Negative cascading effects of a predatory fly larva on an ant-plant protective mutualism

In ant-plant mutualisms mediated by plant-based liquid resources (e.g., extrafloral and pericarpial nectar), third-party exploiters are often present. Usually, the costs of having exploiters species seem to have little to no effect on ant-plant mutualistic interaction. However, little is known about how and to what extent exploiters affect mutualistic interactions. In this study, we evaluated the cascading effects of Rhinoleucophenga myrmecophaga (Diptera) predatory larvae on the outcomes of the interaction between mutualistic ants and a pericarpial nectary (PN)-bearing plant, Cordiera elliptica (Rubiaceae). In a manipulative experiment controlling for the presence of ants and/or predatory larvae, we evaluated arthropods and larvae, plant fitness, and pericarpial nectar production. We found that: (a) the presence of larvae decreased ant abundance by 50% resulting in an increase of arthropod number by 38%; (b) the absence of ants negatively influenced larva development and survival; (c) fruits with larvae and without ants had lower volume and weight, fewer viable seeds, and a higher parasitism rate; and (d) the pericarpial nectar sugar concentration is higher in plants with larvae and without ants. We showed that the outcomes of the multitrophic relationship between ants, R. myrmecophaga larvae, and an PN-bearing plant led to costly cascading effects on this mutualistic relationship. This interaction can range from negative to positive, in which the predatory fly larvae directly and indirectly influence ants number and behavior, and consequently the plant fitness.

Automated summary

This study evaluated the cascading effects of predatory larvae on the outcomes of ant-plant interactions. The presence of larvae decreased ant abundance and the absence of ants negatively influenced larval development and survival. Fruits with larvae and without ants had lower quality and the pericarpial nectar sugar concentration was higher in plants with larvae and without ants.