may depend on the nature of the ant species involved in the mutualism. Aggressive ant species protect dense populations of herbivores, but also deter herbivores of voracious non-hemipteran herbivores. If the indirect positive effects of aggressive ants outweigh the negative effects of the hemipterans, they can benefit host plant fitness. Less aggressive ant species protect smaller populations of hemipterans, but rarely deter non-hemipteran herbivores. Two species of ants that tend the green coffee scale, Coccus viridis, were compared in their aggressiveness, and overall effects on coffee fitness. Azteca instabilis fostered greater populations of C. viridis, and was associated with lower levels of damage due to the coffee berry borer (Hypothenemus hampei) compared to Pheidole synanthropica. Coffee plants inhabited by A. instabilis in 2009 tended to have a greater mean berries per branch in 2010 than coffee plants inhabited by P. synanthropica, or neither ant species. This study emphasizes that to evaluate the long-term effects of ant-hemipteran mutualisms on host plant fitness, the aggressiveness of the ant should be considered. Understanding how the hemipteran-tending ants interact with hemipteran enemies and/or pathogens will give insight as to whether the negative effects of the hemipterans could outweigh the indirect positive effects of the ant on plant fitness.